Prof. Dr. Claus Michael Schneider

Peter Grünberg Institute
University of Duisburg-Essen

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  • Bulk electronic structure of lanthanum hexaboride (La B6) by hard x-ray angle-resolved photoelectron spectroscopy
    Rattanachata, A. and Nicolaï, L.C. and Martins, H.P. and Conti, G. and Verstraete, M.J. and Gehlmann, M. and Ueda, S. and Kobayashi, K. and Vishik, I. and Schneider, C.M. and Fadley, C.S. and Gray, A.X. and Minár, J. and Nemšák, S.
    Physical Review Materials 5 (2021)
    In the last decade rare-earth hexaborides have been investigated for their fundamental importance in condensed matter, and for their applications in advanced technological fields. Among these compounds, LaB6 has a special place, being a traditional d-band metal without additional f bands. In order to understand the bulk electronic structure of the more complex rare-earth hexaborides, in this paper we investigate the bulk electronic structure of LaB6 using tender/hard x-ray photoemission spectroscopy, measuring both core-level and angle-resolved valence-band spectra. Furthermore, we compare the La 3d core level spectrum to cluster model calculations in order to understand the bulklike core-hole screening effects. The results show that the La 3d well-screened peak is at a lower binding energy compared to the main poorly screened peak; the relative intensity between these peaks depends on how strong the hybridization is between La and B atoms. We show that the recoil effect, negligible in the soft x-ray regime, becomes prominent at higher kinetic energies for lighter elements, such as boron, but is still negligible for heavy elements, such as lanthanum. In addition, we report the bulklike band structure of LaB6 determined by tender/hard x-ray angle-resolved photoemission spectroscopy (HARPES). We compare HARPES experimental results to the free-electron final-state calculations and to the more precise one-step photoemission theory including matrix element and phonon excitation effects. The agreement between the features present in the experimental ARPES data and the theoretical calculations is very good. In addition, we consider the nature and the magnitude of phonon excitations in order to interpret HARPES experimental data measured at different temperatures and excitation energies. We demonstrate that the one-step theory of photoemission and HARPES experiments provides, at present, the only approach capable of probing, both experimentally and theoretically, true "bulklike"electronic band structure of rare-earth hexaborides and strongly correlated materials. © 2021 American Physical Society.
    view abstract10.1103/PhysRevMaterials.5.055002
  • Coherent GHz lattice and magnetization excitations in thin epitaxial Ag/Fe/Cr/Fe films
    Cao, D. and Adam, R. and Bürgler, D.E. and Wang, F. and Song, C. and Li, S. and Mikulics, M. and Hardtdegen, H. and Heidtfeld, S. and Greb, C. and Schneider, C.M.
    Physical Review B 104 (2021)
    We excited an epitaxial magnetic Ag/Fe/Cr/Fe multilayer nonthermally and nonoptically with very short (<1 ps) electromagnetic pulses. We detected the synchronous phononic-magnetic response by time-resolved magneto-optical Kerr effect measurements. The Ag/Fe/Cr/Fe multilayer was patterned into a coplanar waveguide transmission line, and the electromagnetic pulses were generated by pulsed-laser illumination of an integrated GaAs photoconductive switch (PCS). The detected magnetic excitations comprise up to four narrow-band high-order modes with the highest frequency reaching 30 GHz. The mode frequencies are independent of both temperature in the range from 16 to 300 K and the applied external magnetic field up to 120 mT. Our analysis shows that the origin of the rigidity of these high-frequency modes is the strong coupling of the magnetic subsystem with the lattice of the Ag/Fe/Cr/Fe multilayer. The exciting electromagnetic pulse generated by the PCS induces, via magnetoelastic coupling, long-lived (ns) standing GHz acoustic waves normal to the Ag/Fe/Cr/Fe film plane. These lattice oscillations in turn couple back and drive the magnetization oscillations via the magnetoelastic coupling. The temperature and field dependence of the damping of the oscillations can be described by inelastic phonon-phonon and phonon-magnon scattering. Our study opens up a possibility of using coherent lattice and magnetization dynamics in ferromagnetic films for spintronic devices at GHz clock rates. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.104.054430
  • Cyclophane with eclipsed pyrene units enables construction of spin interfaces with chemical accuracy
    Metzelaars, M. and Schleicher, S. and Hattori, T. and Borca, B. and Matthes, F. and Sanz, S. and Bürgler, D.E. and Rawson, J. and Schneider, C.M. and Kögerler, P.
    Chemical Science 12 (2021)
    Advanced functionality in molecular electronics and spintronics is orchestrated by exact molecular arrangements at metal surfaces, but the strategies for constructing such arrangements remain limited. Here, we report the synthesis and surface hybridization of a cyclophane that comprises two pyrene groups fastened together by two ferrocene pillars. Crystallographic structure analysis revealed pyrene planes separated by ∼352 pm and stacked in an eclipsed geometry that approximates the rare configuration of AA-stacked bilayer graphene. We deposited this cyclophane onto surfaces of Cu(111) and Co(111) at submonolayer coverage and studied the resulting hybrid entities with scanning tunnelling microscopy (STM). We found distinct characteristics of this cyclophane on each metal surface: on non-magnetic Cu(111), physisorption occurred and the two pyrene groups remained electronically coupled to each other; on ferromagnetic Co(111) nanoislands, chemisorption occurred and the two pyrene groups became electronically decoupled. Spin-polarized STM measurements revealed that the ferrocene groups had spin polarization opposite to that of the surrounding Co metal, while the pyrene stack had no spin polarization. Comparisons to the non-stacked analogue comprising only one pyrene group bolster our interpretation of the cyclophane's STM features. The design strategy presented herein can be extended to realize versatile, three-dimensional platforms in single-molecule electronics and spintronics. © The Royal Society of Chemistry 2021.
    view abstract10.1039/d1sc01036k
  • Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures
    Hajlaoui, M. and Ponzoni, S. and Deppe, M. and Henksmeier, T. and As, D.J. and Reuter, D. and Zentgraf, T. and Springholz, G. and Schneider, C.M. and Cramm, S. and Cinchetti, M.
    Scientific Reports 11 (2021)
    Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures. © 2021, The Author(s).
    view abstract10.1038/s41598-021-98569-6
  • Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure
    Cojocariu, I. and Carlotto, S. and Sturmeit, H.M. and Zamborlini, G. and Cinchetti, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Jugovac, M. and Puschnig, P. and Piamonteze, C. and Casarin, M. and Feyer, V. and Schneider, C.M.
    Chemistry - A European Journal 27 (2021)
    Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature. © 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202004932
  • Fusing pyrene and ferrocene into a chiral, redox-active triangle
    Metzelaars, M. and Sanz, S. and Rawson, J. and Hartmann, R. and Schneider, C.M. and Kögerler, P.
    Chemical Communications 57 (2021)
    A macrocycle that integrates three ferrocene-pyrene dyads in a triangularC2-symmetric arrangement is synthesised as a racemate in a simple one-pot approach. Crystal structural analysis reveals two enantiomeric conformers that pack alternatinglyviaπ-π stacking and interconvert dynamically in solution. Electrochemical investigations indicate weak electrostatic interactions between Fc groups upon oxidation to a mixed valence triangle. © The Royal Society of Chemistry 2021.
    view abstract10.1039/d1cc02191e
  • Generation of terahertz transients from Co2Fe0.4 M0.6 Si-Heusler-alloy/normal-metal nanobilayers excited by femtosecond optical pulses
    Heidtfeld, S. and Adam, R. and Kubota, T. and Takanashi, K. and Cao, D. and Schmitz-Antoniak, C. and Bürgler, D.E. and Wang, F. and Greb, C. and Chen, G. and Komissarov, I. and Hardtdegen, H. and Mikulics, M. and Sobolewski, R. and Suga, S. and Schneider, C.M.
    Physical Review Research 3 (2021)
    We generated pulses of electromagnetic radiation in the terahertz (THz) frequency range by optical excitation of (CFMS)/normal-metal (NM) bilayer structures. The CFMS is a Heusler alloy showing a band gap in one spin channel and is therefore a half metal. We compared the THz emission efficiency in a systematic manner for four different CFMS/NM bilayers, where NM was either Pt, Ta, Cr, or Al. Our measurements show that the THz intensity is highest for a Pt capping. We also demonstrate the tunability of the THz amplitude by varying the magnetic field for all four bilayers. We attribute the THz generation to the inverse spin Hall effect. In order to investigate the role of the interface in THz generation, we measured the spin mixing conductance for each CFMS/NM bilayer using a ferromagnetic resonance method. We found that the spin-orbit coupling cannot completely describe the THz generation in the bilayers and that the spin transmission efficiency of the CFMS/NM interface and the spin diffusion length, as well as the oxidation of the NM layer, play crucial roles in the THz emission process. © 2021 Published by the American Physical Society
    view abstract10.1103/PhysRevResearch.3.043025
  • Insight into intramolecular chemical structure modifications by on-surface reaction using photoemission tomography
    Cojocariu, I. and Feyersinger, F. and Puschnig, P. and Schio, L. and Floreano, L. and Feyer, V. and Schneider, C.M.
    Chemical Communications 57 (2021)
    The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems. © The Royal Society of Chemistry 2021.
    view abstract10.1039/d1cc00311a
  • Magnetization relaxation dynamics in multilayers on pico- And nanosecond timescales
    Wang, F. and Bürgler, D.E. and Adam, R. and Parlak, U. and Cao, D. and Greb, C. and Heidtfeld, S. and Schneider, C.M.
    Physical Review Research 3 (2021)
    We experimentally investigated magnetization relaxation dynamics in the largely unexplored time window extending from few picoseconds up to two nanoseconds following femtosecond laser pulse excitation. We triggered magnetization dynamics in multilayers and measured the resulting magneto-optic response by recording both transient hysteresis loops as well as transients of magnetization dynamics. We observe that the coercive field of the sample is still strongly suppressed even ∼1 ms after the laser excitation, which is three orders of magnitude longer than the recovery time of the magnetization amplitude. In addition, we succeeded to fit the magnetization relaxation data in the entire experimentally observed time window by considering two phenomenological time constants and describing fast (ps) and slow (ns) magnetization relaxation processes, respectively. The fits of the data suggest a magnetic field dependent relaxation slowdown beyond 100 ps after excitation. We observe an explosion of the and values when the magnetization is completely quenched and relaxes intrinsically in the absence of an external magnetic field. We interpret the phenomenological time constants and using an intuitive physical picture based on the Landau-Lifshitz-Bloch model and numerical solutions of the extended three-temperature model. © 2021 Published by the American Physical Society
    view abstract10.1103/PhysRevResearch.3.033061
  • Molecular Level Synthesis of InFeO3and InFeO3/Fe2O3Nanocomposites
    Nahrstedt, V. and Stadler, D. and Fischer, T. and Duchoň, T. and Mueller, D.N. and Schneider, C.M. and Mathur, S.
    Inorganic Chemistry 60 (2021)
    New heterometallic In-Fe alkoxides [InFe(OtBu)4(PyTFP)2] (1), [InFe2(OneoPen)9(Py)] (2), and [InFe3(OneoPen)12] (3) were synthesized and structurally characterized. The arrangement of metal centers in mixed-metal framework was governed by the In:Fe ratio and the coordination preferences of Fe(III) and In(III) centers to be in tetrahedral and octahedral environments, respectively. 3 displayed a star-shaped so-called "Mitsubishi"motif with the central In atom coordinated with three tetrahedral {Fe(OneoPen)4}- anionic units. The deterministic structural influence of the larger In atom was evident in 1 and 2 which displayed the coordination of neutral coligands to achieve the desired coordination number. Thermal decomposition studies of compounds 1-3 under inert conditions with subsequent powder diffraction studies revealed the formation of Fe2O3 and In2O3 in the case of 3 and 2, whereas 1 intriguingly produced elemental In and Fe. In contrary, the thermal decomposition of 1-3 under ambient conditions produced a ternary oxide, InFeO3, with additional Fe2O3 present as a secondary phase in a different stoichiometric ratio predetermined through the In:Fe ratio in 2 and 3. The intimate mixing of different phases in InFeO3/Fe2O3 nanocomposites was confirmed by transmission electron microscopy of solid residues obtained after the decomposition of 1 and 2. The pure InFeO3 particles demonstrated ferromagnetic anomalies around 170 K as determined by temperature-dependent field-cooled and zero-field-cooled magnetization experiments. A first-order magnetic transition with an increase in the ZFC measurements was explained by temperature-induced reduction of the Fe-Fe distance and the corresponding increase in superexchange. © 2021 American Chemical Society.
    view abstract10.1021/acs.inorgchem.0c03425
  • Multistate current-induced magnetization switching in Au/Fe/MgO(001) epitaxial heterostructures
    Gospodarič, P. and Młyńczak, E. and Soldatov, I. and Kákay, A. and Bürgler, D.E. and Plucinski, L. and Schäfer, R. and Fassbender, J. and Schneider, C.M.
    Physical Review Research 3 (2021)
    Magnetization switching using in-plane charge current recently has been widely investigated in heavy metal/ferromagnet bilayers with the switching mechanism usually attributed to the action of the spin-orbit coupling. Here we study in-plane current induced magnetization switching in model epitaxial bilayers that consist of Au(001) and Fe(001) grown on MgO(001). We use the planar Hall effect combined with magnetooptical Kerr effect (MOKE) microscopy to investigate magnetic properties of the bilayers and current-induced switching. We show that a current density beyond 1.4×107 A/cm2 can be employed for reproducible electrical switching of the magnetization between multiple stable states that correspond to different arrangements of magnetic domains with magnetization direction along one of the in-plane easy magnetization axes of the Fe(001) film. Lower current densities result in stable intermediate transversal resistances which are interpreted based on MOKE-microscopy investigations as resulting from the current-induced magnetic domain structure that is formed in the area of the Hall cross. We find that the physical mechanism of the current-induced magnetization switching of the Au/Fe/MgO(001) system at room temperature can be fully explained by the Oersted field, which is generated by the charge current flowing mostly through the Au layer. © 2021 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevResearch.3.023089
  • Nanoscopic Surface Decomposition of Pr0.5Ba0.5CoO3-δPerovskites Turns Performance Descriptors Ambiguous
    Mueller, D.N. and Giesen, M. and Duchoň, T. and Cramm, S. and Gunkel, F. and Jugovac, M. and Zamborlini, G. and Feyer, V. and Schneider, C.M.
    Journal of Physical Chemistry C 125 (2021)
    The surface electronic structure of a material is frequently used to identify simple descriptors for its catalytic efficacy and other properties. To harness the predictive ability of such descriptors, structural and chemical evolutions of the material when exposed to operating conditions, such as oxidizing environments and high temperatures, need to be considered. These evolutions occur at length scales not easily observable, leading to averaging over short-range variations and thus misinterpretation of the property in question. Here, we investigate perovskite Pr0.5Ba0.5CoO3-δ as a prototypical mixed ionic-electronic conductor that exhibits promising catalytic properties toward the oxygen evolution reaction in electrochemical cells, which have been characterized by such descriptors. We employ spatially resolved X-ray absorption spectroscopy and find a Cahn-Hilliard-type decomposition process at sub-micrometer length scales after mere hours at operating or processing conditions. The observation is in contrast to the thermodynamic stability of the Pr0.5Ba0.5CoO3-δ bulk, suggesting the decomposition to be confined to the surface. Our results showcase a considerable lateral inhomogeneity of the surface electronic structure, emphasizing that descriptors derived through spatially averaging techniques have to be heavily scrutinized. ©
    view abstract10.1021/acs.jpcc.1c00976
  • Near total reflection x-ray photoelectron spectroscopy: Quantifying chemistry at solid/liquid and solid/solid interfaces
    Martins, H.P. and Conti, G. and Cordova, I. and Falling, L. and Kersell, H. and Salmassi, F. and Gullikson, E. and Vishik, I. and Baeumer, C. and Naulleau, P. and Schneider, C.M. and Nemsak, S.
    Journal of Physics D: Applied Physics 54 (2021)
    Near total reflection regime has been widely used in x-ray science, specifically in grazing incidence small angle x-ray scattering and in hard x-ray photoelectron spectroscopy (XPS). In this work, we introduce some practical aspects of using near total reflection (NTR) in ambient pressure XPS and apply this technique to study chemical concentration gradients in a substrate/photoresist system. Experimental data are accompanied by x-ray optical and photoemission simulations to quantitatively probe the photoresist and the interface with the depth accuracy of ∼1 nm. Together, our calculations and experiments confirm that NTR XPS is a suitable method to extract information from buried interfaces with highest depth-resolution, which can help address open research questions regarding our understanding of concentration profiles, electrical gradients, and charge transfer phenomena at such interfaces. The presented methodology is especially attractive for solid/liquid interface studies, since it provides all the strengths of a Bragg-reflection standing-wave spectroscopy without the need of an artificial multilayer mirror serving as a standing wave generator, thus dramatically simplifying the sample synthesis. © 2021 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ac2067
  • Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi4Te7 and MnBi6Te10
    Vidal, R.C. and Bentmann, H. and Facio, J.I. and Heider, T. and Kagerer, P. and Fornari, C.I. and Peixoto, T.R.F. and Figgemeier, T. and Jung, S. and Cacho, C. and Büchner, B. and Van Den Brink, J. and Schneider, C.M. and Plucinski, L. and Schwier, E.F. and Shimada, K. and Richter, M. and Isaeva, A. and Reinert, F.
    Physical Review Letters 126 (2021)
    Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi2Te3-terminated surfaces but remains preserved for MnBi2Te4-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination. © 2021 American Physical Society.
    view abstract10.1103/PhysRevLett.126.176403
  • Quantum spin mixing in Dirac materials
    Chen, Y.-J. and Hoffmann, M. and Zimmermann, B. and Bihlmayer, G. and Blügel, S. and Schneider, C.M. and Tusche, C.
    Communications Physics 4 (2021)
    The spin of the electron is nowadays replacing the charge as basic carrier of information not only in spintronics applications, but also in the emerging field of quantum information. Topological quantum materials, where spin-momentum locking is believed to lead to particularly long spin lifetimes, are regarded as a promising platform for such applications. However, spin-orbit coupling, that is essential to all topological matter, at the same time gives rise to spin mixing and decoherence as a major obstacle for quantum computing. Here, we give experimental evidence that hot-spots of spin-mixing and spin-conserving contributions of the spin-orbit operator coexist in an archetypal topological Dirac metal, and that these hot spots can have a strongly anisotropic distribution of their respective wave vectors with respect to the spin quantization direction. Our results can be understood within a theory that takes into account the decomposition of the spin-orbit Hamiltonian into spin-conserving and spin-flip terms, contributing to a better understanding of quantum decoherence in topological materials, in general. © 2021, The Author(s).
    view abstract10.1038/s42005-021-00682-5
  • Reversible redox reactions in metal-supported porphyrin: The role of spin and oxidation state
    Cojocariu, I. and Carlotto, S. and Zamborlini, G. and Jugovac, M. and Schio, L. and Floreano, L. and Casarin, M. and Feyer, V. and Schneider, C.M.
    Journal of Materials Chemistry C 9 (2021)
    On-surface molecular functionalization paved the way for the stabilization of chelated ions in different oxidation and spin states, allowing for the fine control of catalytic and magnetic properties of metalorganic networks. Considering two model systems, a reduced Co(i) and an open-shell Co(ii) metal-supported 2D molecular array, we investigate the interplay between the low valence oxidation and unpaired spin state in the molecular reactivity. We show that the redox reaction taking place at the cobalt tetraphenylporphyrin/Cu(100) interface, stabilizing the low-spin Co(i) state with no unpaired electrons in its valence shell, plays a pivotal role in changing the reactivity. This goes beyond the sole presence of unpaired electrons in the valence state of the Co(ii) metal-organic species, often designated as being responsible for the reactivity towards small molecules like NO and NO2. The reversible Co-NO2interaction, established with the Co(i) leads to the stabilization of the Co(iii) oxidation state. © The Royal Society of Chemistry 2021.
    view abstract10.1039/d1tc02222a
  • Room-Temperature On-Spin-Switching and Tuning in a Porphyrin-Based Multifunctional Interface
    Sturmeit, H.M. and Cojocariu, I. and Windischbacher, A. and Puschnig, P. and Piamonteze, C. and Jugovac, M. and Sala, A. and Africh, C. and Comelli, G. and Cossaro, A. and Verdini, A. and Floreano, L. and Stredansky, M. and Vesselli, E. and Hohner, C. and Kettner, M. and Libuda, J. and Schneider, C.M. and Zamborlini, G. and Cinchetti, M. and Feyer, V.
    Small (2021)
    Molecular interfaces formed between metals and molecular compounds offer a great potential as building blocks for future opto-electronics and spintronics devices. Here, a combined theoretical and experimental spectro-microscopy approach is used to show that the charge transfer occurring at the interface between nickel tetraphenyl porphyrins and copper changes both spin and oxidation states of the Ni ion from [Ni(II), S = 0] to [Ni(I), S = 1/2]. The chemically active Ni(I), even in a buried multilayer system, can be functionalized with nitrogen dioxide, allowing a selective tuning of the electronic properties of the Ni center that is switched to a [Ni(II), S = 1] state. While Ni acts as a reversible spin switch, it is found that the electronic structure of the macrocycle backbone, where the frontier orbitals are mainly localized, remains unaffected. These findings pave the way for using the present porphyrin-based system as a platform for the realization of multifunctional devices where the magnetism and the optical/transport properties can be controlled simultaneously by independent stimuli. © 2021 The Authors. Small published by Wiley-VCH GmbH
    view abstract10.1002/smll.202104779
  • Spin-polarized quantized electronic structure of Fe(001) with symmetry breaking due to the magnetization direction
    Młyńczak, E. and Aguilera, I. and Gospodarič, P. and Heider, T. and Jugovac, M. and Zamborlini, G. and Tusche, C. and Suga, S. and Feyer, V. and Blügel, S. and Plucinski, L. and Schneider, C.M.
    Physical Review B 103 (2021)
    Quantum well states formed by d electrons in metallic thin films are responsible for many fundamental phenomena that oscillate with layer thickness, such as magnetic anisotropy or magnetoresistance. Using momentum microscopy and angle-resolved photoemission, we mapped in unprecedented detail the quantized electronic states of Fe(001) in a broad photon energy range starting from soft x-ray (160 eV) down to vacuum ultraviolet (8.4 eV). We show that it is possible to simulate the experimentally observed photoemission spectra with high accuracy by using the ab initio electronic bulk band structure as the initial state, taking into account that free electron final electronic states are intrinsically broadened along the wave vector direction perpendicular to the sample surface. To simulate the thin-film case, we take into account a subset of the initial electronic states, which results in the reproduction of the quantized electronic structure observed in the experiment. In addition, we present results of the spin-sensitive measurements, which are confronted with the photoemission simulation that takes into account the spin degree of freedom. We demonstrate electronic states that can be responsible for the oscillations of the magnetic anisotropy in Fe(001) thin films with periods of about 5 and 9 monolayers. We show that these quantum well states change position in reciprocal space depending on the magnetization direction. Our photoemission simulation reproduces this effect, which highlights its origin in the relativistic bulk electronic band structure of bcc Fe. We also observed magnetization-dependent spin-orbit gaps with the symmetry lower than the bulk symmetry. We believe that the same method of simulating photoemission spectra might facilitate interpretation of the photoemission intensities measured for other three-dimensional materials, especially when the spin-polarized quantized electronic states are considered. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.103.035134
  • Establishing structure-sensitivity of ceria reducibility: Real-Time observations of surface-hydrogen interactions
    Duchoň, T. and Hackl, J. and Mueller, D.N. and Kullgren, J. and Du, D. and Senanayake, S.D. and Mouls, C. and Gottlob, D.M. and Khan, M.I. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Journal of Materials Chemistry A 8 (2020)
    The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure-reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c9ta11784a
  • Evaluation of molecular orbital symmetry via oxygen-induced charge transfer quenching at a metal-organic interface
    Cojocariu, I. and Sturmeit, H.M. and Zamborlini, G. and Cossaro, A. and Verdini, A. and Floreano, L. and D'Incecco, E. and Stredansky, M. and Vesselli, E. and Jugovac, M. and Cinchetti, M. and Feyer, V. and Schneider, C.M.
    Applied Surface Science 504 (2020)
    Thin molecular films under model conditions are often exploited as benchmarks and case studies to investigate the electronic and structural changes occurring on the surface of metallic electrodes. Here we show that the modification of a metallic surface induced by oxygen adsorption allows the preservation of the geometry of a molecular adlayer, giving access to the determination of molecular orbital symmetries by means of near-edge X-ray absorption fine structure spectroscopy, NEXAFS. As a prototypical example, we exploited nickel tetraphenylporphyrin molecules deposited on a bare and on an oxygen pre-covered Cu(1 0 0) surface. We find that adsorbed atomic oxygen quenches the charge transfer at the metal-organic interface but, in contrast to a thin film sample, maintains the ordered adsorption geometry of the organic molecules. In this way, it is possible to disentangle π* and σ* symmetry orbitals, hence estimate the relative oscillator strength of core level transitions directly from the experimental data, as well as to evaluate and localize the degree of charge transfer in a coupled system. In particular, we neatly single out the σ* contribution associated with the N 1s transition to the mixed N 2px,y-Ni 3dx 2 -y 2 orbital, which falls close to the leading π*-symmetry LUMO resonance. © 2019
    view abstract10.1016/j.apsusc.2019.144343
  • From photoemission microscopy to an “All-in-One” photoemission experiment
    Tusche, C. and Chen, Y.-J. and Plucinski, L. and Schneider, C.M.
    e-Journal of Surface Science and Nanotechnology 18 (2020)
    Photoelectron spectroscopy is our main tool to explore the electronic structure of novel material systems, the properties of which are often determined by an intricate interplay of competing interactions. Elucidating the role of this interactions requires studies over an extensive range of energy, momentum, length, and time scales. We show that immersion lens-based momentum microscopy with spin-resolution is able to combine these seemingly divergent requirements in a unifying experimental approach. We will discuss applications to different areas in information research, for example, resistive switching and spintronics. The analysis of resistive switching phenomena in oxides requires high lateral resolution and chemical selectivity, as the processes involve local redox processes and oxygen vacancy migration. In spintronics topological phenomena are currently a hot topic, which lead to complex band structures and spin textures in reciprocal space. Spin-resolved momentum microscopy is uniquely suited to address these aspects. © 2020 The Japan Society of Vacuum and Surface Science. All rights reserved.
    view abstract10.1380/ejssnt.2020.48
  • Magnetic-Field Enhancement of THz Surface Emission in Highly Resistive GaAs
    Chen, G. and Chakraborty, D. and Cheng, J. and Chimera, C. and Komissarov, I. and Mikulics, M. and Adam, R. and Burgler, D.E. and Schneider, C.M. and Hardtdegen, H. and Sobolewski, R.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2020-November (2020)
    Irradiation of semiconductor surfaces with femtosecond optical laser pulses is one of the common techniques for broadband, free-space THz transient generation. We demonstrate that the amplitude of surface-emitted THz pulses scales linearly with an applied, external, in-plane magnetic field. We studied the effect in several highly resistive GaAs samples and ascribe it to the Lorentz force that additionally accelerates optically excited carriers. © 2020 IEEE.
    view abstract10.1109/IRMMW-THz46771.2020.9370531
  • Memory of professor charles (Chuck) S. Fadley (1941.9−2019.8)
    Daimon, H. and Schneider, C.M. and Mun, B.S.
    e-Journal of Surface Science and Nanotechnology 18 (2020)
    Professor Charles S. Fadley (nicknamed Chuck), who was a global leader in photoelectron spectroscopy using synchrotron radiation, passed away on 1st August 2019 at the age of 77. He was a well-known founder of photoelectron diffraction, and as a front runner in photoelectron spectroscopy using synchrotron radiation he initiated and promoted several novel approaches; such as, photoelectron holography, hard X-ray photoelectron spectroscopy, soft-X-ray standing wave spectroscopy, and more. He contributed to many scientific activities and served scientific communities including this ALC conference. He was an honorable member of the JSPS 141st Committee and a laureate of JSPS 141st Committee Award. This paper summarizes his life to honor his great achievements in science and contributions to scientific communities. © 2020 The Japan Society of Vacuum and Surface Science. All rights reserved.
    view abstract10.1380/EJSSNT.2020.235
  • Molecular anchoring stabilizes low valence Ni(i)TPP on copper against thermally induced chemical changes
    Sturmeit, H.M. and Cojocariu, I. and Jugovac, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Sala, A. and Comelli, G. and Moro, S. and Stredansky, M. and Corva, M. and Vesselli, E. and Puschnig, P. and Schneider, C.M. and Feyer, V. and Zamborlini, G. and Cinchetti, M.
    Journal of Materials Chemistry C 8 (2020)
    Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0tc00946f
  • On space charge effects in laboratory-based photoemission electron microscopy using compact gas discharge extreme ultraviolet sources
    Wilson, D. and Schmitz, C. and Rudolf, D. and Wiemann, C. and Schneider, C.M. and Juschkin, L.
    New Journal of Physics 22 (2020)
    The analysis of electronic and structural properties of surfaces has been greatly advanced by photoemission electron microscopy and spectroscopy techniques. To further improve lateral and energy resolution of the instruments, it is necessary to optimize parameters of the radiation sources employed for photoemission studies (e.g. photon flux, pulse duration, spot size etc). We studied space charge effects observed in an energy-filtering photoemission electron microscope operated with a compact laboratory-scale gas-discharge extreme ultraviolet light source. In this system, we found limits of spatial- and energy-resolution controlled by the source radiation parameters. The pulse repetition rate can be varied in the kHz range and the duration of the EUV emission was measured to be several tens of nanoseconds long, and thereby very different from the standard synchrotron sources typically used for similar experiments. The spatial resolution could be improved by a factor of 5, but only on the expense of the photon density per pulse, which had to be decreased by a factor of 17 in order to reduce the image blur due to space charge effects. Furthermore, we found broadening of the x-ray photoelectron spectroscopy peaks for high photon fluxes. We have also performed a n-body Monte Carlo simulation to evaluate the difference between core-level photoelectrons and secondary electrons with respect to space charge. © 2020 The Author(s).
    view abstract10.1088/1367-2630/abbc29
  • Onset-Time Control of THz Transients Generated by Spintronic Emitters
    Chen, G. and Adam, R. and Burgler, D.E. and Cao, D. and Pericolo, A. and Cheng, J. and Komissarov, I. and Heidtfeld, S. and Gladczuk, L. and Przyslupski, P. and Hardtdegen, H. and Mikulics, M. and Schneider, C.M. and Sobolewski, R.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2020-November (2020)
    We have generated intense electromagnetic transients by femtosecond laser pulse illumination of ferromagnet/metal (F/M) nanobilayers, in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up to 5 THz. We have also observed that upon the increase of the magnetic field, the entire THz transient shifts towards earlier times by up to 110 fs. We ascribe this magnetically tunable onset-time shift to extra acceleration of photoelectrons induced due to the Lorenz force. © 2020 IEEE.
    view abstract10.1109/IRMMW-THz46771.2020.9370970
  • Photoemission electron microscopy of magneto-ionic effects in La0.7Sr0.3MnO3
    Wilhelm, M. and Giesen, M. and Duchoň, T. and Moors, M. and Mueller, D.N. and Hackl, J. and Baeumer, C. and Hamed, M.H. and Cao, L. and Zhang, H. and Petracic, O. and Glöß, M. and Cramm, S. and Nemšák, S. and Wiemann, C. and Dittmann, R. and Schneider, C.M. and Müller, M.
    APL Materials 8 (2020)
    Magneto-ionic control of magnetism is a promising route toward the realization of non-volatile memory and memristive devices. Magneto-ionic oxides are particularly interesting for this purpose, exhibiting magnetic switching coupled to resistive switching, with the latter emerging as a perturbation of the oxygen vacancy concentration. Here, we report on electric-field-induced magnetic switching in a La0.7Sr0.3MnO3 (LSMO) thin film. Correlating magnetic and chemical information via photoemission electron microscopy, we show that applying a positive voltage perpendicular to the film surface of LSMO results in the change in the valence of the Mn ions accompanied by a metal-to-insulator transition and a loss of magnetic ordering. Importantly, we demonstrate that the voltage amplitude provides granular control of the phenomena, enabling fine-tuning of the surface electronic structure. Our study provides valuable insight into the switching capabilities of LSMO that can be utilized in magneto-ionic devices. © 2020 Author(s).
    view abstract10.1063/5.0022150
  • Superconducting NbN and CaFe0.88Co0.12AsF studied by point-contact spectroscopy with a nanoparticle Au array
    Wu, Y.F. and Yu, A.B. and Lei, L.B. and Zhang, C. and Wang, T. and Ma, Y.H. and Huang, Z. and Chen, L.X. and Liu, Y.S. and Schneider, C.M. and Mu, G. and Xiao, H. and Hu, T.
    Physical Review B 101 (2020)
    The point-contact-spectroscopy measurement is a powerful method to detect the superconducting gap and the spin polarization of materials. However, it is difficult to get a stable and clean point contact by conventional techniques. In this work, we fabricate multiple point contacts by depositing Au nanoparticle arrays on the surface of a superconductor through an anodic aluminum oxide patterned shadow mask. We obtained the superconducting gaps of niobium nitride thin film (NbN, Tc=16 K) and iron superconductors CaFe0.88Co0.12AsF single crystals (Ca-1111, Tc=21.3 K) by fitting the point-contact spectroscopy with the Blonder-Tinkham-Klapwijk theory. We found that NbN's gap (Δ) exhibits the BCS-like temperature dependence with Δ≈2.88 meV at 0 K and 2Δ/kBTc≈4.22 in agreement with previous reports. By contrast, Ca-1111 has a multigap structure with Δ1≈1.99 meV and Δ2≈5.01 meV at 0 K, and the ratio between the superconducting gap and Tc is 2Δ1/kBTc=2.2 and 2Δ2/kBTc=5.5, suggesting an unconventional paring mechanism of Ca-1111 also in agreement with previous reports on other Fe-based superconductors. Our multiple point-contacts method thus provides an alternative way to perform measurements of the superconducting gap. © 2020 American Physical Society. ©2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.174502
  • Terahertz Transients Emitted from La-Sr-Mn-O/Metal Nanobilayers Excited by Femtosecond Optical Pulses
    Chen, G. and Gladczuk, L. and Przyslupski, P. and Cheng, J. and Komissarov, I. and Adam, R. and Burgler, D.E. and Heidtfeld, S. and Cao, D. and Mikulics, M. and Hardtdegen, H. and Schneider, C.M. and Sobolewski, R.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2020-November (2020)
    We report on the generation of electromagnetic transients of picosecond duration from La-Sr-Mn-O/Au and La-Sr-Mn-O/lr nanobilayers, illuminated by femtosecond laser pulses in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up 4 THz. The amplitude of THz transients followed the functional dependence of La-Sr-Mn-O magnetization on the magnetic field and was also tunable by varying the intensity of the optical pulses. We ascribe the observed emission of THz transients to the inverse spin Hall effect, earlier demonstrated in metallic ferromagnet/metal spintronic nanobilayers. © 2020 IEEE.
    view abstract10.1109/IRMMW-THz46771.2020.9370435
  • Tunable coupling by means of oxygen intercalation and removal at the strongly interacting graphene/cobalt interface
    Jugovac, M. and Genuzio, F. and Menteş, T.O. and Locatelli, A. and Zamborlini, G. and Feyer, V. and Schneider, C.M.
    Carbon 163 (2020)
    It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene's electronic states. © 2020 Elsevier Ltd
    view abstract10.1016/j.carbon.2020.03.034
  • Two-dimensional electron systems in perovskite oxide heterostructures: Role of the polarity-induced substitutional defects
    Lin, S.-C. and Kuo, C.-T. and Shao, Y.-C. and Chuang, Y.-D. and Geessinck, J. and Huijben, M. and Rueff, J.-P. and Graff, I.L. and Conti, G. and Peng, Y. and Bostwick, A. and Rotenberg, E. and Gullikson, E. and Nem+í+ík, S. and Vailionis, A. and Gauquelin, N. and Verbeeck, J. and Ghiringhelli, G. and Schneider, C.M. and Fadley, C.S.
    Physical Review Materials 4 (2020)
    The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as LaAlO3 and SrTiO3 has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, the fundamental origins of the 2DES are still not understood, e.g., the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation on the depth profile of 2DES-relevant Ti3d interface carriers using depth- and element-specific techniques like standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti 3d interface carriers, which give rise to the 2DES are located within three unit cells from the n-type interface in the SrTiO3 layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first three unit cells of the opposing LaAlO3 layer (Gê+10 +à). Our findings provide a microscopic picture of how the localized and mobile Ti 3d interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the LaAlO3/SrTiO3 interface have disparate explanations as originating from different types of interface carriers. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.115002
  • Vibronic Fingerprints of the Nickel Oxidation States in Surface-Supported Porphyrin Arrays
    Stredansky, M. and Moro, S. and Corva, M. and Jugovac, M. and Zamborlini, G. and Feyer, V. and Schneider, C.M. and Cojocariu, I. and Sturmeit, H.M. and Cinchetti, M. and Verdini, A. and Cossaro, A. and Floreano, L. and Vesselli, E.
    Journal of Physical Chemistry C (2020)
    The 2D self-assembly of Ni-containing tetrapyrroles on Cu(100) allows control of the Ni atom oxidation state, yielding inactive Ni(II) or active Ni(I) upon modification of the molecule-substrate interaction, resembling the behavior of the biochemical counterpart. Ni(I) is indeed the active site of methanogenic bacteria in the tetrahydrocorphin of the F430 coenzyme of methyl-coenzyme reductase. Tuning of the electronic configuration of the Ni atom in the 2D system is accomplished by exploiting the surface trans effect, by analogy to the biologic enzymatic pocket, which is activated by a molecular trans effect. In this report, we identify the vibrational fingerprint of the molecular macrocycle that reflects the actual Ni oxidation state in the 2D system showing that, despite the apparent differences of the two cases, the fact that the Ni-porphin in the F430 pocket is accessible to the reactants but not to the solvent makes the two situations more similar than expected. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c01387
  • A scanning reflection X-ray microscope for magnetic imaging in the EUV range
    Schümmer, A. and Mertins, H.-C. and Schneider, C.M. and Adam, R. and Trellenkamp, S. and Borowski, R. and Bürgler, D.E. and Juschkin, L. and Berges, U.
    Journal of Synchrotron Radiation 26 (2019)
    The mechanical setup of a novel scanning reflection X-ray microscope is presented. It is based on zone plate optics optimized for reflection mode in the EUV spectral range. The microscope can operate at synchrotron radiation beamlines as well as at laboratory-based plasma light sources. In contrast to established X-ray transmission microscopes that use thin foil samples, the new microscope design presented here allows the investigation of any type of bulk materials. Importantly, this permits the investigation of magnetic materials by employing experimental techniques based on X-ray magnetic circular dichroism, X-ray linear magnetic dichroism or the transversal magneto-optical Kerr effect (T-MOKE). The reliable functionality of the new microscope design has been demonstrated by T-MOKE microscopy spectra of Fe/Cr-wedge/Fe trilayer samples. The spectra were recorded at various photon energies across the Fe 3p edge revealing the orientation of magnetic domains in the sample. © International Union of Crystallography 2019.
    view abstract10.1107/S1600577519012219
  • Chemical control of the electrical surface properties in donor-doped transition metal oxides
    Andrä, M. and Bluhm, H. and Dittmann, R. and Schneider, C.M. and Waser, R. and Mueller, D.N. and Gunkel, F.
    Physical Review Materials 3 (2019)
    Donor-doped transition metal oxides such as donor-doped strontium titanate (n-SrTiO3) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of n-SrTiO3, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere (pO2). We correlate the pO2 dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the n-SrTiO3 surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.044604
  • Current-induced domain wall oscillations in a nanowire imaged by time-resolved photoemission electron microscopy
    Khan, M.I. and Cramm, S. and Bürgler, D.E. and Parlak, U. and Nemšák, S. and Gottlob, D.M. and Hackl, J. and Doğanay, H. and Schneider, C.M.
    Journal of Magnetism and Magnetic Materials 476 (2019)
    We study reversible domain wall motion in half-ring Ni 80 Fe 20 nanowires on a nanosecond (ns) timescale in a truly current-induced pump-probe experiment using an energy filtered, aberration-corrected photoemission electron microscope. The X-ray magnetic circular dichroism signal is probed at different time delays before, during and after the current pulse in a stroboscopic mode with circularly polarized synchrotron radiation in the energy range of the Fe L 3 -edge (707 eV). We observe lateral domain wall oscillations with a frequency of ∼0.4 GHz. Comparing the results to a proposed string model, we find that the domain wall oscillations can be described as string-like asymmetric oscillations. © 2019
    view abstract10.1016/j.jmmm.2019.01.003
  • Direct measurement of anisotropic conductivity in a nanolaminated (Mn0.5Cr0.5)2GaC thin film
    Flatten, T. and Matthes, F. and Petruhins, A. and Salikhov, R. and Wiedwald, U. and Farle, M. and Rosen, J. and Bürgler, D.E. and Schneider, C.M.
    Applied Physics Letters 115 (2019)
    The direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic Mn+1AXn (MAX) phase film is presented. A multitip scanning tunneling microscope is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities ρab and ρc. A (Cr0.5Mn0.5)2GaC MAX phase film shows a large anisotropy ratio ρ c / ρ ab = 525 ± 49. This is a consequence of the complex bonding scheme of MAX phases with covalent M-X and metallic M-M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes. © 2019 Author(s).
    view abstract10.1063/1.5115347
  • Electrostatic potential mapping at ferroelectric domain walls by low-temperature photoemission electron microscopy
    Schaab, J. and Shapovalov, K. and Schoenherr, P. and Hackl, J. and Khan, M.I. and Hentschel, M. and Yan, Z. and Bourret, E. and Schneider, C.M. and Nemsák, S. and Stengel, M. and Cano, A. and Meier, D.
    Applied Physics Letters 115 (2019)
    Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM-based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times on the order of 0.1-1 s. © 2019 Author(s).
    view abstract10.1063/1.5117881
  • Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices
    Königshofen, S. and Matthes, F. and Bürgler, D.E. and Schneider, C.M. and Dirksen, E. and Müller, T.J.J.
    Thin Solid Films 680 (2019)
    The growing field of molecular spintronics is an auspicious route to future concepts of data storage and processing. It has been reported that the hybridization of the electronic structures of non-magnetic organic molecules and ferromagnetic transition-metal (FM) surfaces can form new magnetic units, so-called hybrid molecular magnets, with distinct magnetic properties, which promise molecular spintronic devices with extremely high information density and low energy consumption. The investigation and profound understanding of these device concepts require the formation of clean and epitaxial interfaces between the surface of a FM bottom electrode and molecular thin films. This can only be realized under ultra-high vacuum conditions. In addition, the FM electrodes must be grown on an insulating substrate to electrically separate neighboring devices. Here, we report on procedures to realize an entirely in-situ preparation of mesoscopic test devices featuring structurally and chemically well-defined interfaces. Au(111)-buffered Co(0001) electrodes are deposited by molecular-beam epitaxy onto sapphire or mica substrates using a shadow-mask to define the geometry. The surface quality is subsequently characterized by scanning tunneling microscopy (STM) and other surface science analysis tools. 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), which serves as an exemplary molecule, is sublimed through another shadow-mask, and the interface formation in the monolayer regime is also studied by STM. Finally, we deposit a Cu top electrode through yet another shadow-mask to complete a mesoscopic (200 × 200 μm2) test device, which reveals in ex-situ transport measurements for the Co/BNTCDI/Cu junction non-metallic behavior and a resistance-area product of 24 MΩ·μm2 at 10 K. © 2019
    view abstract10.1016/j.tsf.2019.04.021
  • Fast and easy fabrication methodology of Fresnel zone plates for the extreme ultraviolet and soft x-ray regions
    Schümmer, A. and Mertins, H.-C.H. and Schneider, C.M. and Adam, R. and Trellenkamp, S. and Borowski, R. and Juschkin, L. and Berges, U.
    Applied Optics 58 (2019)
    Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15 mm are reported: (i) phase-shifting zone plates made by 190 nm thick PMMA rings on Si 3 N 4 membranes, (ii) absorbing zone plates made by 75 nm thick Au ring structures on Si 3 N 4 , and (iii) freestanding Au rings of 50 nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9 3 μm, which is the theoretical limit resulting from the synchrotron beam parameters at 60 eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance. © 2019 Optical Society of America.
    view abstract10.1364/AO.58.001057
  • High-temperature 2D Fermi surface of SrTiO 3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO 3 (001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.6533
  • High-temperature 2D Fermi surface of SrTiO3 studied by energy-filtered PEEM
    Mathieu, C. and Gonzalez, S. and Lubin, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Surface and Interface Analysis 51 (2019)
    Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry-induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO3(001) at 140 K. The 2DEG is created by soft X-ray irradiation and can be reversibly controlled by a combination of soft X-rays and oxygen partial pressure. © 2018 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.6533
  • Imaging properties of hemispherical electrostatic energy analyzers for high resolution momentum microscopy
    Tusche, C. and Chen, Y.-J. and Schneider, C.M. and Kirschner, J.
    Ultramicroscopy 206 (2019)
    Hemispherical deflection analyzers are the most widely used energy filters for state-of-the-art electron spectroscopy. Due to the high spherical symmetry, they are also well suited as imaging energy filters for electron microscopy. Here, we review the imaging properties of hemispherical deflection analyzers with emphasis on the application for cathode lens microscopy. In particular, it turns out that aberrations, in general limiting the image resolution, cancel out at the entrance and exit of the analyzer. This finding allows more compact imaging energy filters for momentum microscopy or photoelectron emission microscopy. For instance, high resolution imaging is possible, using only a single hemisphere. Conversely, a double pass hemispherical analyzer can double the energy dispersion, which means it can double the energy resolution at certain transmission, or can multiply the transmission at certain energy resolution. © 2019 The Authors
    view abstract10.1016/j.ultramic.2019.112815
  • In-Gap States and Band-Like Transport in Memristive Devices
    Baeumer, C. and Funck, C. and Locatelli, A. and Menteş, T.O. and Genuzio, F. and Heisig, T. and Hensling, F. and Raab, N. and Schneider, C.M. and Menzel, S. and Waser, R. and DIttmann, R.
    Nano Letters 19 (2019)
    Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface. © 2018 American Chemical Society.
    view abstract10.1021/acs.nanolett.8b03023
  • Kink far below the Fermi level reveals new electron-magnon scattering channel in Fe
    Młyńczak, E. and Müller, M.C.T.D. and Gospodarič, P. and Heider, T. and Aguilera, I. and Bihlmayer, G. and Gehlmann, M. and Jugovac, M. and Zamborlini, G. and Tusche, C. and Suga, S. and Feyer, V. and Plucinski, L. and Friedrich, C. and Blügel, S. and Schneider, C.M.
    Nature Communications 10 (2019)
    Many properties of real materials can be modeled using ab initio methods within a single-particle picture. However, for an accurate theoretical treatment of excited states, it is necessary to describe electron-electron correlations including interactions with bosons: phonons, plasmons, or magnons. In this work, by comparing spin- and momentum-resolved photoemission spectroscopy measurements to many-body calculations carried out with a newly developed first-principles method, we show that a kink in the electronic band dispersion of a ferromagnetic material can occur at much deeper binding energies than expected (Eb = 1.5 eV). We demonstrate that the observed spectral signature reflects the formation of a many-body state that includes a photohole bound to a coherent superposition of renormalized spin-flip excitations. The existence of such a many-body state sheds new light on the physics of the electron-magnon interaction which is essential in fields such as spintronics and Fe-based superconductivity. © 2019, The Author(s).
    view abstract10.1038/s41467-019-08445-1
  • Laser-induced acceleration of Helium ions from unpolarized gas jets
    Engin, I. and Chitgar, Z.M. and Deppert, O. and Lucchio, L.D. and Engels, R. and Fedorets, P. and Frydrych, S. and Gibbon, P. and Kleinschmidt, A. and Lehrach, A. and Maier, R. and Prasuhn, D. and Roth, M. and Schlüter, F. and Schneider, C.M. and Stöhlker, T. and Strathmann, K. and Büscher, M.
    Plasma Physics and Controlled Fusion 61 (2019)
    In order to develop a laser-driven spin-polarized 3He-ion beam source available for nuclear-physics experiments as well as for the investigation of polarized nuclear fusion, several challenges have to be overcome. Apart from the provision of a properly polarized 3He gas-jet target, one of the biggest milestones is the demonstration of the general feasibility of laser-induced ion acceleration out of gas-jet targets. Of particular importance is the knowledge about the main ion-emission angles as well as the achievable ion-energy spectra (dependent on the optimal set of laser and target parameters). We report on the results of such a feasibility study performed at PHELIX, GSI Darmstadt. Both 3He- and 4He-gas jets (n gas ∼ 1019 cm-3) were illuminated with high-intensity laser pulses, IL~O(1019,W cm-2). The main ion-emission angles could be identified (±90° with respect to the laser-propagation direction) and the ion-energy spectra for all ion species could be extracted: for the optimal laser and target parameters, the high-energy cut-offs for He2+,1+ ions were 4.65 MeV (with a normalized energy uncertainty of Δ ϵ ϵ-1=0.033) and 3.27 MeV (Δ ϵ ϵ-1=0.055), respectively. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6587/ab4613
  • Magnetic Field-Assisted Chemical Vapor Deposition of Iron Oxide Thin Films: Influence of Field-Matter Interactions on Phase Composition and Morphology
    Stadler, D. and Mueller, D.N. and Brede, T. and Duchoň, T. and Fischer, T. and Sarkar, A. and Giesen, M. and Schneider, C.M. and Volkert, C.A. and Mathur, S.
    Journal of Physical Chemistry Letters 10 (2019)
    Magnetic field-assisted CVD offers a direct pathway to manipulate the evolution of microstructure, phase composition, and magnetic properties of the as-prepared film. We report on the role of applied magnetic fields (0.5 T) during a cold-wall CVD deposition of iron oxide from [FeIII(OtBu)3]2 leading to higher crystallinity, larger particulates, and better out-of-plane magnetic anisotropy, if compared with zero-field depositions. Whereas selective formation of homogeneous magnetite films was observed for the field-assisted process, coexistence of hematite and amorphous iron(III) oxide was confirmed under zero-field conditions. Comparison of the coercive field (11 vs 60 mT) indicated lower defect concentration for the field-assisted process with nearly superparamagnetic behavior. X-ray photoemission electron microscopy (X-PEEM) in absorption mode at the O-K and Fe-L3,2 edges confirmed the selective formation of magnetite (field-assisted) and hematite (zero-field) with coexisting amorphous phases, respectively, emphasizing the importance of field-matter interactions in the phase-selective synthesis of magnetic thin films. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.9b02381
  • Magnetically and optically tunable terahertz radiation from Ta/NiFe/Pt spintronic nanolayers generated by femtosecond laser pulses
    Adam, R. and Chen, G. and Bürgler, D.E. and Shou, T. and Komissarov, I. and Heidtfeld, S. and Hardtdegen, H. and Mikulics, M. and Schneider, C.M. and Sobolewski, R.
    Applied Physics Letters 114 (2019)
    We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation. © 2019 Author(s).
    view abstract10.1063/1.5099201
  • Magnon dispersion in Ni/Co multilayers grown on Cu(100)
    Ibach, H. and Schneider, C.M.
    Physical Review B 99 (2019)
    Motivated by recent studies on current-driven domain-wall motion we have explored the dispersion of spin waves (magnons) in ultrathin nickel/cobalt multilayers. The layers are grown epitaxially on Cu(100) surfaces and consist of n nickel single-atom layers, each topped by a cobalt bilayer with n=2, 3, and 4(n×Ni1Co2). Layers of the type 2×Ni2Co1 are also studied. While parallel to the film plane the magnon dispersion is nearly equal to that of pure cobalt films, magnons with wave vectors perpendicular to the film plane (standing waves) are considerably softened. The softening is attributed to a reduction of the effective interlayer exchange coupling between the two layers next to the interface with the Cu(100) substrate. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.184406
  • Reconfigurable lateral anionic heterostructures in oxide thin films via lithographically defined topochemistry
    Lefler, B.M. and Duchoň, T. and Karapetrov, G. and Wang, J. and Schneider, C.M. and May, S.J.
    Physical Review Materials 3 (2019)
    Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of SrFeO2.5/SrFeO3,SrFeO2.5/SrFeO2F, and SrFeO3/SrFeO2F have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.073802
  • Role of carbon dissolution and recondensation in graphene epitaxial alignment on cobalt
    Jugovac, M. and Genuzio, F. and Gonzalez Lazo, E. and Stojić, N. and Zamborlini, G. and Feyer, V. and Menteş, T.O. and Locatelli, A. and Schneider, C.M.
    Carbon 152 (2019)
    The crystalline quality of the graphene lattice is a crucial parameter that not only rules the electronic and transport properties of the carbon film, but also its interaction with the substrate. Elucidating the effect of different growth pathways on the resulting graphene-substrate structural configurations and the microscopic mechanisms for their formation is, therefore, a goal of utmost importance. By using electron spectro-microscopy with high chemical and structural sensitivity, we image the structural transformation that graphene on cobalt undergoes at temperatures above 500°C, from a rotationally-incoherent, defective layer to a high quality epitaxial one. We find that the transformation takes place via the growth and propagation of mesoscopic carbidic islands. We identify the underlying mechanism for the formation of epitaxial graphene to involve the dissolution and recondensation of carbon within these regions. The activation energy of the process is estimated to be 1.84 ± 0.11 eV, indicating that the carbon detachment is the rate-limiting step. With the aid of theoretical calculations, we show that the martensitic phase transition occurring in cobalt above 420°C does not affect the graphene transformation. These findings help to establish the optimal parameters to grow high-quality graphene epilayers on Co, opening viable routes towards usage in artificially fabricated magnetic heterostructures. © 2019 Elsevier Ltd
    view abstract10.1016/j.carbon.2019.06.033
  • Single-pass STEM-EMCD on a zone axis using a patterned aperture: progress in experimental and data treatment methods
    Thersleff, T. and Schönström, L. and Tai, C.-W. and Adam, R. and Bürgler, D.E. and Schneider, C.M. and Muto, S. and Rusz, J.
    Scientific Reports 9 (2019)
    Measuring magnetic moments in ferromagnetic materials at atomic resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). However, experimental and data processing hurdles currently hamper the realization of this goal. Experimentally, the sample must be tilted to a zone-axis orientation, yielding a complex distribution of magnetic scattering intensity, and the same sample region must be scanned multiple times with sub-atomic spatial registration necessary at each pass. Furthermore, the weak nature of the EMCD signal requires advanced data processing techniques to reliably detect and quantify the result. In this manuscript, we detail our experimental and data processing progress towards achieving single-pass zone-axis EMCD using a patterned aperture. First, we provide a comprehensive data acquisition and analysis strategy for this and other EMCD experiments that should scale down to atomic resolution experiments. Second, we demonstrate that, at low spatial resolution, promising EMCD candidate signals can be extracted, and that these are sensitive to both crystallographic orientation and momentum transfer. © 2019, The Author(s).
    view abstract10.1038/s41598-019-53373-1
  • The new dedicated HAXPES beamline P22 at PETRAIII
    Schlueter, C. and Gloskovskii, A. and Ederer, K. and Schostak, I. and Piec, S. and Sarkar, I. and Matveyev, Y. and Lömker, P. and Sing, M. and Claessen, R. and Wiemann, C. and Schneider, C.M. and Medjanik, K. and Schönhense, G. and Amann, P. and Nilsson, A. and Drube, W.
    AIP Conference Proceedings 2054 (2019)
    A new undulator beamline (P22) for hard X-ray photoelectron spectroscopy (HAXPES) was built at PETRA III (DESY, Hamburg) to meet the increasing demand for HAXPES-based techniques. It provides four special instruments for high-resolution studies of the electronic and chemical structure of functional nano-materials and catalytic interfaces, with a focus on measurements under operando and/or ambient conditions: (i) a versatile solid-state spectroscopy setup with optional wide-angle lens and in-situ electrical characterization, (ii) a HAXPEEM instrument for sub-μm spectro-microscopy applications, (iii) an ambient pressure system (&gt; 1 bar) for operando studies of catalytic reactions and (iv) a time-of-flight spectrometer as a full-field k-microscope for measurements of the 4D spectral function ρ(EB,k). The X-ray optics were designed to deliver high brightness photon flux within the HAXPES energy range 2.4 - 15 keV. An LN 2 -cooled double-crystal monochromator with interchangeable pairs of Si(111) and (311) crystals is optionally combined with a double channel-cut post-monochromator to generate X-rays with variable energy bandpass adapted to the needs of the experiment. Additionally, the beam polarization can be varied using a diamond phase plate integrated into the beamline. Adaptive beam focusing is realized by Be compound refractive lenses and/or horizontally deflecting mirrors down to a spot size of ∼20x17 μm 2 with a flux of up to 1.1x10 13 ph/s (for Si(111) at 6 keV). © 2019 Author(s).
    view abstract10.1063/1.5084611
  • Topotactic Phase Transition Driving Memristive Behavior
    Nallagatla, V.R. and Heisig, T. and Baeumer, C. and Feyer, V. and Jugovac, M. and Zamborlini, G. and Schneider, C.M. and Waser, R. and Kim, M. and Jung, C.U. and Dittmann, R.
    Advanced Materials 31 (2019)
    Redox-based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen-ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen-ion transport properties for resistive-switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X-ray absorption spectromicroscopy, it is demonstrated that the reversible redox-based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5, and the conductive perovskite phase, SrFeO3, gives rise to the resistive-switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric-field-induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111)-grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive-switching mechanism in SrFeOx-based memristive devices within the framework of metal–insulator topotactic phase transitions. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.201903391
  • Tunable Magnetic Phases at Fe 3 O 4 /SrTiO 3 Oxide Interfaces
    Hamed, M.H. and Hinz, R.A. and Lömker, P. and Wilhelm, M. and Gloskovskii, A. and Bencok, P. and Schmitz-Antoniak, C. and Elnaggar, H. and Schneider, C.M. and Müller, M.
    ACS Applied Materials and Interfaces 11 (2019)
    We demonstrate the emergence and control of magnetic phases between magnetite (Fe 3 O 4 ), a ferrimagnetic halfmetal, and SrTiO 3 , a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (T V ) was detected to persist from bulk-like down to ultrathin Fe 3 O 4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe 2+ ions with decreasing film thickness, accompanied by an increase of Fe 3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe 2 O 3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the Î-Fe 2 O 3 that finally leads to stoichiometric and ferrimagnetic Fe 3 O 4 /SrTiO 3 (001) heterointerfaces. © 2019 American Chemical Society.
    view abstract10.1021/acsami.8b20625
  • Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface
    Chandrasena, R.U. and Flint, C.L. and Yang, W. and Arab, A. and Nemšák, S. and Gehlmann, M. and Özdöl, V.B. and Bisti, F. and Wijesekara, K.D. and Meyer-Ilse, J. and Gullikson, E. and Arenholz, E. and Ciston, J. and Schneider, C.M. and Strocov, V.N. and Suzuki, Y. and Gray, A.X.
    Physical Review B 98 (2018)
    Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic LaNiO3 and antiferromagnetic CaMnO3 exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in LaNiO3. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the LaNiO3/CaMnO3 interface. Our measurements reveal an increased concentration of Mn3+ and Ni2+ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the Mn4+-Mn3+ ferromagnetic double exchange and Ni2+-O-Mn4+ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.155103
  • Electronic structure of the dilute magnetic semiconductor G a1-x M nx P from hard x-ray photoelectron spectroscopy and angle-resolved photoemission
    Keqi, A. and Gehlmann, M. and Conti, G. and Nemšák, S. and Rattanachata, A. and Minár, J. and Plucinski, L. and Rault, J.E. and Rueff, J.P. and Scarpulla, M. and Hategan, M. and Pálsson, G.K. and Conlon, C. and Eiteneer, D. and Saw, A.Y. and Gray, A.X. and Kobayashi, K. and Ueda, S. and Dubon, O.D. and Schneider, C.M. and Fadley, C.S.
    Physical Review B 97 (2018)
    We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimental data, as well as theoretical calculations, to understand the role of the Mn dopant in the emergence of ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray, Nat. Mater. 11, 957 (2012)10.1038/nmat3450], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.155149
  • Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide
    Nemšák, S. and Gehlmann, M. and Kuo, C.-T. and Lin, S.-C. and Schlueter, C. and Mlynczak, E. and Lee, T.-L. and Plucinski, L. and Ebert, H. and Di Marco, I. and Minár, J. and Schneider, C.M. and Fadley, C.S.
    Nature Communications 9 (2018)
    The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials. © 2018, The Author(s).
    view abstract10.1038/s41467-018-05823-z
  • Element-selective investigation of femtosecond spin dynamics in NiPd magnetic alloys using extreme ultraviolet radiation
    Gang, S.-G. and Adam, R. and Plötzing, M. and Von Witzleben, M. and Weier, C. and Parlak, U. and Bürgler, D.E. and Schneider, C.M. and Rusz, J. and Maldonado, P. and Oppeneer, P.M.
    Physical Review B 97 (2018)
    We studied femtosecond spin dynamics in NixPd1-x magnetic thin films by optically pumping the system with infrared (1.55 eV) laser pulses and subsequently recording the reflectivity of extreme ultraviolet (XUV) pulses synchronized with the pump pulse train. XUV light in the energy range from 20 to 72 eV was produced by laser high-harmonic generation. The reflectivity of XUV radiation at characteristic resonant energies allowed separate detection of the spin dynamics in the elemental subsystems at the M2,3 absorption edges of Ni (68.0 and 66.2 eV) and N2,3 edges of Pd (55.7 and 50.9 eV). The measurements were performed in transversal magneto-optical Kerr effect geometry. In static measurements, we observed a magnetic signature of the Pd subsystem due to an induced magnetization. Calculated magneto-optical asymmetries based on density functional theory show close agreement with the measured results. Femtosecond spin dynamics measured at the Ni absorption edges indicates that increasing the Pd concentration, which causes a decrease in the Curie temperature TC, results in a drop of the demagnetization time τM, contrary to the τM∼1/TC scaling expected for single-species materials. This observation is ascribed to the increase of the Pd-mediated spin-orbit coupling in the alloy. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.064412
  • Full angular dependence of the spin Hall and ordinary magnetoresistance in epitaxial antiferromagnetic NiO(001)/Pt thin films
    Baldrati, L. and Ross, A. and Niizeki, T. and Schneider, C. and Ramos, R. and Cramer, J. and Gomonay, O. and Filianina, M. and Savchenko, T. and Heinze, D. and Kleibert, A. and Saitoh, E. and Sinova, J. and Kläui, M.
    Physical Review B 98 (2018)
    We report the observation of the three-dimensional angular dependence of the spin Hall magnetoresistance (SMR) in a bilayer of the epitaxial antiferromagnetic insulator NiO(001) and the heavy metal Pt, without any ferromagnetic element. The detected angular-dependent longitudinal and transverse magnetoresistances are measured by rotating the sample in magnetic fields up to 11 T, along three orthogonal planes (xy-, yz-, and xz-rotation planes, where the z axis is orthogonal to the sample plane). The total magnetoresistance has contributions arising from both the SMR and ordinary magnetoresistance. The onset of the SMR signal occurs between 1 and 3 T and no saturation is visible up to 11 T. The three-dimensional angular dependence of the SMR can be explained by a model considering the reversible field-induced redistribution of magnetostrictive antiferromagnetic S and T domains in the NiO(001), stemming from the competition between the Zeeman energy and the elastic clamping effect of the nonmagnetic MgO substrate. From the observed SMR ratio, we estimate the spin mixing conductance at the NiO/Pt interface to be greater than 2×1014Ω-1m-2. Our results demonstrate the possibility to electrically detect the Néel vector direction in stable NiO(001) thin films, for rotations in the xy and xz planes. Moreover, we show that a careful subtraction of the ordinary magnetoresistance contribution is crucial to correctly estimate the amplitude of the SMR. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.024422
  • In Aqua Electrochemistry Probed by XPEEM: Experimental Setup, Examples, and Challenges
    Nemšák, S. and Strelcov, E. and Guo, H. and Hoskins, B.D. and Duchoň, T. and Mueller, D.N. and Yulaev, A. and Vlassiouk, I. and Tselev, A. and Schneider, C.M. and Kolmakov, A.
    Topics in Catalysis 61 (2018)
    Recent developments in environmental and liquid cells equipped with electron transparent graphene windows have enabled traditional surface science spectromicroscopy tools, such as scanning X-ray photoelectron microscopy, X-ray photoemission electron microscopy (XPEEM), and scanning electron microscopy to be applied for studying solid–liquid and liquid–gas interfaces. Here, we focus on the experimental implementation of XPEEM to probe electrified graphene–liquid interfaces using electrolyte-filled microchannel arrays as a new sample platform. We demonstrate the important methodological advantage of these multi-sample arrays: they combine the wide field of view hyperspectral imaging capabilities from XPEEM with the use of powerful data mining algorithms to reveal spectroscopic and temporal behaviors at the level of the individual microsample or the entire array ensemble. © 2018, This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.
    view abstract10.1007/s11244-018-1065-4
  • In situ disentangling surface state transport channels of a topological insulator thin film by gating
    Lüpke, F. and Just, S. and Eschbach, M. and Heider, T. and Młyńczak, E. and Lanius, M. and Schüffelgen, P. and Rosenbach, D. and von den Driesch, N. and Cherepanov, V. and Mussler, G. and Plucinski, L. and Grützmacher, D. and Schneider, C.M. and Tautz, F.S. and Voigtländer, B.
    npj Quantum Materials 3 (2018)
    In the thin film limit, the surface state of a three-dimensional topological insulator gives rise to two parallel conduction channels at the top and bottom surface of the film, which are difficult to disentangle in transport experiments. Here, we present gate-dependent multi-tip scanning tunneling microscope transport measurements combined with photoemission experiments all performed in situ on pristine BiSbTe3 thin films. To analyze the data, we develop a generic transport model including quantum capacitance effects. This approach allows us to quantify the gate-dependent conductivities, charge carrier concentrations, and mobilities for all relevant transport channels of three-dimensional topological insulator thin films (i.e., the two topological surface state channels, as well as the interior of the film). For the present sample, we find that the conductivity in the bottom surface state channel is minimized below a gate voltage of Vgate = −34 V and the top surface state channel dominates the transport through the film. © 2018, The Author(s).
    view abstract10.1038/s41535-018-0116-1
  • Localized segregation of gold in ultrathin Fe films on Au(001)
    Gospodarič, P. and Młyńczak, E. and Eschbach, M. and Gehlmann, M. and Zamborlini, G. and Feyer, V. and Plucinski, L. and Schneider, C.M.
    Physical Review B 97 (2018)
    The growth of up to ten-monolayer-thick Fe films on a Au(001) surface was investigated during deposition at room temperature and during annealing, using low-energy electron diffraction and x-ray photoemission spectroscopy, as well as locally with low-energy electron microscopy and photoemission electron microscopy. The growth proceeds with a submonolayer of Au segregating on the surface of Fe, which is in agreement with previous studies. Annealing was found to be critical for the presence of Au on the Fe surface. Our study identifies a spatially inhomogeneous Au segregation mechanism which proceeds by the formation of cracks in the Fe film, starting at the annealing temperature of 190C, through which Au diffuses towards the surface. As a result, a system with a nonuniform surface electronic structure is obtained. This study shows the necessity to employ spatially resolved techniques to fully understand the growth modes of the layered epitaxial systems. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.97.085409
  • New Developments in Spin-Dependent Photoemission∗
    Schneider, C.M.
    e-Journal of Surface Science and Nanotechnology 16 (2018)
    The electron spin governs many phenomena in modern condensed matter physics, such as magnetism, superconductivity, etc. Often, minute details in the electronic structure determine the physical behavior of a material. Photoelectron emission—being the most established approach to explore electronic structures—is currently entering a new era thanks to a breathtaking development in light sources, spectrometer concepts, and spin detectors. In particular, the evolution in novel highly efficient electron spin polarimeters opens up new experimental opportunities and permits unequaled insights into the electronic structure. This contribution will discuss several examples in this field of spin-dependent interactions and spin-based phenomena. A prominent one refers to the class of topological insulators, where strong spin-orbit coupling (SOC) causes a unique spin-momentum locking around the Dirac cone. Transition metal dichalcogenides consist of quasi-2D layers coupled by v. d. Waals interactions. Here, strong SOC leads to pronounced hybridization effects. We also address fundamental issues in ferromagnetism, e.g., the complex interplay of SOC and exchange interaction, causing characteristic k-, spin- and symmetry-dependent band mixing. Using spin- and time-resolved photoemission we explore ultrafast spin dynamics in ferromagnets driven by strong ultrashort laser pulses. We find the changes in both majority and minority spin states to take place on a 100 fs time scale and to be compatible with band mirroring. In this contribution, we will discuss several new aspects of spin-dependent and spin-resolved photoemission covering both static and dynamic issues of electronic states. © 2018 The Japan Society of Vacuum and Surface Science.
    view abstract10.1380/ejssnt.2018.177
  • New HAXPES Applications at PETRA III
    Schlueter, C. and Gloskovskii, A. and Ederer, K. and Piec, S. and Sing, M. and Claessen, R. and Wiemann, C. and Schneider, C.M. and Medjanik, K. and Schönhense, G. and Amann, P. and Nilsson, A. and Drube, W.
    Synchrotron Radiation News 31 (2018)
    view abstract10.1080/08940886.2018.1483656
  • Nonlocal electron correlations in an itinerant ferromagnet
    Tusche, C. and Ellguth, M. and Feyer, V. and Krasyuk, A. and Wiemann, C. and Henk, J. and Schneider, C.M. and Kirschner, J.
    Nature Communications 9 (2018)
    Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. At the same time we observe regions of anomalously large “waterfall”-like band renormalization, previously only attributed to strong electron correlations in high-TC superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations. © 2018, The Author(s).
    view abstract10.1038/s41467-018-05960-5
  • On the growth mechanisms of polar (100) surfaces of ceria on copper (100)
    Hackl, J. and Duchoň, T. and Gottlob, D.M. and Cramm, S. and Veltruská, K. and Matolín, V. and Nemšák, S. and Schneider, C.M.
    Surface Science 671 (2018)
    We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction. © 2018 Elsevier B.V.
    view abstract10.1016/j.susc.2018.01.008
  • On-surface nickel porphyrin mimics the reactive center of an enzyme cofactor
    Zamborlini, G. and Jugovac, M. and Cossaro, A. and Verdini, A. and Floreano, L. and Lüftner, D. and Puschnig, P. and Feyer, V. and Schneider, C.M.
    Chemical Communications 54 (2018)
    Metal-containing enzyme cofactors achieve their unusual reactivity by stabilizing uncommon metal oxidation states with structurally complex ligands. In particular, the specific cofactor promoting both methanogenesis and anaerobic methane oxidation is a porphyrinoid chelated to a nickel(i) atom via a multi-step biosynthetic path, where nickel reduction is achieved through extensive molecular hydrogenation. Here, we demonstrate an alternative route to porphyrin reduction by charge transfer from a selected copper substrate to commercially available 5,10,15,20-tetraphenyl-porphyrin nickel(ii). X-ray absorption measurements at the Ni L3-edge unequivocally show that NiTPP species adsorbed on Cu(100) are stabilized in the highly reactive Ni(i) oxidation state by electron transfer to the molecular orbitals. Our approach highlights how some fundamental properties of synthetically inaccessible biological cofactors may be reproduced by hybridization of simple metalloporphyrins with metal surfaces, with implications towards novel approaches to heterogenous catalysis. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8cc06739b
  • Optically induced magnetization reversal in [Co/Pt]N multilayers: Role of domain wall dynamics
    Parlak, U. and Adam, R. and Bürgler, D.E. and Gang, S. and Schneider, C.M.
    Physical Review B 98 (2018)
    All-optical switching (AOS) of magnetization in ferri- and ferromagnetic thin films has in recent years attracted a strong interest since it allows magnetization reversal in the absence of applied magnetic field. Here we investigate AOS in [Co/Pt]N multilayers. The coercivity (HC) of the multilayers was tuned either by varying the bilayer repetition number (N) or the sample temperature (T). During the AOS experiments, we first illuminated the multilayers by a sequence of femtosecond laser pulses with varying fluence, light polarization, and repetition rate. The optically affected area was then imaged with magneto-optical Kerr microscopy. Our results indicate that the optical pulses can trigger either AOS or initiate an all-optical domain formation (AODF). The laser fluence required for AOS scales linearly with HC and depends on a precise tuning of laser pulse fluence, repetition rate, and light polarization. Furthermore, the magnetic response of the samples at a varying ambient temperature (down to 50 K) and for different time intervals between subsequent laser pulses point to the crucial role of domain wall dynamics in optical control of magnetization in ferromagnetic multilayers. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.214443
  • Photoelectron microscopy at Elettra: Recent advances and perspectives
    Amati, M. and Barinov, A. and Feyer, V. and Gregoratti, L. and Al-Hada, M. and Locatelli, A. and Mentes, T.O. and Sezen, H. and Schneider, C.M. and Kiskinova, M.
    Journal of Electron Spectroscopy and Related Phenomena 224 (2018)
    The complementary capabilities of the Scanning PhotoElectron Microscopes (SPEM) and X-ray PhotoEmission Electron Microscopes (XPEEM), operated at Elettra, in terms of imaging and micro-spectroscopy have opened unique opportunities to explore properties of functional materials as a function of their morphology and dimensions and to follow modifications in their properties during their operation. This paper describes the present performance of SPEMs and XPEEMs at Elettra, illustrated by selected recent studies relevant to graphene science. Ongoing efforts for implementing SPEM set-ups allowing for in-situ investigations under realistic operating conditions and PEEM set-up for spin-filtered momentum microscopy are outlined and discussed as well. © 2017 Elsevier B.V.
    view abstract10.1016/j.elspec.2017.06.006
  • Quest for magnons in ultrathin nickel films
    Ibach, H. and Schneider, C.M.
    Physical Review B 98 (2018)
    High-momentum spin waves (magnons) in ultrathin films of cobalt and iron have been explored thoroughly using inelastic scattering of low-energy electrons. The search for magnons in ultrathin nickel films failed, however, although high-energy magnons do exist in bulk nickel. The failure might be due to the weak coupling of nickel magnons to scattering electrons. In order to increase the coupling we deposited layers of cobalt onto Ni films and successfully studied the magnons of such films. The acoustic modes show the same dispersion as pure Co films, which is consistent with the nearly identical stiffness of bulk magnons in nickel and cobalt. Standing magnons in Ni films covered with Co are strongly damped at the Ni/Cu(100) interface when their total wave vector exceeds about 3nm-1. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.014413
  • Size limits of magnetic-domain engineering in continuous in-plane exchange-bias prototype films
    Gaul, A. and Emmrich, D. and Ueltzhöffer, T. and Huckfeldt, H. and Doganay, H. and Hackl, J. and Khan, M.I. and Gottlob, D.M. and Hartmann, G. and Beyer, A. and Holzinger, D. and Nemšák, S. and Schneider, C.M. and Gölzhäuser, A. and Reiss, G. and Ehresmann, A.
    Beilstein Journal of Nanotechnology 9 (2018)
    Background: The application of superparamagnetic particles as biomolecular transporters in microfluidic systems for lab-on-a-chip applications crucially depends on the ability to control their motion. One approach for magnetic-particle motion control is the superposition of static magnetic stray field landscapes (MFLs) with dynamically varying external fields. These MFLs may emerge from magnetic domains engineered both in shape and in their local anisotropies. Motion control of smaller beads does necessarily need smaller magnetic patterns, i.e., MFLs varying on smaller lateral scales. The achievable size limit of engineered magnetic domains depends on the magnetic patterning method and on the magnetic anisotropies of the material system. Smallest patterns are expected to be in the range of the domain wall width of the particular material system. To explore these limits a patterning technology is needed with a spatial resolution significantly smaller than the domain wall width. Results: We demonstrate the application of a helium ion microscope with a beam diameter of 8 nm as a mask-less method for local domain patterning of magnetic thin-film systems. For a prototypical in-plane exchange-bias system the domain wall width has been investigated as a function of the angle between unidirectional anisotropy and domain wall. By shrinking the domain size of peri odic domain stripes, we analyzed the influence of domain wall overlap on the domain stability. Finally, by changing the geometry of artificial two-dimensional domains, the influence of domain wall overlap and domain wall geometry on the ultimate domain size in the chosen system was analyzed. Conclusion: The application of a helium ion microscope for magnetic patterning has been shown. It allowed for exploring the fundamental limits of domain engineering in an in-plane exchange-bias thin film as a prototypical system. For two-dimensional domains the limit depends on the domain geometry. The relative orientation between domain wall and anisotropy axes is a crucial parameter and therefore influences the achievable minimum domain size dramatically. © 2018 Gaul et al.
    view abstract10.3762/bjnano.9.276
  • Spin transport in multilayer systems with fully epitaxial NiO thin films
    Baldrati, L. and Schneider, C. and Niizeki, T. and Ramos, R. and Cramer, J. and Ross, A. and Saitoh, E. and Kläui, M.
    Physical Review B 98 (2018)
    We report the generation and transport of thermal spin currents in fully epitaxial γ-Fe2O3/NiO(001)/Pt and Fe3O4/NiO(001)/Pt trilayers. A thermal gradient, perpendicular to the plane of the sample, generates a magnonic spin current in the ferrimagnetic maghemite (γ-Fe2O3) and magnetite (Fe3O4) thin films by means of the spin Seebeck effect. The spin current propagates across the epitaxial, antiferromagnetic insulating NiO layer, before being detected in the Pt layer by the inverse spin Hall effect. The transport of the spin signal is studied as a function of the NiO thickness, temperature, and ferrimagnetic material where the spin current is generated. In epitaxial NiO grown on maghemite, the spin Seebeck signal decays exponentially as a function of the NiO thickness, with a spin-diffusion length for thermally generated magnons of λMSDL=1.6±0.2nm (where MSDL is mean spin-diffusion length), largely independent of temperature. We see no enhancement of the spin-current signal as previously reported for certain temperatures and thicknesses of the NiO. In epitaxial NiO grown on magnetite, the temperature-averaged spin-diffusion length is λMSDL=3.8±0.3nm, and we observe an enhancement of the spin signal when the NiO thickness is 0.8 nm, demonstrating that the growth conditions dramatically affect the spin-transport properties of the NiO even for full epitaxial growth. In contrast to theoretical predictions for coherent spin transport, we do not see vastly different spin-diffusion lengths between epitaxial and polycrystalline NiO layers. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.014409
  • Ultra-High Vacuum Deposition of Pyrene Molecules on Metal Surfaces
    Schleicher, S. and Borca, B. and Rawson, J. and Matthes, F. and Bürgler, D.E. and Kögerler, P. and Schneider, C.M.
    Physica Status Solidi (B) Basic Research 255 (2018)
    Interfacial properties of organic adsorbates featuring aromatic π-orbitals on metal surfaces play an important role for organic electronics and spintronics. Pyrene is a flat aromatic molecule with a size between ultimately small benzene and extended graphene segments. The deposition of pyrene molecules onto clean and reactive surfaces with a sub-monolayer coverage under ultra-high vacuum (UHV) conditions is challenging, since pyrene is a solid with a high vapor pressure. Here, a sublimation procedure under UHV and image pyrene adlayers on in situ prepared Au(111) and Fe/W(110) substrates by means of low-temperature scanning tunneling microscopy is presented. For Au(111), the molecule–surface interaction is weak as indicated by the specific herringbone reconstruction of the Au(111) surface that is visible through the self-assembled pyrene adlayer. Pyrene desorption due to weak intermolecular interaction self-limits the growth to one monolayer (ML). On the more reactive 2–4 ML thick Fe films on W(110), the molecular order of the pyrene adlayer sensitively depends on the Fe thickness-dependent dislocation pattern at the substrate surface. Irregular arrangements occur for 1 ML Fe and near substrate dislocations for 2–4 ML Fe. Self-assembled ordered arrays form predominantly for 2 ML Fe, where the dislocation pattern leaves sufficiently large unperturbed areas between the dislocation lines. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201800235
  • Valence change detection in memristive oxide based heterostructure cells by hard X-ray photoelectron emission spectroscopy
    Kindsmüller, A. and Schmitz, C. and Wiemann, C. and Skaja, K. and Wouters, D.J. and Waser, R. and Schneider, C.M. and Dittmann, R.
    APL Materials 6 (2018)
    The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum. © 2018 Author(s).
    view abstract10.1063/1.5026063
  • Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt
    Eich, S. and Plötzing, M. and Rollinger, M. and Emmerich, S. and Adam, R. and Chen, C. and Kapteyn, H.C. and Murnane, M.M. and Plucinski, L. and Steil, D. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M. and Schneider, C.M. and Mathias, S.
    Science Advances 3 (2017)
    The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization. 2017 © The Authors, some rights reserved.
    view abstract10.1126/sciadv.1602094
  • Bi1Te1 is a dual topological insulator
    Eschbach, M. and Lanius, M. and Niu, C. W. and Mlynczak, E. and Gospodaric, P. and Kellner, J. and Schuffelgen, P. and Gehlmann, M. and Doring, S. and Neumann, E. and Luysberg, M. and Mussler, G. and Plucinski, L. and Morgenstern, M. and Grutzmacher, D. and Bihlmayer, G. and Blugel, S. and Schneider, C. M.
    Nature Communications 8 (2017)
    New three-dimensional (3D) topological phases can emerge in superlattices containing constituents of known two-dimensional topologies. Here we demonstrate that stoichiometric Bi1Te1, which is a natural superlattice of alternating two Bi2Te3 quintuple layers and one Bi bilayer, is a dual 3D topological insulator where a weak topological insulator phase and topological crystalline insulator phase appear simultaneously. By density functional theory, we find Z(2) indices (0; 001) and a non-zero mirror Chern number. We have synthesized Bi1Te1 by molecular beam epitaxy and found evidence for its topological crystalline and weak topological character by spin-and angle-resolved photoemission spectroscopy. The dual topology opens the possibility to gap the differently protected metallic surface states on different surfaces independently by breaking the respective symmetries, for example, by magnetic field on one surface and by strain on another surface.
    view abstract10.1038/ncomms14976
  • Direct Observation of the Band Gap Transition in Atomically Thin ReS2
    Gehlmann, M. and Aguilera, I. and Bihlmayer, G. and Nemšák, S. and Nagler, P. and Gospodarič, P. and Zamborlini, G. and Eschbach, M. and Feyer, V. and Kronast, F. and Młyńczak, E. and Korn, T. and Plucinski, L. and Schüller, C. and Blügel, S. and Schneider, C.M.
    Nano Letters 17 (2017)
    ReS2 is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of this van der Waals compound leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS2 using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS2 are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing the transition from an indirect band gap in bulk ReS2 to a direct gap in the bilayer and the monolayer. We also find a significantly increased effective hole mass in single-layer crystals. Our results establish bilayer ReS2 as an advantageous building block for two-dimensional devices and van der Waals heterostructures. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b00627
  • Electrical resistance of individual defects at a topological insulator surface
    Lüpke, F. and Eschbach, M. and Heider, T. and Lanius, M. and Schüffelgen, P. and Rosenbach, D. and Von Den Driesch, N. and Cherepanov, V. and Mussler, G. and Plucinski, L. and Grützmacher, D. and Schneider, C.M. and Voigtländer, B.
    Nature Communications 8 (2017)
    Three-dimensional topological insulators host surface states with linear dispersion, which manifest as a Dirac cone. Nanoscale transport measurements provide direct access to the transport properties of the Dirac cone in real space and allow the detailed investigation of charge carrier scattering. Here we use scanning tunnelling potentiometry to analyse the resistance of different kinds of defects at the surface of a (Bi 0.53 Sb 0.47) 2 Te 3 topological insulator thin film. We find the largest localized voltage drop to be located at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. Furthermore, we resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analysed by means of a resistor network model. The effect resulting from the voids is found to be small compared with the other defects.
    view abstract10.1038/ncomms15704
  • Evidence for in-gap surface states on the single phase SmB6(001) surface
    Miyamachi, T. and Suga, S. and Ellguth, M. and Tusche, C. and Schneider, C.M. and Iga, F. and Komori, F.
    Scientific Reports 7 (2017)
    Structural and electronic properties of the SmB6(001) single-crystal surface prepared by Ar+ ion sputtering and controlled annealing are investigated by scanning tunneling microscopy. In contrast to the cases of cleaved surfaces, we observe a single phase surface with a non-reconstructed p(1 × 1) lattice on the entire surface at an optimized annealing temperature. The surface is identified as Sm-terminated on the basis of spectroscopic measurements. On a structurally uniform surface, the emergence of the in-gap state, a robust surface state against structural variation, is further confirmed inside a Kondo hybridization gap at 4.4 K by temperature and atomically-resolved spatial dependences of the differential conductance spectrum near the Fermi energy. © 2017 The Author(s).
    view abstract10.1038/s41598-017-12887-2
  • Exploiting micro-scale structural and chemical observations in real time for understanding chemical conversion: LEEM/PEEM studies over CeOx–Cu(111)
    Duchoň, T. and Hackl, J. and Höcker, J. and Veltruská, K. and Matolín, V. and Falta, J. and Cramm, S. and Nemšák, S. and Schneider, C.M. and Flege, J.I. and Senanayake, S.D.
    Ultramicroscopy 183 (2017)
    Proper consideration of length-scales is critical for elucidating active sites/phases in heterogeneous catalysis, revealing chemical function of surfaces and identifying fundamental steps of chemical reactions. Using the example of ceria thin films deposited on the Cu(111) surface, we demonstrate the benefits of multi length-scale experimental framework for understanding chemical conversion. Specifically, exploiting the tunable sampling and spatial resolution of photoemission electron microscopy, we reveal crystal defect mediated structures of inhomogeneous copper–ceria mixed phase that grow during preparation of ceria/Cu(111) model systems. The density of the microsized structures is such that they are relevant to the chemistry, but unlikely to be found during investigation at the nanoscale or with atomic level investigations. Our findings highlight the importance of accessing micro-scale when considering chemical pathways over heteroepitaxially grown model systems. © 2017 Elsevier B.V.
    view abstract10.1016/j.ultramic.2017.05.003
  • Functional electronic inversion layers at ferroelectric domain walls
    Mundy, J.A. and Schaab, J. and Kumagai, Y. and Cano, A. and Stengel, M. and Krug, I.P. and Gottlob, D.M. and Doǧanay, H. and Holtz, M.E. and Held, R. and Yan, Z. and Bourret, E. and Schneider, C.M. and Schlom, D.G. and Muller, D.A. and Ramesh, R. and Spaldin, N.A. and Meier, D.
    Nature Materials 16 (2017)
    Ferroelectric domain walls hold great promise as functional two-dimensional materials because of their unusual electronic properties. Particularly intriguing are the so-called charged walls where a polarity mismatch causes local, diverging electrostatic potentials requiring charge compensation and hence a change in the electronic structure. These walls can exhibit significantly enhanced conductivity and serve as a circuit path. The development of all-domain-wall devices, however, also requires walls with controllable output to emulate electronic nano-components such as diodes and transistors. Here we demonstrate electric-field control of the electronic transport at ferroelectric domain walls. We reversibly switch from resistive to conductive behaviour at charged walls in semiconducting ErMnO 3. We relate the transition to the formation - and eventual activation - of an inversion layer that acts as the channel for the charge transport. The findings provide new insight into the domain-wall physics in ferroelectrics and foreshadow the possibility to design elementary digital devices for all-domain-wall circuitry. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstract10.1038/nmat4878
  • Heisenberg vs. stoner: Probing the microscopic picture of ultrafast demagnetization using high harmonics
    Zusin, D. and Turgut, E. and Legut, D. and Carva, K. and Gentry, C. and Tengdin, P. and Nembach, H. and Shaw, J. and Mathias, S. and Aeschlimann, M. and Schneider, C. M. and Silva, T. and Oppeneer, P. and Grychtol, P. and Kapteyn, H. and Murnane, M.
    Optics InfoBase Conference Papers Part F42-CLEO_QELS 2017 (2017)
    We uncover the multiple mechanisms underlying laser-driven demagnetization in cobalt using a tabletop high harmonic source. Ultrafast magnon excitation, as well as a transient reduction of the exchange splitting, are both important on femtosecond timescales. © OSA 2017.
    view abstract10.1364/CLEO_QELS.2017.FW1H.1
  • Interfacial Electrochemistry in Liquids Probed with Photoemission Electron Microscopy
    Nemšák, S. and Strelcov, E. and Duchoň, T. and Guo, H. and Hackl, J. and Yulaev, A. and Vlassiouk, I. and Mueller, D.N. and Schneider, C.M. and Kolmakov, A.
    Journal of the American Chemical Society 139 (2017)
    Studies of the electrified solid-liquid interfaces are crucial for understanding biological and electrochemical systems. Until recently, use of photoemission electron microscopy (PEEM) for such purposes has been hampered by incompatibility of the liquid samples with ultrahigh vacuum environment of the electron optics and detector. Here we demonstrate that the use of ultrathin electron transparent graphene membranes, which can sustain large pressure differentials and act as a working electrode, makes it possible to probe electrochemical reactions in operando in liquid environments with PEEM. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b07365
  • Magnetic subunits within a single molecule-surface hybrid
    Heß, V. and Friedrich, R. and Matthes, F. and Caciuc, V. and Atodiresei, N. and Bürgler, D.E. and Blügel, S. and Schneider, C.M.
    New Journal of Physics 19 (2017)
    Magnetic molecule-surface hybrids are ideal building blocks for molecular spintronic devices due to their appealing tailorable magnetic properties and nanoscale size. So far, assemblies of interacting molecular-surface hybrids needed for spintronic functionality were generated by depositing aromatic molecules onto transition-metal surfaces, resulting in a random arrangement of hybrid magnets due to the inherent and strong hybridization. Here, we demonstrate the formation of multiple intramolecular subunits within a single molecule-surface hybrid by means of spin-polarized scanning tunneling microscopy experiments and ab initio density functional theory calculations. This novel effect is realized by depositing a polycyclic aromatic molecule on a magnetic surface. A highly asymmetric chiral adsorption position induces different structural, electronic, and magnetic properties in each aromatic ring of the molecule. In particular, the induced molecular spin polarization near the Fermi energy varies among the rings due to site- and spin-dependent molecule-surface hybridization. Our results showcase a possible organic chemistry route of tailoring geometrically well-defined assemblies of magnetically distinguishable subunits in molecule-surface hybrids. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/aa6ece
  • Nanoscale x-ray investigation of magnetic metallofullerene peapods
    Fritz, F. and Westerström, R. and Kostanyan, A. and Schlesier, C. and Dreiser, J. and Watts, B. and Houben, L. and Luysberg, M. and Avdoshenko, S.M. and Popov, A.A. and Schneider, C.M. and Meyer, C.
    Nanotechnology 28 (2017)
    Endohedral lanthanide ions packed inside carbon nanotubes (CNTs) in a one-dimensional assembly have been studied with a combination of high resolution transmission electron microscopy (HRTEM), scanning transmission x-ray microscopy (STXM), and x-ray magnetic circular dichroism (XMCD). By correlating HRTEM and STXM images we show that structures down to 30 nm are resolved with chemical contrast and record x-ray absorption spectra from endohedral lanthanide ions embedded in individual nanoscale CNT bundles. XMCD measurements of an Er3N@C80 bulk sample and a macroscopic assembly of filled CNTs indicate that the magnetic properties of the endohedral Er3+ ions are unchanged when encapsulated in CNTs. This study demonstrates the feasibility of local magnetic x-ray characterisation of low concentrations of lanthanide ions embedded in molecular nanostructures. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa8b4c
  • Optical control of magnetization dynamics in Gd-Fe-Co films with different compositions
    Ohkochi, T. and Fujiwara, H. and Kotsugi, M. and Takahashi, H. and Adam, R. and Sekiyama, A. and Nakamura, T. and Tsukamoto, A. and Schneider, C. M. and Kuroda, H. and Arguelles, E.F. and Sakaue, M. and Kasai, H. and Tsunoda, M. and Suga, S. and Kinoshita, T.
    Applied Physics Express 10 (2017)
    Perpendicularly magnetized ferrimagnetic Gd-Fe-Co thin films with different compositions and multilayer arrangements were subjected to femtosecond laser pulses. The pulses triggered different magnetization dynamics in the various thin films. In the Gd26Fe66Co8 film, which has an angular-momentum-compensation temperature (TA) well above ambient temperature (Texp), monotonic magnetization reversal occurred, whereas the Gd22Fe70Co8 film (where TA is well below Texp) exhibited remarkable wavelike spin modulation with spatial inhomogeneity during relaxation of the laser-induced nonequilibrium state. These findings can enable broad-range tuning of the magneto-optical responses of Gd-Fe-Co alloys, facilitating advances in materials engineering. © 2017 The Japan Society of Applied Physics.
    view abstract10.7567/APEX.10.103002
  • Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3
    Andrä, M. and Dvořák, F. and Vorokhta, M. and Nemšák, S. and Matolín, V. and Schneider, C.M. and Dittmann, R. and Gunkel, F. and Mueller, D.N. and Waser, R.
    APL Materials 5 (2017)
    In this study, we investigated the electronic surface structure of donor-doped strontium titanate. Homoepitaxial 0.5 wt. % donor-doped SrTiO3 thin films were analyzed by in situ near ambient pressure X-ray photoelectron spectroscopy at a temperature of 770 K and oxygen pressures up to 5 mbar. Upon exposure to an oxygen atmosphere at elevated temperatures, we observed a rigid binding energy shift of up to 0.6 eV towards lower binding energies with respect to vacuum conditions for all SrTiO3 core level peaks and the valence band maximum with increasing oxygen pressure. The rigid shift is attributed to a relative shift of the Fermi energy towards the valence band concomitant with a negative charge accumulation at the surface, resulting in a compensating electron depletion layer in the near surface region. Charge trapping effects solely based on carbon contaminants are unlikely due to their irreversible desorption under the given experimental conditions. In addition, simple reoxygenation of oxygen vacancies can be ruled out as the high niobium dopant concentration dominates the electronic properties of the material. Instead, the negative surface charge may be provided by the formation of cation vacancies or the formation of charged oxygen adsorbates at the surface. Our results clearly indicate a pO2-dependent surface space charge formation in donor-doped SrTiO3 in oxidizing conditions. © 2017 Author(s).
    view abstract10.1063/1.4983618
  • Quantum interference effects in molecular spin hybrids
    Esat, T. and Friedrich, R. and Matthes, F. and Caciuc, V. and Atodiresei, N. and Blugel, S. and Burgler, D. E. and Tautz, F. S. and Schneider, C. M.
    Physical Review B 95 (2017)
    We have studied by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) single molecular spin hybrids formed upon chemisorbing a polycyclic aromatic, threefold symmetric hydrocarbon molecule on Co(111) nanoislands. The spin- dependent hybridization between the Co d states and the p orbitals of the molecule leads to a spin- imbalanced electronic structure of the chemisorbed organic molecule. Spin- sensitive measurements reveal that the spin polarization shows intramolecular variations among the different aromatic rings in spite of the highly symmetric adsorption geometry promoted by symmetry matching of the threefold symmetric molecule and the sixfold symmetric Co(111) lattice. Hence the varying degree of spin polarization on the organic molecule does not stem from a different hybridization of the aromatic rings with the Co atoms, but is proposed to be a consequence of the superposition of the spin polarization of the molecule and the spatially modulated spin polarization of the spin-dependent quantum interference pattern of the Co(111) surface state.
    view abstract10.1103/PhysRevB.95.094409
  • Room temperature 2D electron gas at the (001)-SrTiO3 surface
    Gonzalez, S. and Mathieu, C. and Copie, O. and Feyer, V. and Schneider, C.M. and Barrett, N.
    Applied Physics Letters 111 (2017)
    Functional oxides and phenomena such as a 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post-CMOS electronics. Non-invasive techniques are required to study the surface chemistry and electronic structure, underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy to chemistry and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. We present results demonstrating a room temperature 2DEG at the (001)-SrTiO3 surface. The 2DEG is switched on by soft X-ray irradiation. © 2017 Author(s).
    view abstract10.1063/1.5001222
  • Subfilamentary Networks Cause Cycle-to-Cycle Variability in Memristive Devices
    Baeumer, C. and Valenta, R. and Schmitz, C. and Locatelli, A. and Menteş, T.O. and Rogers, S.P. and Sala, A. and Raab, N. and Nemsak, S. and Shim, M. and Schneider, C.M. and Menzel, S. and Waser, R. and Dittmann, R.
    ACS Nano 11 (2017)
    A major obstacle for the implementation of redox-based memristive memory or logic technology is the large cycle-to-cycle and device-to-device variability. Here, we use spectromicroscopic photoemission threshold analysis and operando XAS analysis to experimentally investigate the microscopic origin of the variability. We find that some devices exhibit variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub)filaments and that the active, current-carrying filament may change from cycle to cycle. These findings account for the observed variability in device performance and represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices. © 2017 American Chemical Society.
    view abstract10.1021/acsnano.7b02113
  • Accurate determination of the valence band edge in hard x-ray photoemission spectra using GW theory
    Lischner, J. and Nemšák, S. and Conti, G. and Gloskovskii, A. and Pálsson, G.K. and Schneider, C.M. and Drube, W. and Louie, S.G. and Fadley, C.
    Journal of Applied Physics 119 (2016)
    We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method. © 2016 Author(s).
    view abstract10.1063/1.4947594
  • Bi atoms mobility-driven circular domains at the Bi/InAs(111) interface
    Richter, M.C. and Mariot, J.-M. and Gafoor, M.A. and Nicolaï, L. and Heckmann, O. and Djukic, U. and Ndiaye, W. and Vobornik, I. and Fujii, J. and Barrett, N. and Feyer, V. and Schneider, C.M. and Hricovini, K.
    Surface Science 651 (2016)
    Bi films deposited on InAs(111) A and B sides have been studied by photoemission electron microscopy. A series of snapshots acquired during sequential annealing of the interfaces at temperatures below and above the melting temperature of Bi allowed obtaining a comprehensive image of the topographic and chemical evolutions of the Bi films that are found to be InAs side dependent. On the A side, a morphology of circular patterns controlled by Bi atoms mobility is observed. The patterns are formed on the pristine In-terminated InAs(111) surface covered by a weakly bonded Bi bilayer. On the B side, no particular morphology is observed due to a stronger chemical interaction between Bi and As atoms as evidenced by the spatially-resolved core-level photoelectron spectra. © 2016 Published by Elsevier Inc.
    view abstract10.1016/j.susc.2016.03.032
  • Compact extreme ultraviolet source for laboratory-based photoemission spectromicroscopy
    Schmitz, C. and Wilson, D. and Rudolf, D. and Wiemann, C. and Plucinski, L. and Riess, S. and Schuck, M. and Hardtdegen, H. and Schneider, C.M. and Tautz, F.S. and Juschkin, L.
    Applied Physics Letters 108 (2016)
    We report on the combination of a state-of-the-art energy-filtering photoemission electron microscope with an intense yet compact laboratory-based gas discharge extreme ultraviolet (EUV) light source. Using a photon energy of 71.7 eV from oxygen plasma (O5+ spectral line), we demonstrate element-selective photoelectron imaging in real space and band structure mapping in reciprocal space. Additionally, the high surface sensitivity of the EUV light was used to study the surface oxidation on islands of the phase-change material Ge1Sb2Te4. The EUV light source allows the extension of spectromicroscopy, previously only feasible at synchrotron beamlines, to laboratory-based work. © 2016 Author(s).
    view abstract10.1063/1.4953071
  • Contact-Free Mapping of Electronic Transport Phenomena of Polar Domains in SrMnO3 Films
    Schaab, J. and Krug, I.P. and Doǧanay, H. and Hackl, J. and Gottlob, D.M. and Khan, M.I. and Nemšák, S. and Maurel, L. and Langenberg, E. and Algarabel, P.A. and Pardo, J.A. and Schneider, C.M. and Meier, D.
    Physical Review Applied 5 (2016)
    High-resolution mapping of electronic transport phenomena plays an increasingly important role for the characterization of ferroic domains and their functionality. At present, spatially resolved electronic transport data are commonly gained from local two-point measurements, collected in line-by-line scans with a conducting nanosized probe. Here, we introduce an innovative experimental approach based on low-energy electron microscopy. As a model case, we study polar domains of varying conductance in strained SrMnO3. By a direct comparison with conductive atomic force and electrostatic force microscopy, we reveal that the applied low-energy electron-microscopy experiment can be considered as an inverse I(V) measurement, providing access to the local electronic conductance with nanoscale resolution and short data-acquisition times in the order of 10-102 ms. Low-energy electrons thus hold yet unexplored application opportunities as a minimal-invasive probe for local electronic transport phenomena, opening a promising route towards spatially resolved, high-throughput sampling at the nanoscale. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.5.054009
  • Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3/SrTiO3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission
    Eiteneer, D. and Pálsson, G.K. and Nemšák, S. and Gray, A.X. and Kaiser, A.M. and Son, J. and LeBeau, J. and Conti, G. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Rotenberg, E. and Gullikson, E.M. and Ueda, S. and Kobayashi, K. and Janotti, A. and Van de Walle, C.G. and Blanca-Romero, A. and Pentcheva, R. and Schneider, C.M. and Stemmer, S. and Fadley, C.S.
    Journal of Electron Spectroscopy and Related Phenomena 211 (2016)
    LaNiO3 (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO3 (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO3)0.3(Sr2AlTaO6)0.7 substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La0.7Sr0.3MnO3/SrTiO3 superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results. © 2016 Elsevier B.V.
    view abstract10.1016/j.elspec.2016.04.008
  • Effects of spin excitons on the surface states of SmB6: A photoemission study
    Arab, A. and Gray, A.X. and Nemšák, S. and Evtushinsky, D.V. and Schneider, C. M. and Kim, D.-J. and Fisk, Z. and Rosa, P.F.S. and Durakiewicz, T. and Riseborough, P.S.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We present the results of a high-resolution valence-band photoemission spectroscopic study of SmB6 which shows evidence for a V-shaped density of states of surface origin within the bulk gap. The spectroscopy data are interpreted in terms of the existence of heavy 4f surface states, which may be useful in resolving the controversy concerning the disparate surface Fermi-surface velocities observed in experiments. Most importantly, we find that the temperature dependence of the valence-band spectrum indicates that a small feature appears at a binding energy of about -9 meV at low temperatures. We attribute this feature to a resonance caused by the spin-exciton scattering in SmB6 which destroys the protection of surface states due to time-reversal invariance and spin-momentum locking. The existence of a low-energy spin exciton may be responsible for the scattering, which suppresses the formation of coherent surface quasiparticles and the appearance of the saturation of the resistivity to temperatures much lower than the coherence temperature associated with the opening of the bulk gap. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.235125
  • Element-Specific Probes of Magnetism
    Schneider, C.M.
    Handbook of Surface Science 5 (2016)
    The investigation of modern spintronics and nanomagnetic elements requires magnetic quantities to be determined with element selectivity. In this contribution, we review the most prominent element-specific probes of magnetism based on the excitation with light and give examples for their applications. The various techniques access the magnetic properties through the electronic system and exploit either the photoemission, photoabsorption, or reflection channel to obtain surface- and bulk-related information about the static and dynamic characteristics of complex magnetic systems. © 2016 Elsevier B.V.
    view abstract10.1016/B978-0-444-62634-9.00002-3
  • Employing soft x-ray resonant magnetic scattering to study domain sizes and anisotropy in Co/Pd multilayers
    Bagschik, K. and Frömter, R. and Bach, J. and Beyersdorff, B. and Müller, L. and Schleitzer, S. and Berntsen, M.H. and Weier, C. and Adam, R. and Viefhaus, J. and Schneider, C. M. and Grübel, G. and Oepen, H.P.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    It is demonstrated that the magnetic diffraction pattern of the isotropic disordered maze pattern is well described utilizing a gamma distribution of domain sizes in a one-dimensional model. From the analysis, the mean domain size and the shape parameter of the distribution are obtained. The model reveals an average domain size that is significantly different from the value that is determined from the peak position of the structure factor in reciprocal space. As a proof of principle, a wedge-shaped (CotÅ/Pd10Å)8 multilayer film, that covers the thickness range of the spin-reorientation transition, has been used. By means of soft x-ray resonant magnetic scattering (XRMS) and imaging techniques the thickness-driven evolution of the magnetic properties of the cobalt layers is explored. It is shown that minute changes of the domain pattern concerning domain size and geometry can be investigated and analyzed due to the high sensitivity and lateral resolution of the XRMS technique. The latter allows for the determination of the magnetic anisotropies of the cobalt layers within a thickness range of a few angstroms. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.134413
  • Energetic, spatial, and momentum character of the electronic structure at a buried interface: The two-dimensional electron gas between two metal oxides
    Nemšák, S. and Conti, G. and Gray, A.X. and Palsson, G.K. and Conlon, C. and Eiteneer, D. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Moreschini, L. and Rotenberg, E. and Strocov, V.N. and Kobayashi, M. and Schmitt, T. and Stolte, W. and Ueda, S. and Kobayashi, K. and Gloskovskii, A. and Drube, W. and Jackson, C.A. and Moetakef, P. and Janotti, A. and Bjaalie, L. and Himmetoglu, B. and Van De Walle, C.G. and Borek, S. and Minar, J. and Braun, J. and Ebert, H. and Plucinski, L. and Kortright, J.B. and Schneider, C. M. and Balents, L. and De Groot, F.M.F. and Stemmer, S. and Fadley, C.S.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    The interfaces between two condensed phases often exhibit emergent physical properties that can lead to new physics and novel device applications and are the subject of intense study in many disciplines. We here apply experimental and theoretical techniques to the characterization of one such interesting interface system: the two-dimensional electron gas (2DEG) formed in multilayers consisting of SrTiO3 (STO) and GdTiO3 (GTO). This system has been the subject of multiple studies recently and shown to exhibit very high carrier charge densities and ferromagnetic effects, among other intriguing properties. We have studied a 2DEG-forming multilayer of the form [6unitcells(u.c.)STO/3u.c.ofGTO]20 using a unique array of photoemission techniques including soft and hard x-ray excitation, soft x-ray angle-resolved photoemission, core-level spectroscopy, resonant excitation, and standing-wave effects, as well as theoretical calculations of the electronic structure at several levels and of the actual photoemission process. Standing-wave measurements below and above a strong resonance have been exploited as a powerful method for studying the 2DEG depth distribution. We have thus characterized the spatial and momentum properties of this 2DEG in detail, determining via depth-distribution measurements that it is spread throughout the 6 u.c. layer of STO and measuring the momentum dispersion of its states. The experimental results are supported in several ways by theory, leading to a much more complete picture of the nature of this 2DEG and suggesting that oxygen vacancies are not the origin of it. Similar multitechnique photoemission studies of such states at buried interfaces, combined with comparable theory, will be a very fruitful future approach for exploring and modifying the fascinating world of buried-interface physics and chemistry. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.245103
  • Erratum: Magnetic measurements with atomic-plane resolution
    Rusz, J. and Muto, S. and Spiegelberg, J. and Adam, R. and Tatsumi, K. and Burgler, D. E. and Oppeneer, P. M. and Schneider, C. M.
    Nature Communications 7 (2016)
    view abstract10.1038/ncomms13159
  • Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet
    Mlynczak, E. and Eschbach, M. and Borek, S. and Minár, J. and Braun, J. and Aguilera, I. and Bihlmayer, G. and Döring, S. and Gehlmann, M. and Gospodaric, P. and Suga, S. and Plucinski, L. and Blügel, S. and Ebert, H. and Schneider, C. M.
    Physical Review X 6 (2016)
    We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of ΔE = 100 meV and Δk = 0.1 Å-1. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spinorbit interaction even for elements as light as 3d ferromagnets. These results set a new paradigm for the investigations of spin-orbit effects in the spintronic materials. The same methodology could be used in the bottom-up design of the devices based on the switching of spin-orbit gaps such as electric-field control of magnetic anisotropy or tunneling anisotropic magnetoresistance.
    view abstract10.1103/PhysRevX.6.041048
  • Functional materials for information and energy technology: Insights by photoelectron spectroscopy
    Müller, M. and Nemšák, S. and Plucinski, L. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 208 (2016)
    The evolution of both information and energy technology is intimately connected to complex condensed matter systems, the properties of which are determined by electronic and chemical interactions and processes on a broad range of length and time scales. Dedicated photoelectron spectroscopy and spectromicroscopy experiments can provide important insights into fundamental phenomena and applied functionalities. We discuss some recent methodological developments with application to relevant questions in spintronics, and towards operando studies of resistive switching and electrochemical processes. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2015.08.003
  • Growth, characterization, and transport properties of ternary (Bi1-xSbx)2Te3 topological insulator layers
    Weyrich, C. and Drögeler, M. and Kampmeier, J. and Eschbach, M. and Mussler, G. and Merzenich, T. and Stoica, T. and Batov, I.E. and Schubert, J. and Plucinski, L. and Beschoten, B. and Schneider, C.M. and Stampfer, C. and Grützmacher, D. and Schäpers, T.
    Journal of Physics Condensed Matter 28 (2016)
    Ternary (Bi1-xSbx)2Te3 films with an Sb content between 0 and 100% were deposited on a Si(1 1 1) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by x-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1-xSbx)2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/28/49/495501
  • High resolution electron energy loss spectroscopy of spin waves in ultra-thin cobalt films
    Michel, E. and Ibach, H. and Schneider, C.M.
    Surface and Interface Analysis 48 (2016)
    Electron energy loss spectroscopy has successfully established itself as the experimental method to study exchange-dominated, high-momentum spin waves in ultra-thin films. Because of insufficient energy resolution, previous studies were limited to spin waves in the high energy range and to wave vectors larger than about q|| = 0.4 Å−1. In this regime, spin waves are strongly damped by decay into Stoner excitations. Furthermore, the spin wave signal of a multilayer film consists of several overlapping modes. After implementation of several technical modifications, our electron spectrometer now enables the study of spin waves with an energy resolution down to 2 meV, and thereby the discrimination of several spin wave modes in ultra-thin films as well as the study of the intrinsic width of modes down to 1 meV. Examples are presented for fcc and hcp cobalt films and discussed in terms of current theoretical models. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.6127
  • Impact of Tunnel-Barrier Strength on Magnetoresistance in Carbon Nanotubes
    Morgan, C. and Misiorny, M. and Metten, D. and Heedt, S. and Schäpers, T. and Schneider, C.M. and Meyer, C.
    Physical Review Applied 5 (2016)
    We investigate magnetoresistance in spin valves involving CoPd-contacted carbon nanotubes. Both the temperature and bias-voltage dependence clearly indicate tunneling magnetoresistance as the origin. We show that this effect is significantly affected by the tunnel-barrier strength, which appears to be one reason for the variation between devices previously detected in similar structures. Modeling the data by means of the scattering matrix approach, we find a nontrivial dependence of the magnetoresistance on the barrier strength. Furthermore, an analysis of the spin precession observed in a nonlocal Hanle measurement yields a spin lifetime of τs=1.1 ns, a value comparable with those found in silicon- or graphene-based spin-valve devices. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.5.054010
  • Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO3/La0.7Sr0.3MnO3
    Krug, I.P. and Doganay, H. and Nickel, F. and Gottlob, D.M. and Schneider, C.M. and Morelli, A. and Preziosi, D. and Lindfors-Vrejoiu, I. and Laskowski, R. and Barrett, N.
    Journal of Applied Physics 120 (2016)
    We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO3/La0.7Sr0.3MnO3 (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer. © 2016 Author(s).
    view abstract10.1063/1.4962007
  • Lifetime and mean free path of spin waves in ultrathin cobalt films
    Michel, E. and Ibach, H. and Schneider, C. M. and Santos, D.L.R. and Costa, A.T.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    Miniaturization of magnon-based devices into the nanometer range would require the utilization of exchange-dominated spin waves of nanometer wavelength. In experiment and theory we show that the intrinsic lifetime and mean free path of the homogeneous acoustic spin wave in ultrathin cobalt films is sufficiently long for such applications provided that the films are atomically flat. The presence of surface steps, however, shortens lifetime and mean free path. The experimental data are consistent with a model which assumes that steps act as perfect sinks for spin waves. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.014420
  • Magnetic measurements with atomic-plane resolution
    Rusz, J. and Muto, S. and Spiegelberg, J. and Adam, R. and Tatsumi, K. and Bürgler, D.E. and Oppeneer, P.M. and Schneider, C.M.
    Nature Communications 7 (2016)
    Rapid development of magnetic nanotechnologies calls for experimental techniques capable of providing magnetic information with subnanometre spatial resolution. Available probes of magnetism either detect only surface properties, such as spin-polarized scanning tunnelling microscopy, magnetic force microscopy or spin-polarized low-energy electron microscopy, or they are bulk probes with limited spatial resolution or quantitativeness, such as X-ray magnetic circular dichroism or classical electron magnetic circular dichroism (EMCD). Atomic resolution EMCD methods have been proposed, although not yet experimentally realized. Here, we demonstrate an EMCD technique with an atomic size electron probe utilizing a probe-corrected scanning transmission electron microscope in its standard operation mode. The crucial element of the method is a ramp in the phase of the electron beam wavefunction, introduced by a controlled beam displacement. We detect EMCD signals with atomic-plane resolution, thereby bringing near-atomic resolution magnetic circular dichroism spectroscopy to hundreds of laboratories worldwide. © 2016 The Author(s).
    view abstract10.1038/ncomms12672
  • Magnetic surface domain imaging of uncapped epitaxial FeRh(001) thin films across the temperature-induced metamagnetic transition
    Zhou, X. and Matthes, F. and Bürgler, D.E. and Schneider, C.M.
    AIP Advances 6 (2016)
    The surface magnetic domain structure of uncapped epitaxial FeRh/MgO(001) thin films was imaged by in-situ scanning electron microscopy with polarization analysis (SEMPA) at various temperatures between 122 and 450 K. This temperature range covers the temperature-driven antiferromagnetic-to-ferromagnetic phase transition in the body of the films that was observed in-situ by means of the more depth-sensitive magneto-optical Kerr effect. The SEMPA images confirm that the interfacial ferromagnetism coexisting with the antiferromagnetic phase inside the film is an intrinsic property of the FeRh(001) surface. Furthermore, the SEMPA data display a reduction of the in-plane magnetization occuring well above the phase transition temperature which, thus, is not related to the volume expansion at the phase transition. This observation is interpreted as a spin reorientation of the surface magnetization for which we propose a possible mechanism based on temperature-dependent tetragonal distortion due to different thermal expansion coefficients of MgO and FeRh. © 2016 Author(s).
    view abstract10.1063/1.4940758
  • Photon drag effect in (Bi1-xSbx)2Te3 three-dimensional topological insulators
    Plank, H. and Golub, L.E. and Bauer, S. and Bel'Kov, V.V. and Herrmann, T. and Olbrich, P. and Eschbach, M. and Plucinski, L. and Schneider, C. M. and Kampmeier, J. and Lanius, M. and Mussler, G. and Grützmacher, D. and Ganichev, S.D.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We report on the observation of a terahertz radiation-induced photon drag effect in epitaxially grown n- and p-type (Bi1-xSbx)2Te3 three-dimensional topological insulators with different antimony concentrations x varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a dc electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent can be generated in surface states. It arises due to the dynamical momentum alignment by time- and space-dependent radiation electric field and implies the radiation-induced asymmetric scattering in the electron momentum space. We show that the photon drag current may also be generated in the bulk. Both surface states and bulk photon drag currents behave identically upon variation of such macroscopic parameters as radiation polarization and photocurrent direction with respect to the radiation propagation. This fact complicates the assignment of the trigonal photon drag effect to a specific electronic system. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.125434
  • Quantifying redox-induced Schottky barrier variations in memristive devices via in operando spectromicroscopy with graphene electrodes
    Baeumer, C. and Schmitz, C. and Marchewka, A. and Mueller, D.N. and Valenta, R. and Hackl, J. and Raab, N. and Rogers, S.P. and Khan, M.I. and Nemsak, S. and Shim, M. and Menzel, S. and Schneider, C.M. and Waser, R. and Dittmann, R.
    Nature Communications 7 (2016)
    The continuing revolutionary success of mobile computing and smart devices calls for the development of novel, cost-And energy-efficient memories. Resistive switching is attractive because of, inter alia, increased switching speed and device density. On electrical stimulus, complex nanoscale redox processes are suspected to induce a resistance change in memristive devices. Quantitative information about these processes, which has been experimentally inaccessible so far, is essential for further advances. Here we use in operando spectromicroscopy to verify that redox reactions drive the resistance change. A remarkable agreement between experimental quantification of the redox state and device simulation reveals that changes in donor concentration by a factor of 2-3 at electrode-oxide interfaces cause a modulation of the effective Schottky barrier and lead to >2 orders of magnitude change in device resistance. These findings allow realistic device simulations, opening a route to less empirical and more predictive design of future memory cells. © The Author(s) 2016.
    view abstract10.1038/ncomms12398
  • Quantitative spectromicroscopy from inelastically scattered photoelectrons in the hard X-ray range
    Renault, O. and Zborowski, C. and Risterucci, P. and Wiemann, C. and Grenet, G. and Schneider, C.M. and Tougaard, S.
    Applied Physics Letters 109 (2016)
    We demonstrate quantitative, highly bulk-sensitive x-ray photoelectron emission microscopy by analysis of inelastically scattered photoelectrons in the hard X-ray range, enabling elemental depth distribution analysis in deeply buried layers. We show results on patterned structures used in electrical testing of high electron mobility power transistor devices with an epitaxial Al0.25Ga0.75N channel and a Ti/Al metal contact. From the image series taken over an energy range of up to 120 eV in the Ti 1s loss feature region and over a typical 100 μm field of view, one can accurately retrieve, using background analysis together with an optimized scattering cross-section, the Ti depth distribution from 14 nm up to 25 nm below the surface. The method paves the way to multi-elemental, bulk-sensitive 3D imaging and investigation of phenomena at deeply buried interfaces and microscopic scales by photoemission. © 2016 Author(s).
    view abstract10.1063/1.4955427
  • Quantum transport in carbon nanotubes covalently functionalized with magnetic molecules
    Schnee, M. and Besson, C. and Frielinghaus, R. and Lurz, C. and Kögerler, P. and Schneider, C.M. and Meyer, C.
    Physica Status Solidi (B) Basic Research 253 (2016)
    We report on quantum transport measurements of a carbon nanotube (CNT) quantum dot that is functionalized with magnetic {Mn4} complexes. The coupling between the spin-5/2 MnII ions within each {Mn4} complex is predominantly antiferromagnetic. Coulomb diamond measurements at T = 4 K reveal that the covalent attachment of the complexes to the CNT framework has only little influence on the carbon nanotube quantum dot. Surprisingly, a strong increase of noise is observed upon cooling the sample to T = 30 mK. Time traces of the current taken at a diamond edge reveal a random telegraph signal. We attribute this to fluctuations of molecular spin states of the attached {Mn4} complexes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201600292
  • Quasi 2D electronic states with high spin-polarization in centrosymmetric MoS2 bulk crystals
    Gehlmann, M. and Aguilera, I. and Bihlmayer, G. and Młyńczak, E. and Eschbach, M. and Döring, S. and Gospodarič, P. and Cramm, S. and Kardynał, B. and Plucinski, L. and Blügel, S. and Schneider, C.M.
    Scientific Reports 6 (2016)
    Time reversal dictates that nonmagnetic, centrosymmetric crystals cannot be spin-polarized as a whole. However, it has been recently shown that the electronic structure in these crystals can in fact show regions of high spin-polarization, as long as it is probed locally in real and in reciprocal space. In this article we present the first observation of this type of compensated polarization in MoS2 bulk crystals. Using spin- and angle-resolved photoemission spectroscopy (ARPES), we directly observed a spin-polarization of more than 65% for distinct valleys in the electronic band structure. By additionally evaluating the probing depth of our method, we find that these valence band states at the point in the Brillouin zone are close to fully polarized for the individual atomic trilayers of MoS2, which is confirmed by our density functional theory calculations. Furthermore, we show that this spin-layer locking leads to the observation of highly spin-polarized bands in ARPES since these states are almost completely confined within two dimensions. Our findings prove that these highly desired properties of MoS2 can be accessed without thinning it down to the monolayer limit.
    view abstract10.1038/srep26197
  • Schottky barrier measurements on individual GaAs nanowires by X-ray photoemission microscopy
    di Mario, L. and Turchini, S. and Zamborlini, G. and Feyer, V. and Tian, L. and Schneider, C.M. and Rubini, S. and Martelli, F.
    Applied Surface Science 386 (2016)
    We present measurements of the Schottky barrier height on individual GaAs nanowires by means of x-ray photoelectron emission microscopy (XPEEM). Values of 0.73 and 0.51 eV, averaged over the entire wires, were measured on Cu-covered n-doped and p-doped GaAs nanowires, respectively, in agreement with results obtained on bulk material. Our measurements show that XPEEM can become a feasible and reliable investigation tool of interface formation at the nanoscale and pave the way towards the study of size-dependent effects on semiconductor-based structures. © 2016 Elsevier B.V.
    view abstract10.1016/j.apsusc.2016.06.002
  • Spatial coherence determination from the Fourier analysis of a resonant soft X-ray magnetic speckle pattern
    Bagschik, K. and Frömter, R. and Müller, L. and Roseker, W. and Bach, J. and Staeck, P. and Thönnißen, C. and Schleitzer, S. and Berntsen, M.H. and Weier, C. and Adam, R. and Viefhaus, J. and Schneider, C. M. and Grübel, G. and Oepen, H.P.
    Optics Express 24 (2016)
    We present a method to determine the two-dimensional spatial coherence of synchrotron radiation in the soft X-ray regime by analyzing the Fourier transform of the magnetic speckle pattern from a ferromagnetic film in a multidomain state. To corroborate the results, a Young's double-pinhole experiment has been performed. The transverse coherence lengths in vertical and horizontal direction of both approaches are in a good agreement. The method presented here is simple and gives a direct access to the coherence properties of synchrotron radiation without nanostructured test objects. ©2016 Optical Society of America.
    view abstract10.1364/OE.24.023162
  • Spin-hybrids: A single-molecule approach to spintronics
    Bürgler, D.E. and Heß, V. and Esat, T. and Fahrendorf, S. and Matthes, F. and Schneider, C. M. and Besson, C. and Monakhov, K.Y. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Atodiresei, N. and Caciuc, V. and Blügel, S.
    e-Journal of Surface Science and Nanotechnology 14 (2016)
    Molecular spintronics aims at exploiting and controlling spin-dependent transport processes at the molecular level. Achieving this aim requires not only appropriate molecules, molecular structures and preparation procedures. Equally important is the understanding and engineering of the electronic and spin-dependent interactions between different molecular species, molecule and substrate, as well as molecule and electrodes. These interactions may not only determine the spin-dependent functionality of the molecular structures, but also their integrity on the substrate. Likewise, there may be also a modification of the surface properties below and in the vicinity of a molecule. We have investigated several molecules on different metallic surfaces, among them magnetic Nd doubledecker phthalocyanines, a cubane-type {Ni4} complex with single-molecule magnet properties, and a nonmagnetic triazine-based molecule. For NdPc2 molecules adsorbed on a Cu(100) surface, our scanning tunneling microscopy and spectroscopy studies show specific electronic states of the molecule-substrate complex. We find that the electric field between STM tip and sample must be taken into account to properly describe the electronic states associated with the upper Pc ligand. © 2016 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2016.17
  • Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation
    Plötzing, M. and Adam, R. and Weier, C. and Plucinski, L. and Eich, S. and Emmerich, S. and Rollinger, M. and Aeschlimann, M. and Mathias, S. and Schneider, C.M.
    Review of Scientific Instruments 87 (2016)
    The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales. © 2016 Author(s).
    view abstract10.1063/1.4946782
  • Surface analysis of the Heusler Ni49.7Mn29.1Ga21.2 Alloy: The composition, phase transition, and twinned microstructure of martensite
    Horáková, K. and Cháb, V. and Heczko, O. and Drchal, V. and Fekete, L. and Honolka, J. and Kopeček, J. and Kudrnovský, J. and Polyak, Y. and Sajdl, P. and Vondráček, M. and Lančok, J. and Feyer, V. and Wiemann, C. and Schneider, C.M.
    Journal of Applied Physics 120 (2016)
    Surface analysis was used to study the dynamics of the martensitic transformation on macro- and mesoscopic scales. The chemical state, morphology, and magnetic and surface structure were monitored at particular stages of the phase transition. At room temperature, the martensitic phase of the Ni49.7Mn29.1Ga21.2 (100) single crystal exhibited macroscopic a/c twinning and a corresponding magnetic domain structure characterized by magnetization vector in and out of the surface plane. Induced by radiation heating, the transformation from martensite to austenite takes place separately at the surface and in the bulk. Its dynamics depend on the history of the sample treatment which affects the crystallographic orientation of twins and minor changes of the surface stoichiometry. The interfaces (twin planes) between twin variants in the martensitic phase were noticeable also in the austenitic phase, thanks to the shape memory effect of this material. © 2016 Author(s).
    view abstract10.1063/1.4962648
  • Verification of redox-processes as switching and retention failure mechanisms in Nb:SrTiO3/metal devices
    Baeumer, C. and Raab, N. and Menke, T. and Schmitz, C. and Rosezin, R. and Müller, P. and Andrä, M. and Feyer, V. and Bruchhaus, R. and Gunkel, F. and Schneider, C.M. and Waser, R. and Dittmann, R.
    Nanoscale 8 (2016)
    Nanoscale redox reactions in transition metal oxides are believed to be the physical foundation of memristive devices, which present a highly scalable, low-power alternative for future non-volatile memory devices. The interface between noble metal top electrodes and Nb-doped SrTiO3 single crystals may serve as a prominent but not yet well-understood example of such memristive devices. In this report, we will present experimental evidence that nanoscale redox reactions and the associated valence change mechanism are indeed responsible for the resistance change in noble metal/Nb-doped SrTiO3 junctions with dimensions ranging from the micrometer scale down to the nanometer regime. Direct verification of the valence change mechanism is given by spectromicroscopic characterization of switching filaments. Furthermore, it is found that the resistance change over time is driven by the reoxidation of a previously oxygen-deficient region. The retention times of the low resistance states, accordingly, can be dramatically improved under vacuum conditions as well as through the insertion of a thin Al2O3 layer which prevents this reoxidation. These insights finally confirm the resistive switching mechanism at these interfaces and are therefore of significant importance for the study and application of memristive devices based on Nb-doped SrTiO3 as well as systems with similar switching mechanisms. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6nr00824k
  • Adsorption phenomena of cubane-type tetranuclear Ni(II) complexes with neutral, thioether-functionalized ligands on Au(111)
    Heß, V. and Matthes, F. and Bürgler, D.E. and Monakhov, K.Y. and Besson, C. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Schneider, C.M.
    Surface Science 641 (2015)
    Abstract The controlled and intact deposition of molecules with specific properties onto surfaces is an emergent field impacting a wide range of applications including catalysis, molecular electronics, and quantum information processing. One strategy is to introduce grafting groups functionalized to anchor to a specific surface. While thiols and disulfides have proven to be quite effective in combination with gold surfaces, other S-containing groups have received much less attention. Here, we investigate the surface anchoring and organizing capabilities of novel charge-neutral heterocyclic thioether groups as ligands of polynuclear nickel(II) complexes. We report on the deposition of a cubane-type {Ni<inf>4</inf>} (= [Ni(μ<inf>3</inf>-Cl)Cl(HL·S)]<inf>4</inf>) single-molecule magnet from dichloromethane solution on a Au(111) surface, investigated by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction, both immediately after deposition and after subsequent post-annealing. The results provide strong evidence for partial decomposition of the coordination complex upon deposition on the Au(111) surface that, however, leaves the magnetic {Ni<inf>4</inf>Cl<inf>4n</inf>} (n = 1 or 2) core intact. Only post-annealing above 480 K induces further decomposition and fragmentation of the {Ni<inf>4</inf>Cl<inf>4n</inf>} core. The detailed insight into the chemisorption-induced decomposition pathway not only provides guidelines for the deposition of thioether-functionalized Ni(II) complexes on metallic surfaces but also reveals opportunities to use multidentate organic ligands decorated with thioether groups as transporters for highly unstable inorganic structures onto conducting surfaces, where they are stabilized retaining appealing electronic and magnetic properties. © 2015 Elsevier B.V.
    view abstract10.1016/j.susc.2015.06.026
  • Aqueous solution/metal interfaces investigated in operando by photoelectron spectroscopy
    Karslioʇlu, O. and Nemšák, S. and Zegkinoglou, I. and Shavorskiy, A. and Hartl, M. and Salmassi, F. and Gullikson, E.M. and Ng, M.L. and Rameshan, Ch. and Rude, B. and Bianculli, D. and Cordones, A.A. and Axnanda, S. and Crumlin, E.J. and Ross, P.N. and Schneider, C.M. and Hussain, Z. and Liu, Z. and Fadley, C.S. and Bluhm, H.
    Faraday Discussions 180 (2015)
    We describe a new in operando approach for the investigation of heterogeneous processes at solid/liquid interfaces with elemental and chemical specificity which combines the preparation of thin liquid films using the meniscus method with standing wave ambient pressure X-ray photoelectron spectroscopy [Nemšák et al., Nat. Commun., 5, 5441 (2014)]. This technique provides information about the chemical composition across liquid/solid interfaces with sub-nanometer depth resolution and under realistic conditions of solution composition and concentration, pH, as well as electrical bias. In this article, we discuss the basics of the technique and present the first results of measurements on KOH/Ni interfaces. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5fd00003c
  • Avalanche-Discharge-Induced Electrical Forming in Tantalum Oxide-Based Metal-Insulator-Metal Structures
    Skaja, K. and Baumer, C. and Peters, O. and Menzel, S. and Moors, M. and Du, H. C. and Bornhofft, M. and Schmitz, C. and Feyer, V. and Jia, C. L. and Schneider, C. M. and Mayer, J. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 (2015)
    Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5- x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5- x interface of the dendrite-like structure.
    view abstract10.1002/adfm.201502767
  • Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates
    Tirpanci, Ş. and Bürgler, D.E. and Schneider, C.M. and Rameev, B. and Aktaş, B.
    Microelectronic Engineering 140 (2015)
    Electron beam writing and imaging of nanoscale structures on highly insulating substrates severely suffer from charging effects, which cause reduction in pattern resolution, positioning precision, and imaging quality. Conductive layers deposited above or below the resist layer can effectively reduce charge accumulation, but often give rise to contamination impairing the physical and chemical properties of functional nanostructures. Here we deal with top and bottom contacted, sub-micron-sized nanopillars made from multilayer stacks comprising ferromagnetic and non-magnetic materials for the study of current-induced magnetization dynamics. We show how the charging effects in a previously established fabrication process for single-crystalline nanopillars by H. Dassow et al. (2006) [1] can be significantly reduced by using the bottom electrode layer as charge dissipater and only isolating and disconnecting the bottom electrodes from ground after the fabrication of the delicate nanopillar structure by electron beam lithography. The modified process is successfully applied to Co<inf>2</inf>MnSi/Ag/Co<inf>2</inf>MnSi(001) multilayer stacks grown on highly insulating MgO substrates. Ellipsoidal nanopillars with a cross-section of 75 × 120 nm2 reveal 2% giant magnetoresistance and angular dependent magnetization behavior due to the magnetic anisotropy of the elliptical nanomagnets. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2015.05.007
  • Controlled covalent binding of antiferromagnetic tetramanganese complexes to carbon nanotubes
    Frielinghaus, R. and Besson, C. and Houben, L. and Saelhoff, A.-K. and Schneider, C.M. and Meyer, C.
    RSC Advances 5 (2015)
    We report on the covalent functionalization of carbon nanotubes with tetramanganese complexes. The process is based on ligand exchange with carboxylic groups on the preliminarily oxidized tubes and does not fundamentally affect the antiferromagnetic coupling between the manganese ions of the complex. We present detailed analysis of the oxidation process and demonstrate that the degree of functionalization directly relates to the density of carboxylic groups. The coverage of a carbon nanotube can be decreased enough to enable fabrication of electronic devices incorporating an individual molecule. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5ra14983e
  • Element selective investigation of spin dynamics in magnetic multilayers
    Rudolf, D. and La-o-Vorakiat, C. and Battiato, M. and Adam, R. and Grychtol, P. and Shaw, J.M. and Turgut, E. and Maldonado, P. and Mathias, S. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Kapteyn, H.C. and Murnane, M.M. and Oppeneer, P.M. and Schneider, C.M.
    Springer Proceedings in Physics 159 (2015)
    Our understanding of ultrafast switching processes in novel spin-based electronics depends on our detailed knowledge of interactions between spin, charge and phonons in magnetic structures. We present element-selective studies, using extreme ultraviolet (XUV) light, to gain insight into spin dynamics in exchange coupled magnetic multilayers on the femtosecond time scale. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_95
  • Femtosecond-laser-induced modifications in Co/Pt multilayers studied with tabletop resonant magnetic scattering
    Weier, C. and Adam, R. and Rudolf, D. and Frömter, R. and Grychtol, P. and Winkler, G. and Kobs, A. and Oepen, H.P. and Kapteyn, H.C. and Murnane, M.M. and Schneider, C. M.
    EPL 109 (2015)
    We characterize the magnetic domain structure of Co/Pt multilayer films on length scales below one hundred nanometers using resonant magnetic scattering and magnetic force microscopy. The extreme ultraviolet light for the scattering experiment is created by a laser-based high-order harmonic generation source. After illumination with intense ultrashort infrared laser pulses, we observe pronounced changes in the magnetic structure and morphology. This study points out the importance of a detailed analysis of the different laser-induced modifications of a magnetic thin film that influence the scattering patterns. © Copyright EPLA, 2015.
    view abstract10.1209/0295-5075/109/17001
  • FMR Investigations of Two-dimensional Periodic Arrays of Disc-shaped Co Particles at Different Temperatures
    Martyanov, O.N. and Balaev, D.A. and Pylypenko, O.V. and Odnodvorets, L.V. and Chernov, S.V. and Nepijko, S.A. and Elmers, H.-J. and Schneider, C.M. and Schönhense, G.
    Journal of Superconductivity and Novel Magnetism 28 (2015)
    Using ferromagnetic resonance method, we performed measurements of two-dimensional periodic arrays of disc-shaped cobalt particles with different diameters a=450 and 900 nm and the distance between them l=2a and 3a with temperature variation in the range T = 140 −300 K. The first derivative of the microwave absorption spectrum was registered. With the increase of T additional peaks on both sides from the main peak that move aside from it, all peaks show an increase of the intensity and a decrease of the width. Dependences of the resonance field on T shows saturation-like behavior with increasing temperature. They move to higher temperatures and show sharper behavior with a increase and l decrease, respectively. An increase of a leads to the intensity decrease and width increase of all three adsorption peaks. © 2015, Springer Science+Business Media New York.
    view abstract10.1007/s10948-015-3178-3
  • Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201500851
  • Interference effects in T-MOKE spectra of Fe thin films at the 3p edges - Theory and experiment
    Legut, D. and Tesch, M.F. and Oppeneer, P.M. and Mertins, H.-Ch. and Jansing, Ch. and Gilbert, M. and Gaupp, A. and Bürgler, D.E. and Schneider, C. M.
    Acta Physica Polonica A 127 (2015)
    We present combined first-principle calculations and experimental results of the transversal magneto-optical Kerr effect (T-MOKE) of thin Fe films across the 3p edges using linearly polarized synchrotron radiation. We show that the experimental T-MOKE spectra at the 3p edges of Fe exhibit clear signals that are strongly influenced by interference effects. Ab initio calculated T-MOKE asymmetry spectra confirm the importance of interference effects. The comparison of experimental with calculated spectra reveals some differences that we attribute to metal/metal interface roughness that is not taken into account in the calculations.
    view abstract10.12693/APhysPolA.127.466
  • Microscopic analysis of the composition driven spin-reorientation transition in NixPd1-x/Cu(001)
    Gottlob, D.M. and Doğanay, H. and Nickel, F. and Cramm, S. and Krug, I.P. and Nemšák, S. and Schneider, C.M.
    Ultramicroscopy 159 (2015)
    The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified. © 2015 Elsevier B.V.
    view abstract10.1016/j.ultramic.2015.05.022
  • Observation by resonant angle-resolved photoemission of a critical thickness for 2-dimensional electron gas formation in SrTiO3 embedded in GdTiO3
    Nemšák, S. and Conti, G. and Palsson, G.K. and Conlon, C. and Cho, S. and Rault, J.E. and Avila, J. and Asensio, M.-C. and Jackson, C.A. and Moetakef, P. and Janotti, A. and Bjaalie, L. and Himmetoglu, B. and Van De Walle, C.G. and Balents, L. and Schneider, C.M. and Stemmer, S. and Fadley, C.S.
    Applied Physics Letters 107 (2015)
    For certain conditions of layer thickness, the interface between GdTiO3 (GTO) and SrTiO3 (STO) in multilayer samples has been found to form a two-dimensional electron gas (2DEG) with very interesting properties including high mobilities and ferromagnetism. We have here studied two trilayer samples of the form [2 nm GTO/1.0 or 1.5 unit cells STO/10 nm GTO] as grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7, with the STO layer thicknesses being at what has been suggested is the critical thickness for 2DEG formation. We have studied these with Ti-resonant angle-resolved and angle-integrated photoemission and find that the spectral feature in the spectra associated with the 2DEG is present in the 1.5 unit cell sample, but not in the 1.0 unit cell sample. We also observe through core-level spectra additional states in Ti and Sr, with the strength of a low-binding-energy state for Sr being associated with the appearance of the 2DEG, and we suggest it to have an origin in final-state core-hole screening. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4936936
  • Realization of a vertical topological p-n junction in epitaxial Sb2Te3/Bi2Te3 heterostructures
    Eschbach, M. and Młyńczak, E. and Kellner, J. and Kampmeier, J. and Lanius, M. and Neumann, E. and Weyrich, C. and Gehlmann, M. and Gospodarič, P. and Döring, S. and Mussler, G. and Demarina, N. and Luysberg, M. and Bihlmayer, G. and Schäpers, T. and Plucinski, L. and Blügel, S. and Morgenstern, M. and Schneider, C.M. and Grützmacher, D.
    Nature Communications 6 (2015)
    Three-dimensional (3D) topological insulators are a new state of quantum matter, which exhibits both a bulk band structure with an insulating energy gap as well as metallic spin-polarized Dirac fermion states when interfaced with a topologically trivial material. There have been various attempts to tune the Dirac point to a desired energetic position for exploring its unusual quantum properties. Here we show a direct experimental proof by angle-resolved photoemission of the realization of a vertical topological p-n junction made of a heterostructure of two different binary 3D TI materials Bi2Te3 and Sb2Te3 epitaxially grown on Si(111). We demonstrate that the chemical potential is tunable by about 200meV when decreasing the upper Sb2Te3 layer thickness from 25 to 6 quintuple layers without applying any external bias. These results make it realistic to observe the topological exciton condensate and pave the way for exploring other exotic quantum phenomena in the near future. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms9816
  • Reversible temperature-driven domain transition in bistable Fe magnetic nanostrips grown on Ru(0001)
    Quesada, A. and Monti, M. and Krug, I.P. and Rougemaille, N. and Nickel, F. and Gottlob, D.M. and Doganay, H. and N'Diaye, A.T. and Chen, G. and Serrano, A. and McCarty, K.F. and Fernández, J.F. and Schneider, C. M. and Schmid, A.K. and De La Figuera, J.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    High-aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multidomain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.024416
  • Spectromicroscopic insights for rational design of redox-based memristive devices
    Baeumer, C. and Schmitz, C. and Ramadan, A.H.H. and Du, H. and Skaja, K. and Feyer, V. and Muller, P. and Arndt, B. and Jia, C.-L. and Mayer, J. and De Souza, R.A. and Schneider, C. M. and Waser, R. and Dittmann, R.
    Nature Communications 6 (2015)
    The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably. © 2015 Macmillan Publishers Limited.
    view abstract10.1038/ncomms9610
  • Spectroscopic XPEEM of highly conductive SI-doped GaN wires
    Renault, O. and Morin, J. and Tchoulfian, P. and Chevalier, N. and Feyer, V. and Pernot, J. and Schneider, C. M.
    Ultramicroscopy 159 (2015)
    Using soft X-ray photoelectron emission microscopy (XPEEM), complemented by scanning Auger microscopy (SAM) and scanning capacitance microscopy, we have quantitatively studied the incorporation of silicon and band bending at the surface (m-facet) of an individual, highly conductive Si-doped GaN micro-wires (Tchoulfian et al., Applied Physics Letters 102 (12), 2013). Electrically active n-dopants Si atoms in Ga interstitial sites are detected as nitride bonding states in the high-resolution Si2p core level spectra, and represent only a small fraction ( < 10%) of the overall Si surface concentration measured by SAM. The derived carrier concentration of 2 x 10(21) at cm(-3) is in reasonable agreement with electrical measurements. A consistent surface band bending of similar to 1 eV is directly evidenced by surface photovoltage measurements. Such an approach combining different surface-sensitive microscopies is of interest for studying other heavily doped semiconducting wires. (C) 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.ultramic.2015.05.007
  • Spin waves in ultrathin hexagonal cobalt films on W(110), Cu(111), and Au(111) surfaces
    Michel, E. and Ibach, H. and Schneider, C. M.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Spin wave spectra of ultrathin epitaxial cobalt films deposited on W(110), Cu(111), and Au(111) surfaces are studied in the wave-vector regime between 0.1Å-1 and 0.7Å-1 using inelastic electron scattering with 6 meV energy resolution. Up to three different spin wave modes are resolved for wave vectors q<inf>∥</inf> < 0.35Å-1. The modes are identified as the acoustic mode and standing modes with one and two nodes inside the film. The relative weight of the modes in a particular spectrum may depend critically on the electron impact energy. For larger wave vectors beyond q<inf>∥</inf> >0.35Å-1 and layers thicker than five atom layers the separate modes merge into a single, broad loss feature. Since the shape and position of the loss feature depend on the electron impact energy, a separation into different modes is nevertheless possible for not too large wave vectors. The spin wave dispersion curves of films grown on W(110) agree with those observed on Cu(111) if one takes into account that on copper the cobalt grows in islands so that the mean height of the islands is higher than the nominal coverage. On films grown on Au(111) the low wave vector spin waves are buried in the high elastic diffuse scattering caused by the considerable disorder in the films. The broader appearance of the spectra at higher wave vectors compared to films grown on W(110) and Cu(111) is quantitatively accounted for by disorder-induced kinematic broadening. Because of the granular growth on copper and gold primarily the spin wave spectrum of cobalt films on W(110) is amenable to quantitative theoretical analysis. Such an analysis is not available at present. We show however, that the dispersion curves are incompatible with the Heisenberg model as long as only a single, layer-independent exchange coupling constant is invoked. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.024407
  • Spin-torque-induced dynamics at fine-split frequencies in nano-oscillators with two stacked vortices
    Sluka, V. and Kákay, A. and Deac, A.M. and Bürgler, D.E. and Schneider, C.M. and Hertel, R.
    Nature Communications 6 (2015)
    The gyrotropic rotation around the equilibrium position constitutes the fundamental excitation of magnetic vortices in nanostructures. The frequency of this mode varies with material and sample geometry, but is independent of the vortex handedness and its core direction. Here, we demonstrate that this degeneracy is lifted in a spin-torque oscillator containing two vortices stacked on top of each other. When driven by spin-polarized currents, such devices exhibit a set of dynamic modes with discretely split frequencies, each corresponding to a specific combination of vorticities and relative core polarities. The fine splitting occurs even in the absence of external fields, demonstrating that such devices can function as zero-field, multi-channel, nano-oscillators for communication technologies. It also facilitates the detection of the relative core polarization and allows for the eight non-degenerate configurations to be distinguished electrically, which may enable the design of multi-state memory devices based on double-vortex nanopillars. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms7409
  • Subnanometre-wide electron channels protected by topology
    Pauly, C. and Rasche, B. and Koepernik, K. and Liebmann, M. and Pratzer, M. and Richter, M. and Kellner, J. and Eschbach, M. and Kaufmann, B. and Plucinski, L. and Schneider, C.M. and Ruck, M. and Van Den Brink, J. and Morgenstern, M.
    Nature Physics 11 (2015)
    Helical locking of spin and momentum and prohibited backscattering are the key properties of topologically protected states. They are expected to enable novel types of information processing by providing pure spin currents, or fault tolerant quantum computation by using the Majorana fermions at interfaces of topological states with superconductors. So far, the required helical conduction channels used to realize Majorana fermions are generated through the application of an axial magnetic field to conventional semiconductor nanowires. Avoiding the magnetic field enhances the possibilities for circuit design significantly. Here, we show that subnanometre-wide electron channels with natural helicity are present at surface step edges of the weak topological insulator Bi 14 Rh 3 I 9 (ref.). Scanning tunneling spectroscopy reveals the electron channels to be continuous in both energy and space within a large bandgap of 200 meV, evidencing its non-trivial topology. The absence of these channels in the closely related, but topologically trivial compound Bi 13 Pt 3 I 7 corroborates the channels'topological nature. The backscatter-free electron channels are a direct consequence of Bi 14 Rh 3 I 9 's structure: a stack of two-dimensional topologically insulating, graphene-like planes separated by trivial insulators. We demonstrate that the surface of Bi 14 Rh 3 I 9 can be engraved using an atomic force microscope, allowing networks of protected channels to be patterned with nanometre precision. © 2015 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nphys3264
  • The geometric and electronic structure of TCNQ and TCNQ+Mn on Ag(0 0 1) and Cu(0 0 1) surfaces
    Feyer, V. and Graus, M. and Nigge, P. and Zamborlini, G. and Acres, R.G. and Schöll, A. and Reinert, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 204 (2015)
    Copper and silver surfaces can be used as model systems to study structure formation and interfacial bonding upon adsorption of organic molecules. We have investigated the geometric and electronic structure of ordered monolayers of TCNQ on Cu(0 0 1) and Ag(0 0 1) and of TCNQ+Mn on Ag(0 0 1) surfaces by LEED and photoelectron momentum microscopy. While TCNQ forms an incommensurable superstructure on Cu(0 0 1), two coverage-dependant, commensurable superstructures are established on Ag(0 0 1). Subsequent adsorption of Mn on top of TCNQ/Ag(0 0 1) results in the formation of a long-range ordered mixed metal-organic superstructure, which is also commensurable with the Ag(0 0 1) substrate. The photoelectron spectroscopy (PES) data shows a filling of the TCNQ LUMO by charge transfer from the substrate for all investigated interfaces and the coadsorption of Mn leads to an energy shift of the TCNQ HOMO and LUMO of 230 meV with respect to TCNQ/Ag(0 0 1). The characteristic angle-dependent intensity pattern of the TCNQ LUMO in PES was utilized to investigate the azimuthal orientation of the molecules in the respective unit cells. The angle-resolved PES data was further analyzed to identify lateral band dispersion effects in the adsorbate layers, but no significant dispersion was observed. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2015.02.010
  • Tuning the Dirac point to the Fermi level in the ternary topological insulator (Bi1-xSbx)2Te3
    Kellner, J. and Eschbach, M. and Kampmeier, J. and Lanius, M. and Młyńczak, E. and Mussler, G. and Holländer, B. and Plucinski, L. and Liebmann, M. and Grützmacher, D. and Schneider, C.M. and Morgenstern, M.
    Applied Physics Letters 107 (2015)
    In order to stabilize Majorana excitations within vortices of proximity induced topological superconductors, it is mandatory that the Dirac point matches the Fermi level rather exactly, such that the conventionally confined states within the vortex are well separated from the Majorana-type excitation. Here, we show by angle resolved photoelectron spectroscopy that (Bi1-xSbx)2Te3 thin films with x = 0.94 prepared by molecular beam epitaxy and transferred in ultrahigh vacuum from the molecular beam epitaxy system to the photoemission setup match this condition. The Dirac point is within 10 meV around the Fermi level, and we do not observe any bulk bands intersecting the Fermi level. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4938394
  • Tuning the orbital ordering in La0.7Sr0.3MnO3 thin films in all-oxide hybrids
    Doʇanay, H. and Krug, I.P. and Schubert, J. and Cramm, S. and Gottlob, D.M. and Nickel, F. and Schneider, C. M.
    EPL 109 (2015)
    This work presents a systematic characterization including x-ray diffractometry, SQUID magnetometry and x-ray absorption spectroscopy on all-oxide ferromagnetic/ferroelectric heterosystem BaTiO<inf>3</inf>/La<inf>0.7</inf>Sr<inf>0.3</inf>MnO<inf>3</inf>/SrTiO<inf>3</inf>(001) fabricated by pulsed-laser deposition. The key aspect of this study is to provide accurate information about differences in electronic and structural properties in LSMO thin films as a function of the oxygen pressure during BTO growth. X-ray absorption spectroscopy experiments at the Mn L<inf>3,2</inf> edge have revealed the conservation of Mn3+/Mn4+ mixed valency configuration of the LSMO films near the interface while tuning the oxygen pressure of overlayer BTO growth. The existing Mn3+ ions favor an in-plane e<inf>g</inf> orbital ordering, reducing the in-plane strain and promoting room temperature ferromagntism in the LSMO film. Furthermore, diffraction experiments showed that the out-of plane lattice parameter of BTO reduces with increasing oxygen pressure consistent with the x-ray linear dichrosim at Ti edge showing less tetragonal symmetry, although the chemical environment of the Ti ions was not changed notably. We demonstrated a way to control magnetic properties and orbital ordering in LSMO thin films while optimizing the ferroelectric properties of BTO overlayer films, which are promising results in terms of magnetoelectric applications using functional heterosystems. Copyright © EPLA, 2015.
    view abstract10.1209/0295-5075/109/67007
  • Ultrafast, Element-specific magnetization dynamics of multi-constituent magnetic materials by use of high-harmonic generation
    Silva, T.J. and Turgut, E. and Mathias, S. and La-O-Vorakiat, C. and Grychtol, P. and Adam, R. and Rudolf, D. and Nembach, H.T. and Aeschlimann, M. and Schneider, C.M. and Kapteyn, H.C. and Murnane, M.M. and Shaw, J.M.
    Springer Proceedings in Physics 159 (2015)
    We have studied femtosecond magnetization dynamics probed by extreme ultraviolet pulses from high-harmonic generation, with element-selectivity and ultrafast time resolution. By use of this technique, we identify the microscopic processes that drive magnetization dynamics on femtosecond timescales. Here, we concentrate on controlling superdiffusive spin-currents in magnetic multilayers. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_93
  • "conductive" yttria-stabilized zirconia as an epitaxial template for oxide heterostructures
    Caspers, C. and Gloskovskii, A. and Drube, W. and Schneider, C.M. and Müller, M.
    Journal of Applied Physics 115 (2014)
    We report an in situ thermochemical treatment that significantly increases the macroscopic electrical conductivity of insulating yttria-stabilized zirconia (YSZ) (001) single-crystalline substrates. We demonstrate the high-quality surface crystalline structure of the resulting "conductive" cYSZ (001) by low- and high-energy electron diffraction. Soft- and hard X-ray photoemission spectroscopy measurements reveal a sizable reduction of Zr cations to a metallic state and their homogeneous distribution within the cYSZ. We discuss the correlation between the microscopic chemical processes leading to the increased macroscopic metallicity. Finally, the heteroepitaxial growth of a functional magnetic oxide model system, ultrathin EuO on cYSZ (001), was demonstrated. cYSZ (001) thereby enables both high quality oxide heteroepitaxy and the advanced sample characterization by high electron-fluence characterization techniques. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4863803
  • Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study
    Feyer, V. and Graus, M. and Nigge, P. and Wießner, M. and Acres, R.G. and Wiemann, C. and Schneider, C.M. and Schöll, A. and Reinert, F.
    Surface Science 621 (2014)
    We present a comprehensive study of the adsorption behavior of iron phthalocyanine on the low-index crystal faces of silver. By combining measurements of the reciprocal space by means of photoelectron momentum mapping and low energy electron diffraction, the real space adsorption geometries are reconstructed. At monolayer coverage ordered superstructures exist on all studied surfaces containing one molecule in the unit cell in case of Ag(100) and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt angle of the molecules against the high symmetry directions of the substrate is derived from the photoelectron momentum maps. A comparative analysis of the momentum patterns on the substrates with different symmetry indicates that both constituents of the twofold degenerate FePc lowest unoccupied molecular orbital are occupied by charge transfer from the substrate at the interface. © 2013 Elsevier B.V.
    view abstract10.1016/j.susc.2013.10.020
  • Band alignment at memristive metal-oxide interfaces investigated by hard x-ray photoemission spectroscopy
    Lenser, C. and Köhl, A. and Patt, M. and Schneider, C. M. and Waser, R. and Dittmann, R.
    Physical Review B - Condensed Matter and Materials Physics 90 (2014)
    The electronic structure and band alignment at metal/oxide interfaces for nonvolatile memory applications are investigated by hard x-ray photoelectron spectroscopy (HAXPES) and DC transport measurements, using acceptor doped SrTiO3 as a model memristive oxide. Metal-insulator-metal (MIM) structures with a noble metal (Pt) top electrode form a Schottky barrier and exhibit rectifying properties, while a reactive metal (Ti) as top electrode shows symmetric I(V) characteristics and a flat band situation at the interface. The transition from rectifying to ohmic I(V) relations with increasing Ti thickness is discussed with respect to the electrochemical reaction at the interface, the band alignment at the electrode/oxide interface, and the slope of the energy bands across the MIM structure. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.90.115312
  • Bulk mixed ion electron conduction in amorphous gallium oxide causes memristive behaviour
    Aoki, Y. and Wiemann, C. and Feyer, V. and Kim, H.-S. and Schneider, C.M. and Ill-Yoo, H. and Martin, M.
    Nature Communications 5 (2014)
    In thin films of mixed ionic electronic conductors sandwiched by two ion-blocking electrodes, the homogeneous migration of ions and their polarization will modify the electronic carrier distribution across the conductor, thereby enabling homogeneous resistive switching. Here we report non-filamentary memristive switching based on the bulk oxide ion conductivity of amorphous GaOx (x~1.1) thin films. We directly observe reversible enrichment and depletion of oxygen ions at the blocking electrodes responding to the bias polarity by using photoemission and transmission electron microscopies, thus proving that oxygen ion mobility at room temperature causes memristive behaviour. The shape of the hysteresis I-V curves is tunable by the bias history, ranging from narrow counter figure-eight loops to wide hysteresis, triangle loops as found in the mathematically derived memristor model. This dynamical behaviour can be attributed to the coupled ion drift and diffusion motion and the oxygen concentration profile acting as a state function of the memristor. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms4473
  • Bulk sensitive hard x-ray photoemission electron microscopy
    Patt, M. and Wiemann, C. and Weber, N. and Escher, M. and Gloskovskii, A. and Drube, W. and Merkel, M. and Schneider, C.M.
    Review of Scientific Instruments 85 (2014)
    Hard x-ray photoelectron spectroscopy (HAXPES) has now matured into a well-established technique as a bulk sensitive probe of the electronic structure due to the larger escape depth of the highly energetic electrons. In order to enable HAXPES studies with high lateral resolution, we have set up a dedicated energy-filtered hard x-ray photoemission electron microscope (HAXPEEM) working with electron kinetic energies up to 10 keV. It is based on the NanoESCA design and also preserves the performance of the instrument in the low and medium energy range. In this way, spectromicroscopy can be performed from threshold to hard x-ray photoemission. The high potential of the HAXPEEM approach for the investigation of buried layers and structures has been shown already on a layered and structured SrTiO3 sample. Here, we present results of experiments with test structures to elaborate the imaging and spectroscopic performance of the instrument and show the capabilities of the method to image bulk properties. Additionally, we introduce a method to determine the effective attenuation length of photoelectrons in a direct photoemission experiment. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4902141
  • Detection of the magnetocrystalline anisotropy in x-ray magnetic linear dichroism reflection spectra across the Fe 3p and 2p edges
    Mertins, H.-C. and Legut, D. and Tesch, M. and Jansing, C. and Gilbert, M. and Gaupp, A. and Oppeneer, P.M. and Bürgler, D.E. and Schneider, C. M. and Berges, U.
    IEEE Transactions on Magnetics 50 (2014)
    The magnetocrystalline anisotropy of X-ray magnetic linear dichroism (XMLD) reflection spectra measured on single-crystalline bcc Fe films across the 3p and 2p edges are presented. The XMLD spectra were obtained from a series of reflection spectra by aligning the electric field vector of linearly polarized undulator radiation with respect to the crystal axes. Our results show the presence of a huge magnetocrystalline anisotropy in the XMLD reflection spectra. The XMLD signal is further investigated as a function of the Fe film thickness in Au/Fe/Ag/GaAs layered systems. Simulations of the reflection spectra reveal the influences of interference effects, which can enhance or diminish the XMLD signals. The measured spectra are in good agreement with ab initio calculated spectra. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2014.2321632
  • Epitaxial Cu(001) films grown on a Cr/Ag/Fe/GaAs(001) buffer system
    Gottlob, D.M. and Jansen, T. and Hoppe, M. and Bürgler, D.E. and Schneider, C.M.
    Thin Solid Films 562 (2014)
    We present a procedure to prepare single-crystalline, high-purity Cu(001) films (templates) suitable as substrates for subsequent epitaxial thin-film growth. The template films were grown in a dedicated molecular-beam epitaxy system on a Cr/Ag/Fe/GaAs(001) buffer layer system. Low-energy electron diffraction and X-ray diffraction were applied to determine the surface orientation and the epitaxial relationship between all layers of the stack. Post-annealing at moderate temperatures enhances the quality of the film as shown by low-energy electron diffraction and atomic force microscopy. X-ray photoemission and Auger electron spectroscopy confirm that no atoms of the buffer layers diffuse into the Cu film during the initial preparation and the post-annealing treatment. The completed Cu(001) template system can be exposed to air and afterwards refurbished by Ar+-ion bombardment and annealing, enabling the transfer between vacuum systems. The procedure provides suitable conductive thin film templates for studies of epitaxial thin films, e.g. on the magnetic and magnetotransport properties of Co and Ni based films and multilayers. © 2014 Elsevier B.V.
    view abstract10.1016/j.tsf.2014.04.078
  • Erratum: Generation of circularly polarized radiation from a compact plasma-based extreme ultraviolet light source for tabletop X-ray magnetic circular dichroism studies (Rev. Sci. Instrum. (2014) 85 ((103110))?
    Wilson, D. and Rudolf, D. and Weier, C. and Adam, R. and Winkler, G. and Frömter, R. and Danylyuk, S. and Bergmann, K. and Grützmacher, D. and Schneider, C.M. and Juschkin, L.
    Review of Scientific Instruments 85 (2014)
    view abstract10.1063/1.4902976
  • Exploring interlayer Dirac cone coupling in commensurately rotated few-layer graphene on SiC(000-1)
    Mathieu, C. and Conrad, E.H. and Wang, F. and Rault, J.E. and Feyer, V. and Schneider, C.M. and Renault, O. and Barrett, N.
    Surface and Interface Analysis 46 (2014)
    We investigate electronic band-structure images in reciprocal space of few-layer graphene epitaxially grown on SiC(000-1). In addition to the observation of commensurate rotation angles of the graphene layers, the k-space images recorded near the Fermi edge highlight structures originating from diffraction of the Dirac cones due to the relative rotation of adjacent layers. The 21.9° and 27° rotation angles between two sheets of graphene are responsible for a periodic pattern that can be described with a superlattice unit cells. The superlattice generates replicas of Dirac cones with smaller wave vectors, because of a Brillouin zone folding. © 2014 John Wiley & Sons, Ltd.
    view abstract10.1002/sia.5541
  • Generation of circularly polarized radiation from a compact plasma-based extreme ultraviolet light source for tabletop X-ray magnetic circular dichroism studies
    Wilson, D. and Rudolf, D. and Weier, C. and Adam, R. and Winkler, G. and Frömter, R. and Danylyuk, S. and Bergmann, K. and Grützmacher, D. and Schneider, C.M. and Juschkin, L.
    Review of Scientific Instruments 85 (2014)
    Generation of circularly polarized light in the extreme ultraviolet (EUV) spectral region (about 25 eV 250 eV) is highly desirable for applications in spectroscopy and microscopy but very challenging to achieve in a small-scale laboratory. We present a compact apparatus for generation of linearly and circularly polarized EUV radiation from a gas-discharge plasma light source between 50 eV and 70 eV photon energy. In this spectral range, the 3p absorption edges of Fe (54 eV), Co (60 eV), and Ni (67 eV) offer a high magnetic contrast often employed for magneto-optical and electron spectroscopy as well as for magnetic imaging. We simulated and designed an instrument for generation of linearly and circularly polarized EUV radiation and performed polarimetric measurements of the degree of linear and circular polarization. Furthermore, we demonstrate first measurements of the X-ray magnetic circular dichroism at the Co 3p absorption edge with a plasma-based EUV light source. Our approach opens the door for laboratory-based, element-selective spectroscopy of magnetic materials and spectro-microscopy of ferromagnetic domains. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4897491
  • Imaging and characterization of conducting ferroelectric domain walls by photoemission electron microscopy
    Schaab, J. and Krug, I.P. and Nickel, F. and Gottlob, D.M. and Doǧ Anay, H. and Cano, A. and Hentschel, M. and Yan, Z. and Bourret, E. and Schneider, C.M. and Ramesh, R. and Meier, D.
    Applied Physics Letters 104 (2014)
    High-resolution X-ray photoemission electron microscopy (X-PEEM) is a well-established method for imaging ferroelectric domain structures. Here, we expand the scope of application of X-PEEM and demonstrate its capability for imaging and investigating domain walls in ferroelectrics with high spatial resolution. Using ErMnO3 as test system, we show that ferroelectric domain walls can be visualized based on photo-induced charging effects and local variations in their electronic conductance can be mapped by analyzing the energy distribution of photoelectrons. Our results open the door for non-destructive, contact-free, and element-specific studies of the electronic and chemical structure at domain walls in ferroelectrics. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4879260
  • Influence of the crystal structure of thin Co films on X-ray magnetic linear dichroism - Comparison of ab initio theory and reflectometry experiments
    Legut, D. and Tesch, M.F. and Mertins, H.-C. and Gilbert, M.C. and Jansing, C. and Bürgler, D.E. and Schneider, C.M. and Gaupp, A. and Hamrle, J. and Oppeneer, P.M.
    Journal of Applied Physics 115 (2014)
    We report an investigation of the influence of the crystal structure of Co thin films on the X-ray magnetic linear dichroism (XMLD) spectrum. We compare XMLD spectra measured in reflection at the 3p-edges for two distinct orientations of the magnetization in the crystalline Co film with ab initio calculated spectra. The latter was computed for the face-centered cubic as well as the hexagonal-close packed crystal structures of Co. We find that the XMLD signal is strongly dependent on the magnetization direction with respect to the crystal axes as well as strongly influenced by the crystal structure. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4869292
  • Insights into nanoscale electrochemical reduction in a memristive oxide: The role of three-phase boundaries
    Lenser, C. and Patt, M. and Menzel, S. and Köhl, A. and Wiemann, C. and Schneider, C.M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 24 (2014)
    The nanoscale electro-reduction in a memristive oxide is a highly relevant field for future non-volatile memory materials. Photoemission electron microscopy is used to identify the conducting filaments and correlate them to structural features of the top electrode that indicate a critical role of the three phase boundary (electrode-oxide-ambient) for the electro-chemical reduction. Based on simulated temperature profiles, the essential role of Joule heating through localized currents for electro-reduction and morphology changes is demonstrated. The three-phase boundary between electrode, oxide and ambient is shown to play a crucial role for the electroreduction in resistively switchable devices fabricated from Fe-doped SrTiO3. Nanoscale chemical mapping by X-ray photoemission electron microscopy, combined with simulated temperature profiles of the filaments, provide evidence that localized Joule heating at the electrode edge is essential for the formation of conducting filaments. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201304233
  • Intensities of surface spin wave excitations in inelastic electron scattering
    Rajeswari, J. and Michel, E. and Ibach, H. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The intensity of surface spin wave excitations in inelastic electron scattering is measured as function of electron energy for fcc and hexagonal close packed (hcp) cobalt layers. Intensities are converted into scattering probabilities with the help of a recently established calibration of our spectrometer. The scattering probability as function of energy exhibits a peak around 7 and 3 eV for fcc and hcp cobalt, respectively, and decays to immeasurably small values at energies above 10 to 15 eV in stark contrast to theoretical predictions. By comparison to phonon scattering the peaks in the scattering probabilities at low energies are tentatively attributed to image potential induced resonances. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.075438
  • Magnetocrystalline anisotropy in x-ray magnetic linear dichroism at the 3 p edges of crystalline Fe thin films
    Tesch, M.F. and Legut, D. and Mertins, H.-Ch. and Gilbert, M.C. and Jansing, C. and Hamrle, J. and Rusz, J. and Oppeneer, P.M. and Bürgler, D.E. and Schneider, C.M. and Gaupp, A. and Berges, U.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    X-ray magnetic linear dichroism spectra measured in reflection (XMLD-R) on crystalline bcc Fe thin films across the 3p absorption edges are reported. A series of measurements with varying orientation of the electric field vector of the linear polarized synchrotron radiation with respect to the crystal axes reveals a strong magnetocrystalline anisotropy in the XMLD-R spectra. The spectra agree well with theoretical spectra calculated within the framework of the density-functional theory accounting for the spin-orbital and exchange splitting of the 3p semicore states on an equal footing. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.140404
  • Photoemission electron microscopy and scanning electron microscopy of magnetospirillum magnetotacticum 's magnetosome chains
    Keutner, C. and Von Bohlen, A. and Berges, U. and Espeter, P. and Schneider, C.M. and Westphal, C.
    Analytical Chemistry 86 (2014)
    Magnetotactic bacteria are of great interdisciplinary interest, since a vast field of applications from magnetic recording media to medical nanorobots is conceivable. A key feature for a further understanding is the detailed knowledge about the magnetosome chain within the bacteria. We report on two preparation procedures suitable for UHV experiments in reflective geometry. Further, we present the results of scanning electron microscopy, as well as the first photoemission electron microscopy experiments, both accessing the magnetosomes within intact magnetotactic bacteria and compare these to scanning electron microscopy data from the literature. From the images, we can clearly identify individual magnetosomes within their chains. © 2014 American Chemical Society.
    view abstract10.1021/ac502050j
  • Quantitative characterization of nanoscale polycrystalline magnets with electron magnetic circular dichroism
    Muto, S. and Rusz, J. and Tatsumi, K. and Adam, R. and Arai, S. and Kocevski, V. and Oppeneer, P.M. and Bürgler, D.E. and Schneider, C.M.
    Nature Communications 5 (2014)
    Electron magnetic circular dichroism (EMCD) allows the quantitative, element-selective determination of spin and orbital magnetic moments, similar to its well-established X-ray counterpart, X-ray magnetic circular dichroism (XMCD). As an advantage over XMCD, EMCD measurements are made using transmission electron microscopes, which are routinely operated at sub-nanometre resolution, thereby potentially allowing nanometre magnetic characterization. However, because of the low intensity of the EMCD signal, it has not yet been possible to obtain quantitative information from EMCD signals at the nanoscale. Here we demonstrate a new approach to EMCD measurements that considerably enhances the outreach of the technique. The statistical analysis introduced here yields robust quantitative EMCD signals. Moreover, we demonstrate that quantitative magnetic information can be routinely obtained using electron beams of only a few nanometres in diameter without imposing any restriction regarding the crystalline order of the specimen. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms4138
  • Reply to "comment on 'ultrafast demagnetization measurements using extreme ultraviolet light: Comparison of electronic and magnetic contributions' "
    Turgut, E. and Grychtol, P. and La-O-Vorakiat, C. and Adams, D.E. and Kapteyn, H.C. and Murnane, M.M. and Mathias, S. and Aeschlimann, M. and Schneider, C. M. and Shaw, J.M. and Nembach, H.T. and Silva, T.J.
    Physical Review X 3 (2014)
    In the following, we show that the conclusions of our article titled "Ultrafast Demagnetization Measurements Using Extreme Ultraviolet Light: Comparison of Electronic and Magnetic Contributions"are correct. The Comment of Vodungbo et al. argues that a unique determination of the refractive index variation over time is not possible using the data set presented in our paper. Furthermore, it was suggested that the lack of uniqueness allows for the possibility of a very specific time-dependent trajectory of the refractive index in the complex plane that could give rise to a large nonmagnetic modulation of the measured asymmetry, in spite of a negligible change in the s-polarized reflectivity. In this Reply, we conclusively show that any nonmagnetic contribution to the measured asymmetry is indeed negligible (< 2%), below the noise level of the magnetic-asymmetry measurements. First, we use a few additional measurements to unambiguously rule out the presence of any nonmagnetic contributions to the signal. Second, we show that the scenario proposed by Vodungbo et al. would require both exotic time and energy dependences of the refractive index near the M edge that are extremely unlikely (virtually impossible) in real materials. Thus, the conclusions of our original article are preserved.
    view abstract10.1103/PhysRevX.3.038002
  • Room-temperature high-frequency transport of Dirac fermions in epitaxially grown Sb2 Te3 - And Bi2 Te3 -based topological insulators
    Olbrich, P. and Golub, L.E. and Herrmann, T. and Danilov, S.N. and Plank, H. and Bel'Kov, V.V. and Mussler, G. and Weyrich, Ch. and Schneider, C. M. and Kampmeier, J. and Grützmacher, D. and Plucinski, L. and Eschbach, M. and Ganichev, S.D.
    Physical Review Letters 113 (2014)
    We report on the observation of photogalvanic effects in epitaxially grown Sb2Te3 and Bi2Te3 three-dimensional (3D) topological insulators (TI). We show that asymmetric scattering of Dirac fermions driven back and forth by the terahertz electric field results in a dc electric current. Because of the "symmetry filtration" the dc current is generated by the surface electrons only and provides an optoelectronic access to probe the electron transport in TI, surface domains orientation, and details of electron scattering in 3D TI even at room temperature. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.113.096601
  • Standing Spin waves in ultrathin magnetic films: A method to test for layer-dependent exchange coupling
    Rajeswari, J. and Ibach, H. and Schneider, C. M.
    Physical Review Letters 112 (2014)
    We introduce a method to test theoretical models for the layer dependence of exchange coupling constants in ultrathin magnetic films. The method is based on the observation of high-energy and high-momentum standing spin wave modes using high-resolution electron energy loss spectroscopy. Experimental data are presented for 5-8 layers of fcc cobalt deposited on Cu(100). The power of the method is illustrated by comparison to two theoretical studies predicting rather different results concerning the ratio of the interlayer and intralayer exchange coupling constants near the surface. Only the theory with a large interlayer coupling shows sufficient energy spreading in the layer dependence of the dispersion curves to match the experimental data. We furthermore discuss the reason for the surprising success of the simple nearest-neighbor Heisenberg model with a single exchange constant matched to experiment. © 2014 American Physical Society.
    view abstract10.1103/PhysRevLett.112.127202
  • Wide-range structural and chemical stability of the magnetic oxide NiFe2O4 Grown by O2-assisted pulsed laser deposition
    Hoppe, M. and Gorgoi, M. and Schneider, C. M. and Muller, M.
    IEEE Transactions on Magnetics 50 (2014)
    We present a study of the structural and chemical properties of the magnetic insulator NiFe2O4 on conductive Nb-doped SrTiO3 (001) substrates. Special regard is given to the dependence of the thin film properties on the O2:Ar ratio during pulsed laser deposition. Using stoichiometric NiFe2O4 target material and varying the O2 partial pressure from 0% to 100%, we find a nonzero oxygen threshold for heteroepitaxial growth and a stoichiometric Fe:Ni cation distribution. Moreover, our study clearly demonstrates that NiFe2O4 thin films grow with high quality over a wide range of oxygen partial pressures. These optimized NiFe2O4/SrTiO3 heterostructures are envisioned as efficient spin filter tunnel contacts for room temperature application. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2014.2322378
  • Accessing 4f-states in single-molecule spintronics
    Fahrendorf, S. and Atodiresei, N. and Besson, C. and Caciuc, V. and Matthes, F. and Blügel, S. and Kögerler, P. and Bürgler, D.E. and Schneider, C.M.
    Nature Communications 4 (2013)
    Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states. Here we show how a judicious choice of the molecular spin centres determines these critical molecule-electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations. Our results showcase how tailoring molecular orbitals can yield all-electrically controlled spintronic device concepts. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms3425
  • Band offsets in complex-oxide thin films and heterostructures of SrTiO 3/LaNiO3 and SrTiO3/GdTiO3 by soft and hard X-ray photoelectron spectroscopy
    Conti, G. and Kaiser, A.M. and Gray, A.X. and Nemšák, S. and Pálsson, G.K. and Son, J. and Moetakef, P. and Janotti, A. and Bjaalie, L. and Conlon, C.S. and Eiteneer, D. and Greer, A.A. and Keqi, A. and Rattanachata, A. and Saw, A.Y. and Bostwick, A. and Stolte, W.C. and Gloskovskii, A. and Drube, W. and Ueda, S. and Kobata, M. and Kobayashi, K. and Van De Walle, C.G. and Stemmer, S. and Schneider, C.M. and Fadley, C.S.
    Journal of Applied Physics 113 (2013)
    The experimental determination of valence band offsets (VBOs) at interfaces in complex-oxide heterostructures using conventional soft x-ray photoelectron spectroscopy (SXPS, hν 1500 eV) and reference core-level binding energies can present challenges because of surface charging when photoelectrons are emitted and insufficient probing depth to clearly resolve the interfaces. In this paper, we compare VBOs measured with SXPS and its multi-keV hard x-ray analogue (HXPS, hν &gt; 2000 eV). We demonstrate that the use of HXPS allows one to minimize charging effects and to probe more deeply buried interfaces in heterostructures such as SrTiO3/LaNiO3 and SrTiO3/GdTiO 3. The VBO values obtained by HXPS for these interfaces are furthermore found to be close to those determined by first-principles calculations. © 2013 American Institute of Physics.
    view abstract10.1063/1.4795612
  • Controlling the competition between optically induced ultrafast spin-flip scattering and spin transport in magnetic multilayers
    Turgut, E. and La-O-Vorakiat, C. and Shaw, J.M. and Grychtol, P. and Nembach, H.T. and Rudolf, D. and Adam, R. and Aeschlimann, M. and Schneider, C. M. and Silva, T.J. and Murnane, M.M. and Kapteyn, H.C. and Mathias, S.
    Physical Review Letters 110 (2013)
    The study of ultrafast dynamics in magnetic materials provides rich opportunities for greater fundamental understanding of correlated phenomena in solid-state matter, because many of the basic microscopic mechanisms involved are as-yet unclear and are still being uncovered. Recently, two different possible mechanisms have been proposed to explain ultrafast laser induced magnetization dynamics: spin currents and spin-flip scattering. In this work, we use multilayers of Fe and Ni with different metals and insulators as the spacer material to conclusively show that spin currents can have a significant contribution to optically induced magnetization dynamics, in addition to spin-flip scattering processes. Moreover, we can control the competition between these two processes, and in some cases completely suppress interlayer spin currents as a sample undergoes rapid demagnetization. Finally, by reversing the order of the Fe/Ni layers, we experimentally show that spin currents are directional in our samples, predominantly flowing from the top to the bottom layer. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.110.197201
  • Effect of contact geometry on magnetoresistance in CoPd-contacted carbon nanotubes
    Morgan, C. and Metten, D. and Schneider, C.M. and Meyer, C.
    Physica Status Solidi (B) Basic Research 250 (2013)
    We present local magnetoresistance measurements on carbon nanotubes contacted with the ferromagnetic alloy Co50 Pd50. A sample with contacts of dimensionsthickness=10nm, length=4μm exhibited the typical sharp switching expected at the coercive fields of the contacts. However, a sample with the same in-plane geometry but a thickness of 40nm displayed a complicated switching behavior indicative of gradual domain wall rotation. In both samples, the contact widths were varied to control shape anisotropy. Further characterization of the contacts and similar contact-like structures via remanent state magnetic force microscopy suggested a single or few domain structure in the 10nm thick contacts. The thicker contacts exhibited a complex domain structure characteristic of a hard or intermediate magnetic axis along the long axis of the contact, despite the effect of shape anisotropy. This result is discussed with respect to the influence of both thickness and in-plane geometry on competing anisotropy contributions within the device. The characterization allows for the pinpointing of precise dimensions that will determine the magnetic easy axis of contacts, thereby controlling magnetotransport. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201300068
  • Electronic structure, surface morphology, and topologically protected surface states of Sb2Te3 thin films grown on Si(111)
    Plucinski, L. and Herdt, A. and Fahrendorf, S. and Bihlmayer, G. and Mussler, G. and Döring, S. and Kampmeier, J. and Matthes, F. and Bürgler, D.E. and Grützmacher, D. and Blügel, S. and Schneider, C.M.
    Journal of Applied Physics 113 (2013)
    We have performed a combined spectroscopy and microscopy study on surfaces of Sb2Te3/Si(111) thin films exposed to air and annealed under ultra-high vacuum conditions. Scanning tunneling microscopy images, with atomic resolution present in most areas of such processed surfaces, show a significant amount of impurities and defects. Scanning tunneling spectroscopy reveals the bulk band gap of ∼ 170 meV centered ∼ 65 meV above the Fermi level. This intrinsic p-type doping behavior is confirmed by high-resolution angle-resolved photoemission spectra, which show the dispersions of the lower Dirac cone and the spectral weight of the bulk valence bands crossing the Fermi level. Spin-polarized photoemission revealed up to ∼15% in-plane spin polarization for photoelectrons related to the topologically protected Dirac cone states near the Fermi level, and up to ∼40% for several states at higher binding energies. The results are interpreted using ab initio electronic structure simulations and confirm the robustness of the time-reversal symmetry protected topological surface states in Sb2 Te3 in the presence of impurities and defects. © 2013 American Institute of Physics.
    view abstract10.1063/1.4789353
  • Erratum: Observation of boron diffusion in an annealed Ta/CoFeB/MgO magnetic tunnel junction with standing-wave hard x-ray photoemission (Applied Physics Letters (2012) 101 (202402))
    Greer, A.A. and Gray, A.X. and Kanai, S. and Kaiser, A.M. and Ueda, S. and Yamashita, Y. and Bordel, C. and Palsson, G. and Maejima, N. and Yang, S.-H. and Conti, G. and Kobayashi, K. and Ikeda, S. and Matsukura, F. and Ohno, H. and Schneider, C.M. and Kortright, J.B. and Hellman, F. and Fadley, C.S.
    Applied Physics Letters 102 (2013)
    view abstract10.1063/1.4773064
  • Exploring the XPS limit in soft and hard x-ray angle-resolved photoemission using a temperature-dependent one-step theory
    Braun, J. and Minár, J. and Mankovsky, S. and Strocov, V.N. and Brookes, N.B. and Plucinski, L. and Schneider, C.M. and Fadley, C.S. and Ebert, H.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    We present a theory of temperature-dependent photoemission which accurately describes phonon effects in soft and hard x-ray angle-resolved photoemission. Our approach is based on a fully relativistic one-step theory of photoemission that quantitatively reproduces the effects of phonon-assisted transitions beyond the usual k-conserving dipole selection rules which lead to the so-called XPS limit in the hard x-ray and/or high temperature regime. Vibrational atomic displacements have been included using the coherent potential approximation in analogy to the treatment of disordered alloys. The applicability of this alloy analogy model is demonstrated by direct comparison to experimental soft x-ray data from W(110) showing very satisfying agreement. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.205409
  • Full field electron spectromicroscopy applied to ferroelectric materials
    Barrett, N. and Rault, J.E. and Wang, J.L. and Mathieu, C. and Locatelli, A. and Mentes, T.O. and Niño, M.A. and Fusil, S. and Bibes, M. and Barthélémy, A. and Sando, D. and Ren, W. and Prosandeev, S. and Bellaiche, L. and Vilquin, B. and Petraru, A. and Krug, I.P. and Schneider, C.M.
    Journal of Applied Physics 113 (2013)
    The application of PhotoEmission Electron Microscopy (PEEM) and Low Energy Electron Microscopy (LEEM) techniques to the study of the electronic and chemical structures of ferroelectric materials is reviewed. Electron optics in both techniques gives spatial resolution of a few tens of nanometres. PEEM images photoelectrons, whereas LEEM images reflected and elastically backscattered electrons. Both PEEM and LEEM can be used in direct and reciprocal space imaging. Together, they provide access to surface charge, work function, topography, chemical mapping, surface crystallinity, and band structure. Examples of applications for the study of ferroelectric thin films and single crystals are presented. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4801968
  • Generation of Ultrashort and Coherent Synchrotron Radiation Pulses at DELTA
    Khan, S. and Bahnsen, F. and Cramm, S. and Döring, S. and Grewe, J. and Höner, M. and Huck, H. and Huck, M. and Molo, R. and Plucinski, L. and Schick, A. and Schneider, C.M. and Ungelenk, P.
    Synchrotron Radiation News 26 (2013)
    Pump-probe experiments to study ultrafast dynamic phenomena such as electron transfer, lattice vibrations, phase transitions, chemical reactions, or spin dynamics require two short radiation pulses as well as good control of the time delay between them. The first pulse to excite ("pump") the sample under study is usually a femtosecond laser pulse in the near-visible regime. For the second pulse to analyze ("probe") the state of the sample as a function of the delay, however, light with shorter and tunable wavelength would be desirable. Conventional synchrotron light sources produce pulses with a typical duration of 30-100 ps (FWHM), given by the electron bunch length in a storage ring, which is not well suited for ultrafast studies. The bunch length can be reduced to a few picoseconds in the so-called low-α mode (e.g., [1]) by lowering the momentum compaction factor α of the storage ring. Radiation pulses in the femtosecond range, however, are obtained more easily by extracting synchrotron light from a small fraction of the electron distribution, rather than the whole bunch, which can be achieved with the laser-based methods described below. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/08940886.2013.791213
  • Hard x-ray photoemission spectroscopy on the trilayer system MgO/Au/Fe using standing-wave excitation
    Döring, S. and Schönbohm, F. and Berges, U. and Bürgler, D.E. and Schneider, C.M. and Gorgoi, M. and Schäfers, F. and Westphal, C.
    Journal of Physics D: Applied Physics 46 (2013)
    The trilayer system MgO/Au monolayer/Fe was investigated by hard x-ray photoemission experiments in combination with the standing-wave technique. The insertion of the Au layer into the Fe/MgO tunnel junction provides an additional handle to influence the properties of the interface. The recently explored method of standing-wave excited hard x-ray photoemission was used to investigate both the structural properties and chemical states of the interfacial layers in one experiment. The results show that the Au monolayer does not grow as a closed layer, but intermixes strongly with the Fe below. This behaviour results in a very sharp interface between the Au/Fe and the MgO layer on top. However, the XPS spectra show no hint for a formation of FeO at the interface. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/46/37/375001
  • Heteroepitaxy and ferromagnetism of EuO/MgO (001): A route towards combined spin- and symmetry-filter tunneling
    Caspers, C. and Gloskovskij, A. and Drube, W. and Schneider, C.M. and Müller, M.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    We demonstrate the high-quality heteroepitaxy of ultrathin EuO films with bulklike ferromagnetism directly on MgO (001), an important step towards combining the spin-filter tunnel effect in magnetic insulators and symmetry-filter tunneling through single-crystalline MgO barriers. Despite a large compressive lattice mismatch, EuO grows fully relaxed on MgO (001) and adopts its bulk lattice parameter from the first monolayer on. This initial heteroepitaxial growth mode is discussed in terms of different electrostatic atomic configurations of the oxides interface. Single-crystalline EuO/MgO (001) thus can be envisioned as highly effective double spin-selective tunnel barriers for spintronics applications. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.245302
  • Large wave vector surface spin waves of the nanomartensitic phase in ultrathin iron films on Cu(100)
    Rajeswari, J. and Ibach, H. and Schneider, C. M.
    EPL 101 (2013)
    It is generally accepted that thin-film magnetism is strongly affected by even small structural modifications. Much less is known about the influence of structure on magnetic excitations, in particular, spin waves. Using electron energy loss spectroscopy we have studied the dispersion of large wave vector surface spin waves of a system for which details of the structure became known only recently, namely ultrathin iron films grown on Cu(100) surfaces. We find the spin wave dispersion to be nearly identical to the dispersion reported for bcc Fe(110) layers grown on W(110). We therefore conclude that the observed spin wave signal stems from the "nanomartensitic" phase of Fe/Cu(100) and that this phase is not merely a surface phase but encompasses the deeper layers. © Copyright EPLA, 2013.
    view abstract10.1209/0295-5075/101/17003
  • Magnetic spectroscopies
    Schneider, C.M. and Fadley, C.S.
    Journal of Electron Spectroscopy and Related Phenomena 189 (2013)
    view abstract10.1016/j.elspec.2013.05.002
  • Momentum-resolved electronic structure at a buried interface from soft X-ray standing-wave angle-resolved photoemission
    Gray, A.X. and Minár, J. and Plucinski, L. and Huijben, M. and Bostwick, A. and Rotenberg, E. and Yang, S.-H. and Braun, J. and Winkelmann, A. and Conti, G. and Eiteneer, D. and Rattanachata, A. and Greer, A.A. and Ciston, J. and Ophus, C. and Rijnders, G. and Blank, D.H.A. and Doennig, D. and Pentcheva, R. and Kortright, J.B. and Schneider, C. M. and Ebert, H. and Fadley, C.S.
    EPL 104 (2013)
    Angle-resolved photoemission spectroscopy (ARPES) is a powerful technique for the study of electronic structure, but it lacks a direct ability to study buried interfaces between two materials. We address this limitation by combining ARPES with soft X-ray standing-wave (SW) excitation (SWARPES), in which the SW profile is scanned through the depth of the sample. We have studied the buried interface in a prototypical magnetic tunnel junction La0.7Sr 0.3MnO3/SrTiO3. Depth-and momentum-resolved maps of Mn 3d eg and t2g states from the central, bulk-like and interface-like regions of La0.7Sr0.3MnO 3 exhibit distinctly different behavior consistent with a change in the Mn bonding at the interface. We compare the experimental results to state-of-the-art density-functional and one-step photoemission theory, with encouraging agreement that suggests wide future applications of this technique. © Copyright EPLA, 2013.
    view abstract10.1209/0295-5075/104/17004
  • Observation of large wave vector interface spin waves: Ni(100)/fcc Co(100) and Cu(100)/Co(100)
    Rajeswari, J. and Ibach, H. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    Using electron energy loss spectroscopy, we have probed the spin waves of fcc(100) cobalt after capping with 1-3 atom layers of pseudomorphic nickel or up to 12 atom layers of copper. The intensity decay of the spin wave signal is quantitatively described by the mean-free path of the incident and the scattered electron within the capping layer. The observed spin waves are therefore localized at the cobalt side of the Co/Ni and Co/Cu interfaces. Compared to the free cobalt surface, the interface spin waves are downshifted in frequency. The effect is attributed to a reduced exchange interaction between cobalt atoms at the interface. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.235415
  • Parallel carbon nanotube quantum dots and their interactions
    Goß, K. and Leijnse, M. and Smerat, S. and Wegewijs, M.R. and Schneider, C.M. and Meyer, C.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    We present quantum transport measurements of interacting parallel quantum dots formed in the strands of a carbon nanotube rope. In this molecular quantum dot system, transport is dominated by one quantum dot, while additional resonances from parallel side dots appear, which exhibit a weak gate coupling. This differential gating effect provides a tunability of the quantum dot system with only one gate electrode and provides control over the carbon nanotube strand that carries the current. By tuning the system to different states, we use quantum transport as a spectroscopic tool to investigate the interdot coupling and show a route to distinguish among various side dots. By comparing the experimental data with master-equation calculations, we identify conditions for the tunneling rates that are required in order to observe different manifestations of the interdot coupling in the transport spectra. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.035424
  • Polarization sensitive surface band structure of doped BaTiO 3(001)
    Rault, J.E. and Dionot, J. and Mathieu, C. and Feyer, V. and Schneider, C. M. and Geneste, G. and Barrett, N.
    Physical Review Letters 111 (2013)
    We present a spatial and wave-vector resolved study of the electronic structure of micron sized ferroelectric domains at the surface of a BaTiO 3(001) single crystal. The n-type doping of the BaTiO3 is controlled by in situ vacuum and oxygen annealing, providing experimental evidence of a surface paraelectric-ferroelectric transition below a critical doping level. Real space imaging of photoemission threshold, core level and valence band spectra show contrast due to domain polarization. Reciprocal space imaging of the electronic structure using linearly polarized light provides unambiguous evidence for the presence of both in- and out-of-plane polarization with two- and fourfold symmetry, respectively. The results agree well with first principles calculations. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.127602
  • Polyoxometalate-stabilized, water dispersible Fe2Pt magnetic nanoparticles
    Seemann, K.M. and Bauer, A. and Kindervater, J. and Meyer, M. and Besson, C. and Luysberg, M. and Durkin, P. and Pyckhout-Hintzen, W. and Budisa, N. and Georgii, R. and Schneider, C.M. and Kögerler, P.
    Nanoscale 5 (2013)
    Magnetic Fe2Pt core-shell nanoparticles with 2 nm cores were synthesized with a monolayer coating of silicotungstate Keggin clusters. The core-shell composition is substantiated by structural analysis performed using high-resolution scanning transmission electron microscopy (HR-STEM) and small angle X-ray scattering (SAXS) in a liquid suspension. The molecular metal oxide cluster shell introduces an enhanced dispersibility of the magnetic Fe-Pt core-shell nanoparticles in aqueous media and thereby opens up new routes to nanoparticle bio-functionalization, for example, using pre-functionalized polyoxometalates. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3nr33374d
  • Spin-polarization limit in Bi2Te3 Dirac cone studied by angle- and spin-resolved photoemission experiments and ab initio calculations
    Herdt, A. and Plucinski, L. and Bihlmayer, G. and Mussler, G. and Döring, S. and Krumrain, J. and Grützmacher, D. and Blügel, S. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The magnitude of electron spin polarization in topologically protected surface states is an important parameter with respect to spintronics applications. In order to analyze the warped spin texture in Bi 2Te3 thin films, we combine angle- and spin-resolved photoemission experiments with theoretical ab initio calculations. We find an in-plane spin polarization of up to ∼45% in the topologically protected Dirac cone states near the Fermi level. The Fermi surface of the Dirac cone state is warped and shows an out-of-plane spin polarization of ∼15%. These findings are in quantitative agreement with dedicated simulations which find electron density of the Dirac cone delocalized over the first quintuple layer with spin reversal occurring in the surface atomic layer. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.035127
  • Structure and magnetization in CoPd thin films and nanocontacts
    Morgan, C. and Schmalbuch, K. and García-Sánchez, F. and Schneider, C.M. and Meyer, C.
    Journal of Magnetism and Magnetic Materials 325 (2013)
    We present results showing the structural and magnetic properties of MBE-grown extended films and nanostructured elements of various CoPd alloys. X-ray diffraction studies show that the thin films are polycrystalline, yet exhibit a strong preferential growth orientation along the (111) direction. Magnetic force microscopy and SQUID are used to gain an understanding of the magnetic behavior of the CoPd system with respect to competing anisotropy contributions, based on temperature-dependent SQUID data, collected between 4 and 300 K. The idea and potential implications of using CoPd as a contact material to achieve spin injection in carbon nanotube-based devices is discussed. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmmm.2012.07.052
  • The electronic structure of spintronic materials as seen by spin-polarized angle-resolved photoemission
    Plucinski, L. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 189 (2013)
    The key quantity in spintronic devices is the spin polarization of the current flowing through the various device components, which in turn is closely determined by the components' electronic structure. Modern spin- and angle-resolved photoemission spectroscopy (spin-ARPES) can map the details of the spin-polarized electronic structure in many novel material systems - both magnetic and nonmagnetic. In order to separate close-lying electronic states, however, an improvement in energy and angular resolution as well as information depth is still mandatory. We review several types of modern photoemission spectrometers capable of spin analysis and discuss the application of the technique for several physical systems including ferromagnetic thin films and topological insulators. © 2013 Elsevier B.V.
    view abstract10.1016/j.elspec.2013.05.001
  • Time-resolved magnetic imaging in an aberration-corrected, energy-filtered photoemission electron microscope
    Nickel, F. and Gottlob, D.M. and Krug, I.P. and Doganay, H. and Cramm, S. and Kaiser, A.M. and Lin, G. and Makarov, D. and Schmidt, O.G. and Schneider, C.M.
    Ultramicroscopy 130 (2013)
    We report on the implementation and usage of a synchrotron-based time-resolving operation mode in an aberration-corrected, energy-filtered photoemission electron microscope. The setup consists of a new type of sample holder, which enables fast magnetization reversal of the sample by sub-ns pulses of up to 10. mT. Within the sample holder current pulses are generated by a fast avalanche photo diode and transformed into magnetic fields by means of a microstrip line. For more efficient use of the synchrotron time structure, we developed an electrostatic deflection gating mechanism capable of beam blanking within a few nanoseconds. This allows us to operate the setup in the hybrid bunch mode of the storage ring facility, selecting one or several bright singular light pulses which are temporally well-separated from the normal high-intensity multibunch pulse pattern. © 2013 Elsevier B.V.
    view abstract10.1016/j.ultramic.2013.03.005
  • Ultrafast element-specific magnetization dynamics of complex magnetic materials on a table-top
    Mathias, S. and La-O-Vorakiat, C. and Shaw, J.M. and Turgut, E. and Grychtol, P. and Adam, R. and Rudolf, D. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Schneider, C.M. and Kapteyn, H.C. and Murnane, M.M.
    Journal of Electron Spectroscopy and Related Phenomena 189 (2013)
    We review recent progress in femtosecond magnetization dynamics probed by extreme ultraviolet pulses from high-harmonic generation. In a transverse magneto-optical Kerr geometry, we established an ultrafast, element-specific experimental capability - on a table-top - for the measurement of magnetization dynamics in complex multi-sublattice magnets and multilayer magnetic structures. We show that this newly introduced technique is an artifact-free magnetic sensor, with only negligible non-magnetic (optical) contributions from the transient variation of the refractive index due to the presence of a non equilibrium hot-electron distribution. We then use these new experimental capabilities of ultrahigh time-resolution, combined with element-specific simultaneous probing, to disentangle important microscopic processes that drive magnetization dynamics on femtosecond timescales. We elucidate the role of exchange interaction on magnetization dynamics in strongly exchange-coupled alloys, and the role of photo-induced superdiffusive spin currents in magnetic multilayer stacks. © 2012 Elsevier B.V.
    view abstract10.1016/j.elspec.2012.11.013
  • Ultrathin magnetic oxide EuO films on Si(001) using SiOx passivation-Controlled by hard x-ray photoemission spectroscopy
    Caspers, C. and Flade, S. and Gorgoi, M. and Gloskovskii, A. and Drube, W. and Schneider, C.M. and Müller, M.
    Journal of Applied Physics 113 (2013)
    We present the chemical and structural optimization of ultrathin magnetic oxide EuO films on silicon. By applying a controlled in situ passivation of the Si(001) surface with SiOx in the monolayer regime, metallic silicide contaminations at the interface can be effectively reduced down to a sub-monolayer coverage, as was carefully quantified by interface-sensitive hard x-ray photoemission spectroscopy. Heteroepitaxial growth of EuO on Si(001) is sustained for this ultrathin SiOx-passivation, and bulk-near magnetic properties are observed for the 4 nm-thin EuO films. Our successful combination of chemically and structurally optimized EuO/Si(001) heterostructures by ultrathin in situ SiOx passivation makes this system promising for an application as alternative spin functional tunnel contacts in spin-FETs. © 2013 American Institute of Physics.
    view abstract10.1063/1.4795010
  • X-ray magneto-optical polarization spectroscopy: An analysis from the visible region to the x-ray regime
    Tesch, M.F. and Gilbert, M.C. and Mertins, H.-Ch. and Bürgler, D.E. and Berges, U. and Schneider, C. M.
    Applied Optics 52 (2013)
    An ultra-high vacuum compatible multipurpose chamber for magneto-optical reflection and transmission experiments with polarization analysis on magnetic systems is introduced. It is applicable in a broad photon energy range from the visible to the soft x-ray regime and for a wide angular range from grazing to normal incidence. It exploits a novel magnetization device based on rotating permanent magnets, which generates tuneable magnetic fields up to 570 mT in longitudinal, transverse and polar geometry. The unique combination of these features enables the feasibility of all typical magneto-optical spectroscopy techniques as T-MOKE, L-MOKE, P-MOKE, x-ray magneto optical linear dichroism, x-ray magnetic circular dichroism in reflection and Kerr polarization- spectroscopy, which is demonstrated for Co with focus on the Co 3p edges. © 2013 Optical Society of America.
    view abstract10.1364/AO.52.004294
  • Antiferromagnetic coupling across silicon regulated by tunneling currents
    Gareev, R.R. and Schmid, M. and Vancea, J. and Back, C.H. and Schreiber, R. and Bürgler, D. and Schneider, C.M. and Stromberg, F. and Wende, H.
    Applied Physics Letters 100 (2012)
    We report on the enhancement of antiferromagnetic coupling in epitaxial Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using the ballistic electron magnetic microscopy, we established that the hot-electron collector current reflects magnetization alignment and the magnetocurrent exceeds 200% at room temperature. The saturation magnetic field for the collector current corresponding to the parallel alignment of magnetizations rises up with the tunneling current, thus demonstrating stabilization of the antiparallel alignment and increasing antiferromagnetic coupling. We connect the enhancement of antiferromagnetic coupling with local dynamic spin torques mediated by spin-polarized tunneling electrons. © 2012 American Institute of Physics.
    view abstract10.1063/1.3675872
  • Bulk electronic structure of the dilute magnetic semiconductor Ga 1-x Mn x As through hard X-ray angle-resolved photoemission
    Gray, A.X. and Minár, J. and Ueda, S. and Stone, P.R. and Yamashita, Y. and Fujii, J. and Braun, J. and Plucinski, L. and Schneider, C.M. and Panaccione, G. and Ebert, H. and Dubon, O.D. and Kobayashi, K. and Fadley, C.S.
    Nature Materials 11 (2012)
    A detailed understanding of the origin of the magnetism in dilute magnetic semiconductors is crucial to their development for applications. Using hard X-ray angle-resolved photoemission (HARPES) at 3.2 keV, we investigate the bulk electronic structure of the prototypical dilute magnetic semiconductor Ga 0.97 Mn 0.03 As, and the reference undoped GaAs. The data are compared to theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Distinct differences are found between angle-resolved, as well as angle-integrated, valence spectra of Ga 0.97 Mn 0.03 As and GaAs, and these are in good agreement with theory. Direct observation of Mn-induced states between the GaAs valence-band maximum and the Fermi level, centred about 400 meV below this level, as well as changes throughout the full valence-level energy range, indicates that ferromagnetism in Ga 1-x Mn x As must be considered to arise from both p-d exchange and double exchange, thus providing a more unifying picture of this controversial material. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nmat3450
  • Covalent functionalization of carbon nanotubes with tetramanganese complexes
    Meyer, C. and Besson, C. and Frielinghaus, R. and Saelhoff, A.-K. and Flötotto, H. and Houben, L. and Kögerler, P. and Schneider, C.M.
    Physica Status Solidi (B) Basic Research 249 (2012)
    We present first results on the covalent chemical functionalization of single-walled carbon nanotubes with polynuclear {Mn4} coordination complexes. Raman spectra prove that the reaction can only be achieved for tubes which have been oxidized to create carboxylic groups. HRTEM is used to show that the reaction can be carried out directly on a substrate as well. Analysis of the D/G intensity ratio for different oxidation times shows that it is possible to reduce the amount of defects created. This is important for the future application of this material in transport devices. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201200135
  • Deflection gating for time-resolved x-ray magnetic circular dichroism-photoemission electron microscopy using synchrotron radiation
    Wiemann, C. and Kaiser, A.M. and Cramm, S. and Schneider, C.M.
    Review of Scientific Instruments 83 (2012)
    In this paper, we present a newly developed gating technique for a time-resolving photoemission microscope. The technique makes use of an electrostatic deflector within the microscopes electron optical system for fast switching between two electron-optical paths, one of which is used for imaging, while the other is blocked by an aperture stop. The system can be operated with a switching time of 20 ns and shows superior dark current rejection. We report on the application of this new gating technique to exploit the time structure in the injection bunch pattern of the synchrotron radiation source BESSY II at Helmholtz-Zentrum Berlin for time-resolved measurements in the picosecond regime. © 2012 American Institute of Physics.
    view abstract10.1063/1.4729603
  • Detection of filament formation in forming-free resistive switching SrTiO 3 devices with Ti top electrodes
    Stille, S. and Lenser, Ch. and Dittmann, R. and Koehl, A. and Krug, I. and Muenstermann, R. and Perlich, J. and Schneider, C.M. and Klemradt, U. and Waser, R.
    Applied Physics Letters 100 (2012)
    We investigated the influence of Ti top electrodes on the resistive switching properties of SrTiO 3 thin film devices. Above a Ti layer thickness of 5 nm, the initial resistance is strongly reduced, giving rise to forming-free devices. Hard x-ray photoemission experiments reveal the Ti layer to be composed of several oxide phases, induced by the redox-reaction at the Ti/SrTiO 3 interface. Grazing incidence small angle x-ray scattering measurements indicate that the reduction of the SrTiO 3 thin film occurs in a filamentary way. We attribute this behavior to the preferential reduction of SrTiO 3 thin films along highly defective areas. © 2012 American Institute of Physics.
    view abstract10.1063/1.4724108
  • Disentangling the physical contributions to the electrical resistance in magnetic domain walls: A multiscale study
    Seemann, K.M. and Garcia-Sanchez, F. and Kronast, F. and Miguel, J. and Kákay, A. and Schneider, C. M. and Hertel, R. and Freimuth, F. and Mokrousov, Y. and Blügel, S.
    Physical Review Letters 108 (2012)
    We analyze the origin of the electrical resistance arising in domain walls of perpendicularly magnetized materials by considering a superposition of anisotropic magnetoresistance and the resistance implied by the magnetization chirality. The domain wall profiles of L1 0-FePd and L1 0-FePt are determined by micromagnetic simulations based on which we perform first-principles calculations to quantify electron transport through the core and closure region of the walls. The wall resistance, being twice as high in L1 0-FePd than in L1 0-FePt, is found to be clearly dominated in both cases by a high gradient of magnetization rotation, which agrees well with experimental observations. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.077201
  • Electronic structure changes across the metamagnetic transition in FeRh via hard X-ray photoemission
    Gray, A.X. and Cooke, D.W. and Krüger, P. and Bordel, C. and Kaiser, A.M. and Moyerman, S. and Fullerton, E.E. and Ueda, S. and Yamashita, Y. and Gloskovskii, A. and Schneider, C. M. and Drube, W. and Kobayashi, K. and Hellman, F. and Fadley, C.S.
    Physical Review Letters 108 (2012)
    Stoichiometric FeRh undergoes a temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) transition at ∼350K. In this Letter, changes in the electronic structure accompanying this transition are investigated in epitaxial FeRh thin films via bulk-sensitive valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 5.95keV. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density-functional theory. Changes in the 2p core levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results indicate that significant electronic structure changes over the entire valence-band region are involved in this AFM-FM transition. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.257208
  • Expanding the view into complex material systems: From micro-ARPES to nanoscale HAXPES
    Schneider, C.M. and Wiemann, C. and Patt, M. and Feyer, V. and Plucinski, L. and Krug, I.P. and Escher, M. and Weber, N. and Merkel, M. and Renault, O. and Barrett, N.
    Journal of Electron Spectroscopy and Related Phenomena 185 (2012)
    The analysis of chemical and electronic states in complex and nanostructured material systems requires electron spectroscopy to be carried out with nanometer lateral resolution, i.e. nanospectroscopy. This goal can be achieved by combining a parallel imaging photoelectron emission microscope with a bandpass energy filter. In this contribution we describe selected experiments employing a dedicated spectromicroscope - the NanoESCA. This instrument has a particular emphasis on the spectroscopic aspects and enables laterally resolved photoelectron spectroscopy from the VUV up into the hard X-ray regime. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2012.08.003
  • Magnetization and magnetization reversal in epitaxial Fe/Cr/Co asymmetric spin-valve systems
    Brüssing, F. and Toperverg, B. and Zhernenkov, K. and Devishvili, A. and Zabel, H. and Wolff, M. and Theis-Bröhl, K. and Wiemann, C. and Kaiser, A. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    We have investigated asymmetric Fe/Cr/Co/Cr superlattices with two magnetic layers of Fe and Co, which are different with respect to their magnetic properties: magnetization, coercivity, and magnetic anisotropy. The magnetic layers are weakly coupled via a mediating Cr spacer layer providing an antiferromagnetic alignment of adjacent layers. The magnetic structure of these spin-valve-like Fe/Cr/Co/Cr superlattices was analyzed from the remanent state up to saturation via polarized neutron scattering and polarized neutron reflectivity (PNR). Furthermore, the domain structure in remanence was imaged via polarized x-ray photoemission electron microscopy (XPEEM). This analysis reveals that the Co magnetization strongly affects the Fe domain structure, while the layer magnetization is collinear from the remanent antiparallel state up to the ferromagnetic saturation state. However, for certain Co layer thicknesses, the as-grown remanent state exhibits a noncollinear antiferromagnetic spin structure, which cannot be recovered after applying a magnetic field. However, the noncollinear structure is reproducible with freshly grown superlattices. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.174409
  • Microscopic and spectroscopic studies of light-induced magnetization switching of GdFeCo facilitated by photoemission electron microscopy
    Ohkochi, T. and Fujiwara, H. and Kotsugi, M. and Tsukamoto, A. and Arai, K. and Isogami, S. and Sekiyama, A. and Yamaguchi, J. and Fukushima, K. and Adam, R. and Schneider, C. M. and Nakamura, T. and Kodama, K. and Masakiyo, T. and Kinoshita, T. and Suga, S.
    Japanese Journal of Applied Physics 51 (2012)
    By means of soft X-ray photoemission electron microscopy (PEEM), we have for the first time observed spatially and temporally resolved elementspecific magnetization switching driven by circularly polarized femtosecond laser pulses. We have confirmed that the magnetization switching depending on the helicity of the circularly polarized laser occurs in the region where the spin temperature is raised appropriately. It is also found that the electronic states in the irradiated region remain unchanged even after millions magnetization reversal cycles. © 2012 The Japan Society of Applied Physics.
    view abstract10.1143/JJAP.51.073001
  • Monitoring structural influences on quantum transport in InAs nanowires
    Frielinghaus, R. and Flöhr, K. and Sladek, K. and Weirich, T.E. and Trellenkamp, S. and Hardtdegen, H. and Schäpers, T. and Schneider, C.M. and Meyer, C.
    Applied Physics Letters 101 (2012)
    A sample design that allows for quantum transport and transmission electron microscopy (TEM) on individual suspended nanostructures is used to investigate moderately n-type doped InAs nanowires (NWs). The nanowires were grown by metal organic vapor phase epitaxy. Universal conductance fluctuations in the nanowires are investigated at temperatures down to 0.35 K. These fluctuations show two different temperature dependences. The very same nanowire segments investigated in transport are subsequently analyzed by TEM revealing crystal phase mixing. However, we find no correspondence between the atomic structure of the wires and the temperature dependences of the conductance fluctuations. © 2012 American Institute of Physics.
    view abstract10.1063/1.4742326
  • Observation and implications of magnetic domains in lateral spin valves
    Mennig, J. and Matthes, F. and Bürgler, D.E. and Schneider, C.M.
    Journal of Applied Physics 111 (2012)
    Co/Cu/Co lateral spin valves (LSV), with Co being the topmost layer, are in situ prepared and measured under ultrahigh vacuum conditions. The clean process yields a non-local spin signal of 0.9 mω. Scanning electron microscopy with polarization analysis (SEMPA) reveals domain structures in both magnetic electrodes that depend on the LSV dimensions. The spin signal correlates to SEMPA images as well as the anisotropic magnetoresistance of both Co magnets, revealing a strong impact of multi-domain states on the spin signal. © 2012 American Institute of Physics.
    view abstract10.1063/1.3671415
  • Observation of boron diffusion in an annealed Ta/CoFeB/MgO magnetic tunnel junction with standing-wave hard x-ray photoemission
    Greer, A.A. and Gray, A.X. and Kanai, S. and Kaiser, A.M. and Ueda, S. and Yamashita, Y. and Bordel, C. and Palsson, G. and Maejima, N. and Yang, S.-H. and Conti, G. and Kobayashi, K. and Ikeda, S. and Matsukura, F. and Ohno, H. and Schneider, C.M. and Kortright, J.B. and Hellman, F. and Fadley, C.S.
    Applied Physics Letters 101 (2012)
    The CoFeB/MgO system shows promise as a magnetic tunnel junction with perpendicular magnetization and low critical current densities for spin-torque driven magnetization switching. The distribution of B after annealing is believed to be critical to performance. We have studied the distribution of B in a Ta/Co0.2Fe0.6B0.2/MgO sample annealed at 300°C for 1 h with standing-wave hard x-ray photoemission spectroscopy (SW-HXPS). Comparing experimental rocking curve data to x-ray optical calculations indicates diffusion of 19.5 of the B uniformly into the MgO and of 23.5 into a thin TaB interface layer. SW-HXPS is effective for probing depth distributions in such spintronic structures. © 2012 American Institute of Physics.
    view abstract10.1063/1.4766351
  • Permalloy and Co 50Pd 50 as ferromagnetic contacts for magnetoresistance measurements in carbon nanotube-based transport structures
    Morgan, C. and Schneider, C.M. and Meyer, C.
    Journal of Applied Physics 111 (2012)
    In this paper, magnetoresistance (MR) measurements on carbon nanotube (CNT) 2-terminal spin-valve devices are presented. Results from samples with both permalloy (Py) and CoPd contacts show repeatable MR switching. In performing gate-dependent MR measurements on the Py-contacted CNTs, two distinct transport regimes are identified, and their transport behavior is discussed with respect to optimizing MR. Results from the first CoPd-contacted CNTs indicate a stable magnetic response with a higher magnitude than that of a Py-contacted nanotube in the same transport regime. © 2012 American Institute of Physics.
    view abstract10.1063/1.3673841
  • Probing buried layers by photoelectron spectromicroscopy with hard x-ray excitation
    Wiemann, C. and Patt, M. and Cramm, S. and Escher, M. and Merkel, M. and Gloskovskii, A. and Thiess, S. and Drube, W. and Schneider, C.M.
    Applied Physics Letters 100 (2012)
    We report about a proof-of-principle experiment which explores the perspectives of performing hard x-ray photoemission spectromicroscopy with high lateral resolution. Our results obtained with an energy-filtered photoemission microscope at the PETRA III storage ring facility using hard x-ray excitation up to 6.5 keV photon energy demonstrate that it is possible to obtain selected-area x-ray photoemission spectra from regions less than 500 nm in diameter. © 2012 American Institute of Physics.
    view abstract10.1063/1.4722940
  • Probing the timescale of the exchange interaction in a ferromagnetic alloy
    Mathias, S. and La-O-Vorakiat, C. and Grychtol, P. and Granitzka, P. and Turgut, E. and Shaw, J.M. and Adam, R. and Nembach, H.T. and Siemens, M.E. and Eich, S. and Schneider, C.M. and Silva, T.J. and Aeschlimann, M. and Murnane, M.M. and Kapteyn, H.C.
    Proceedings of the National Academy of Sciences of the United States of America 109 (2012)
    The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
    view abstract10.1073/pnas.1201371109
  • Quenched Slonczewski windmill in spin-torque vortex oscillators
    Sluka, V. and Kákay, A. and Deac, A.M. and Bürgler, D.E. and Hertel, R. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    We present a combined analytical and numerical study on double-vortex spin-torque nano-oscillators and describe a mechanism that suppresses the windmill modes. The magnetization dynamics is dominated by the gyrotropic precession of the vortex in one of the ferromagnetic layers. In the other layer, the vortex gyration is strongly damped. The dominating layer for the magnetization dynamics is determined by the sign of the product between sample current and the chiralities. Measurements on Fe/Ag/Fe nanopillars support these findings. The results open up a new perspective for building high quality-factor spin-torque oscillators operating at selectable, well-separated frequency bands. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.214422
  • Scaling potential of local redox processes in memristive SrTiO 3 thin-film devices
    Dittmann, R. and Muenstermann, R. and Krug, I. and Park, D. and Menke, T. and Mayer, J. and Besmehn, A. and Kronast, F. and Schneider, C. M. and Waser, R.
    Proceedings of the IEEE 100 (2012)
    In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO 3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO 3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference. © 1963-2012 IEEE.
    view abstract10.1109/JPROC.2012.2188771
  • Surface spin waves of fcc cobalt films on Cu(100): High-resolution spectra and comparison to theory
    Rajeswari, J. and Ibach, H. and Schneider, C.M. and Costa, A.T. and Santos, D.L.R. and Mills, D.L.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    With the help of a recently developed electron energy-loss spectrometer we have studied the surface spin waves on an eight-monolayer cobalt film deposited on Cu(100) surfaces with unprecedented energy resolution. Standing waves of the bulk of the film are discovered in the region of small wave vectors q a< 0.35 A -1. The dispersion of surface spin waves is isotropic even for large q a. Because of the high-energy resolution and the complete characterization of the electron optical properties of the spectrometer reliable data for the linewidth of the surface spin waves are obtained. As a byproduct the dispersion of the Rayleigh surface phonon was measured. Data are compared to theoretical spin wave spectra extracted from calculations of the transverse spin susceptibility based on an ab initio electronic structure that incorporates both the metallic substrate and the magnetic film. The calculation takes fully into account the itinerant nature of the electrons responsible for the magnetic moments. The agreement between theoretical and experimental spin wave energies and linewidths is remarkably good. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.165436
  • Temperature-driven nucleation of ferromagnetic domains in FeRh thin films
    Baldasseroni, C. and Bordel, C. and Gray, A.X. and Kaiser, A.M. and Kronast, F. and Herrero-Albillos, J. and Schneider, C.M. and Fadley, C.S. and Hellman, F.
    Applied Physics Letters 100 (2012)
    The evolution of ferromagnetic (FM) domains across the temperature-driven antiferromagnetic (AF) to FM phase transition in uncapped and capped epitaxial FeRh thin films was studied by x-ray magnetic circular dichroism and photoemission electron microscopy. The coexistence of the AF and FM phases was evidenced across the broad transition and the different stages of nucleation, growth, and coalescence were directly imaged. The FM phase nucleates into single domain islands and the width of the transition of an individual nucleus is sharper than that of the transition in a macroscopic average. © 2012 American Institute of Physics.
    view abstract10.1063/1.4730957
  • TetraMag: A compact magnetizing device based on eight rotating permanent magnets
    Gilbert, M. and Mertins, H.-Ch. and Tesch, M. and Berges, O. and Feilbach, H. and Schneider, C.M.
    Review of Scientific Instruments 83 (2012)
    In this paper we describe a novel magnetizing device based on eight rotatable permanent magnets arranged in a quadrupolar configuration, which is termed the TetraMag. TetraMag creates stable and homogeneous magnetic fields at the sample position with a resolution of 0.02 mT tunable between -570 mT and 570 mT. The field direction is continuously rotatable between 0 and 360 within the sample plane, while the field strength is maintained. A simplified mathematical description of TetraMag is developed leading to magnetic field calculations which are in good agreement with the experimental results. This versatile device avoids electrical energy dissipation, cooling mechanisms, and hysteresis effects known from classical electromagnets. It is ultrahigh vacuum compatible and it offers a completely free optical path over 180 for magneto-optical experiments. It is suitable for scattering experiments with synchrotron radiation and neutrons and may be employed in a large class of magnetization experiments. © 2012 American Institute of Physics.
    view abstract10.1063/1.3684877
  • Ultrafast demagnetization measurements using extreme ultraviolet light: Comparison of electronic and magnetic contributions
    La-O-Vorakiat, C. and Turgut, E. and Teale, C.A. and Kapteyn, H.C. and Murnane, M.M. and Mathias, S. and Aeschlimann, M. and Schneider, C. M. and Shaw, J.M. and Nembach, H.T. and Silva, T.J.
    Physical Review X 2 (2012)
    Ultrashort pulses of extreme ultraviolet light from high-harmonic generation are a new tool for probing coupled charge, spin, and phonon dynamics with element specificity, attosecond pump-probe synchronization, and time resolution of a few femtoseconds in a tabletop apparatus. In this paper, we address an important question in magneto-optics that has implications for understanding magnetism on the fastest time scales: Is the signal from the transverse magneto-optical Kerr effect at the M 2,3 edges of a magnetic material purely magnetic or is it perturbed by nonmagnetic artifacts? Our measurements demonstrate conclusively that transverse magneto-optical Kerr measurements at the M 2,3 edges sensitively probe the magnetic state, with almost negligible contributions from the transient variation of the refractive index by the nonequilibrium hot-electron distribution. In addition, we compare pump-probe demagnetization dynamics measured by both high harmonics and conventional visible-wavelength magneto-optics and find that the measured demagnetization times are in agreement.
    view abstract10.1103/PhysRevX.2.011005
  • Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
    Rudolf, D. and La-O-Vorakiat, C. and Battiato, M. and Adam, R. and Shaw, J.M. and Turgut, E. and Maldonado, P. and Mathias, S. and Grychtol, P. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Kapteyn, H.C. and Murnane, M.M. and Schneider, C.M. and Oppeneer, P.M.
    Nature Communications 3 (2012)
    Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro-or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms2029
  • A new nanospectroscopy tool with synchrotron radiation: NanoESCA@Elettra
    Wiemann, C. and Patt, M. and Krug, I.P. and Weber, N.B. and Escher, M. and Merkel, M. and Schneider, C. M.
    e-Journal of Surface Science and Nanotechnology 9 (2011)
    We describe first experiences with a novel spectromicroscopy set-up - NanoESCA@Elettra - which has been installed at the nanospectroscopy soft x-ray beamline at Elettra (Trieste). The system features an energy-filtered photoemission microscope with a 30 kV immersion lens system and a double-hemispherical energy analyzer. The instrument provides both real space and k-space mapping modes. Experiments on nanostructured samples with laboratory gas discharge sources show a lateral resolution of less than 50 nm and an energy resolution of better than 200 meV. We have also performed first tests of the instrument with synchrotron radiation. © 2011 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2011.395
  • An electron energy loss spectrometer designed for studies of electronic energy losses and spin waves in the large momentum regime
    Ibach, H. and Rajeswari, J. and Schneider, C.M.
    Review of Scientific Instruments 82 (2011)
    Based on 143° electrostatic deflectors we have realized a new spectrometer for electron energy loss spectroscopy which is particularly suitable for studies on surface spin waves and other low energy electronic energy losses. Contrary to previous designs high resolution is maintained even for diffuse inelastic scattering due to a specific management of the angular aberrations in combination with an angle aperture. The performance of the instrument is demonstrated with high resolution energy loss spectra of surface spin waves on a cobalt film deposited on the Cu(100) surface. © 2011 American Institute of Physics.
    view abstract10.1063/1.3670731
  • Band mapping in x-ray photoelectron spectroscopy: An experimental and theoretical study of W(110) with 1.25 keV excitation
    Papp, C. and Plucinski, L. and Minar, J. and Braun, J. and Ebert, H. and Schneider, C.M. and Fadley, C.S.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Angle-resolved photoemission spectroscopy (ARPES) has generally been carried out at energies below ∼150 eV, but there is growing interest in going to higher energies so as to achieve greater bulk sensitivity. To this end, we have measured ARPES spectra from a tungsten (110) crystal in a plane containing the [100], [110], and [010] directions with a photon energy of 1253.6 eV. The experimental data are compared to free-electron final-state calculations in an extended zone scheme with no inclusion of matrix elements, as well as highly accurate one-step theory including matrix elements. Both models provide further insight into such future higher-energy ARPES measurements. Special effects occurring in a higher-energy ARPES experiment, such as photon momentum, phonon-induced zone averaging effects, and the degree of cryogenic cooling required are discussed, together with qualitative predictions via appropriate Debye-Waller factors for future experiments with a number of representative elements being presented. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.045433
  • Chemical stability of the magnetic oxide EuO directly on silicon observed by hard x-ray photoemission spectroscopy
    Caspers, C. and Müller, M. and Gray, A.X. and Kaiser, A.M. and Gloskovskii, A. and Fadley, C.S. and Drube, W. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We present a detailed study of the electronic structure and chemical state of high-quality stoichiometric EuO and O-rich Eu1O1+x thin films grown directly on silicon without any buffer layer using hard x-ray photoemission spectroscopy (HAXPES). We determine the EuO oxidation state from a consistent quantitative peak analysis of 4f valence band and 3d core-level spectra. The results prove that nearly ideal, stoichiometric, and homogeneous EuO thin films can be grown on silicon, with a uniform depth distribution of divalent Eu cations. Furthermore, we identify the chemical stability of the EuO/silicon interface from Si 2p core-level photoemission. This work clearly demonstrates the successful integration of high-quality EuO thin films directly on silicon, opening up the pathway for the future incorporation of this functional magnetic oxide into silicon-based spintronic devices. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.205217
  • Comprehensive characterization of an individual carbon nanotube transport device
    Frielinghaus, R. and Goß, K. and Trellenkamp, S. and Houben, L. and Schneider, C.M. and Meyer, C.
    Physica Status Solidi (B) Basic Research 248 (2011)
    We present a comprehensive characterization of an individual multiwalled carbon nanotube transport device combining electron microscopy and Raman spectroscopy with electrical measurements. Each method gives complementary information that mutually help to interpret each other. A sample design that allows for combining these investigation methods on individual carbon nanotube devices is introduced. This offers a direct correlation of transport features and shifts of Raman modes with structural properties as, e.g. the contact interface and the morphology of the nanotube. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201100081
  • Controlling magnetic properties of EuS-based spin valve structures on Si(001)
    Müller, M. and Schreiber, R. and Schneider, C. M.
    IEEE Transactions on Magnetics 47 (2011)
    We report on the growth and magnetic characterization of the ferromagnetic insulator Europium Sulfide (EuS) on (001)-oriented Silicon substrates. The influence of the EuS film thickness on the coercive field and Curie temperature was systematically investigated with regard to a potential application of thin EuS films as spin filter tunnel barriers. In a further step, we fabricated EuS/Si(001) spin valve structures with both ferromagnetic Gd and exchange-biased CoO/Co counter electrodes. An independent magnetic switching of the EuS barrier and the ferromagnetic layers was accomplished by eliminating intermediate magnetic exchange couplings. Our results clearly demonstrate the feasibility to employ thin EuS ferromagnetic insulator films as spin filter tunnel barriers on Silicon(001) in spin valve structures for future magnetotransport experiments. © 2011 IEEE.
    view abstract10.1109/TMAG.2011.2106767
  • Determination of layer-resolved composition, magnetization, and electronic structure of an Fe/MgO tunnel junction by standing-wave core and valence photoemission
    Yang, S.-H. and Balke, B. and Papp, C. and Döring, S. and Berges, U. and Plucinski, L. and Westphal, C. and Schneider, C.M. and Parkin, S.S.P. and Fadley, C.S.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    Spin-dependent tunneling across a highly textured MgO insulating barrier has received much attention due to its potential applications in various spintronic devices. However, the interfacial magnetic and electronic structure of a prototypical realization of this in Fe/MgO/Fe and the effective band gap of the MgO layer are still under debate. In order to resolve these issues, we have employed standing-wave excited core and valence photoemission, as well as core-level magnetic circular dichroism (MCD) in photoemission, to study the Fe/MgO interface with subnanometer depth resolution. For our synthetic procedure, we show that the Fe/MgO interface is linearly intermixed in composition over a length of ∼8 (∼4 monolayers) and that there is a magnetic dead layer ∼2-3 thick. The unambiguous extraction of depth-resolved density of states (DOS) reveals that the interfacial layer composition is mostly metallic and nonmagnetic FeOx, with x ≅ 1, which accounts for a smaller magnetoresistance compared to theoretical predictions. The formation of the magnetic dead layer (FeO) at the interface should also reduce the tunneling spin polarization. The analysis of our data also shows a clear valence band edge of ultrathin MgO layer at ∼3.5 eV below the Fermi level (EF) that is very close to that of single crystal bulk MgO. An analysis that does not consider the interdiffused region separately exhibits the valence band edge for MgO layer ∼1.3 eV below EF, which is significantly closer to the MgO barrier height estimated from magnetotransport measurements and further suggests that the Fe/MgO interdiffusion effectively reduces the MgO band gap. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.184410
  • Electronic structure of EuO spin filter tunnel contacts directly on silicon
    Caspers, C. and Müller, M. and Gray, A.X. and Kaiser, A.M. and Gloskovskii, A. and Fadley, C.S. and Drube, W. and Schneider, C.M.
    Physica Status Solidi - Rapid Research Letters 5 (2011)
    We present an electronic structure study of a magnetic oxide/ semiconductor model system, EuO on silicon, which is dedicated for efficient spin injection and spin detection in silicon-based spintronics devices. A combined electronic structure analysis of Eu core levels and valence bands using hard X-ray photoemission spectroscopy was performed to quantify the nearly ideal stoichiometry of EuO "spin filter" tunnel barriers directly on silicon, and the absence of silicon oxide at the EuO/Si interface. These results provide evidence for the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Hard X-ray photoemission spectroscopy of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Hard X-ray photoemission spectroscopy reveals the nearly ideal stoichiometry of EuO spin filter tunnel barriers grown directly on silicon, and the absence of silicon oxide formation at the EuO/Si interface. These results demonstrate the successful integration of a magnetic oxide tunnel barrier with silicon, paving the way for the future integration of magnetic oxides into functional spintronics devices. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssr.201105403
  • FERRUM: A new highly efficient spin detector for electron spectroscopy
    Escher, M. and Weber, N.B. and Merkel, M. and Plucinski, L. and Schneider, C. M.
    e-Journal of Surface Science and Nanotechnology 9 (2011)
    To overcome the restrictions of conventional spin detectors (Mott, SPLEED) low energy electron scattering at a Fe(100)-p(1×1)O surface was first proposed by Bertacco and Ciccacci [Phys. Rev. B 59, 4207 (1999)] as a system for highly efficient spin detection. We developed a new instrument based on that scheme. The iron is prepared in-situ on a W(100) crystal without the need for separate sample preparation and exchange when using a MgO substrate. Our working point at 6.5 eV scattering energy is consistent with the above paper by Bertacco and Ciccacci. Results obtained with UPS on an Fe film are shown. With a Sherman-function of approx. 30% and a reflectivity of up to 10% the figure of merit (FOM) is more than one magnitude higher compared to conventional detectors. The FOM shows no degradation over days in UHV. © 2011 The Surface Science Society of Japan.
    view abstract10.1380/ejssnt.2011.340
  • Hard x-ray photoemission using standing-wave excitation applied to the MgO/Fe interface
    Döring, S. and Schönbohm, F. and Berges, U. and Schreiber, R. and Bürgler, D.E. and Schneider, C.M. and Gorgoi, M. and Schäfers, F. and Papp, C. and Balke, B. and Fadley, C.S. and Westphal, C.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    Many applications in electronics and spintronics rely on interfaces, which are buried a few nanometers deep and thus are hardly accessible in real devices except for invasive techniques. Here, we report on hard x-ray photoemission spectroscopy combined with the x-ray standing-wave technique as a noninvasive method to access buried interfaces with a depth resolution of a few and enhanced interface sensitivity. Within these experiments, the film thicknesses and also the thicknesses of the intermixing layers are determined. We extend the data analysis scheme previously developed for soft x-rays to the hard x-ray regime and apply the method to buried epitaxial Fe/MgO interfaces, which play a crucial role in magnetic tunnel junctions and their applications. It was found that there was no detectable intermixing or reaction of the Fe and MgO layers at the interface. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.165444
  • Index assignment of a carbon nanotube rope using tip-enhanced Raman spectroscopy
    Goß, K. and Peica, N. and Thomsen, C. and Maultzsch, J. and Schneider, C.M. and Meyer, C.
    Physica Status Solidi (B) Basic Research 248 (2011)
    We used tip-enhanced Raman spectroscopy to study the diameter-dependent Raman modes in a contacted carbon nanotube (CNT) rope. We show that with the near-field tip enhancement a large number of nanotubes within a rope can be identified, even if the nanotube modes cannot be distinguished in the far-field signal. Several metallic and semiconducting nanotubes can be identified and assigned to nanotube families. Additionally, we provide a tentative chiral index assignment. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201100161
  • Injection locking of single-vortex and double-vortex spin-torque oscillators
    Bürgler, D.E. and Sluka, V. and Lehndorff, R. and Deac, A.M. and Kákay, A. and Hertel, R. and Schneider, C. M.
    Proceedings of SPIE - The International Society for Optical Engineering 8100 (2011)
    Spin-torque oscillators are a promising application for the spin-transfer torque effect. The major challenge lies in pushing their microwave output power to useful levels, e.g. by operating an array of spin-torque oscillators in a synchronized, phase-locked mode. Our experiments on metallic, GMR-type nanopillars focus on the influence of external high-frequency signals on the current-driven vortex dynamics in single-vortex and double-vortex spintorque oscillators. For both cases we observe injection locking behavior according to the nonlinear oscillator theory, which is a prerequisite for the synchronization of spin-torque oscillators via microwaves in common electrical electrodes. © 2011 SPIE.
    view abstract10.1117/12.892395
  • Layer-selective studies of an anti-ferromagnetically coupled multilayer by resonant magnetic reflectivity in the extreme ultraviolet range
    Grychtol, P. and Adam, R. and Kaiser, A.M. and Cramm, S. and Bürgler, D.E. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 184 (2011)
    A multilayer comprising a 5 nm Ni80Fe20 and a 10 nm Co40Fe60 layer separated by a 0.6 nm Cr layer was investigated by resonant magnetic reflectivity measurements of horizontally polarized light in the extreme ultraviolet spectral range (EUV). By exploiting the transversal magneto-optical Kerr effect (T-MOKE) at the M absorption edges of iron, cobalt and nickel (54 eV, 60 eV and 67 eV) a magnetic contrast as large as 30% can be obtained near a Brewster angle of about 45 °. Energy dependent scans of the magnetic asymmetry as well as magneto-optical hysteresis loops were recorded to study the magneto-optical response and to determine whether the switching behavior of individual layers in the strongly anti-ferromagnetically coupled multilayer system can be probed. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2010.09.012
  • Magnetic properties of EuS spin filter tunnel contacts to silicon
    Müller, M. and Schreiber, R. and Schneider, C.M.
    Journal of Applied Physics 109 (2011)
    We investigate the magnetic properties of the ferromagnetic insulator EuS in view of its potential in spin-filter tunnel contacts to silicon. We prepared thin polycrystalline EuS films directly on (001) oriented Si substrates that show well-defined magnetic properties down to the monolayer regime. Addressing the question of magnetic coupling between a EuS magnetic tunnel barrier and a CoOCo magnetic electrode, we succeeded in realizing an independent magnetic switching behavior in this spin-valve-type system. These results are important prerequisites for future spin-dependent transport experiments. © 2011 American Institute of Physics.
    view abstract10.1063/1.3549609
  • MBE growth optimization of topological insulator Bi2Te 3 films
    Krumrain, J. and Mussler, G. and Borisova, S. and Stoica, T. and Plucinski, L. and Schneider, C.M. and Grützmacher, D.
    Journal of Crystal Growth 324 (2011)
    We investigated the growth of the topological insulator Bi 2Te3 on Si(1 1 1) substrates by means of molecular-beam epitaxy (MBE). The substrate temperature as well as the Bi and Te beam-equivalent pressure (BEP) was varied in a large range. The structure and morphology of the layers were studied using X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM). The layer-by-layer growth mode with quintuple layer (QL) as an unit is accomplished on large plateaus if the MBE growth takes place in a Te overpressure. At carefully optimized MBE growth parameters, we obtained atomically smooth, single-crystal Bi 2Te3 with large area single QL covering about 75% of the layer surface. Angular-resolved photoelectron spectroscopy reveals a linear energy dispersion of charge carriers at the surface, evidencing topologically insulating properties of the Bi2Te3 epilayers. © 2011 Elsevier B.V.
    view abstract10.1016/j.jcrysgro.2011.03.008
  • Nano and picosecond magnetization dynamics of weakly coupled CoFe/Cr/NiFe trilayers studied by a multitechnique approach
    Kaiser, A.M. and Schöppner, C. and Römer, F.M. and Hassel, C. and Wiemann, C. and Cramm, S. and Nickel, F. and Grychtol, P. and Tieg, C. and Lindner, J. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We present results on the magnetization dynamics in heterostructures of the CoFe/Cr/NiFe type. We have employed a combination of different layer-selective methods covering a broad range from quasistatic hysteresis measurements by x-ray magnetic circular dichroism (XMCD), over time-resolved photoemission electron microscopy (PEEM) at subnanosecond timescales to high-frequency ferromagnetic resonance (FMR) experiments. With increasing driving frequency, we found a different influence of the coupling between the two ferromagnetic layers on the dynamic behavior. Employing the spatial resolution of the PEEM method, we have been able to discern various dynamic responses in different regions of the sample that could be attributed to magnetodynamic processes with a different degree of coupling. In conjunction with the complementary FMR and XMCD measurements, we attribute the inhomogeneous influence of interlayer coupling to a shift from domain-wall-motion-dominated dynamics at low frequencies to precession-dominated dynamics at higher frequencies. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.134406
  • Observation of spin filtering in magnetic insulator contacts to silicon
    Müller, M. and Luysberg, M. and Schneider, C.M.
    Applied Physics Letters 98 (2011)
    The magnetic insulator EuS is used to create a spin-selective and conductivity-matched tunnel contact to silicon, in analogy to a conventional ferromagnetic metal/semiconductor configuration employed for spin injection. The spin filter efficiency of such a magnetic "spin filter" tunnel barrier is quantified using an adjacent Co ferromagnetic electrode as spin detector in a spin valve-type structure. This previously unobserved magnetoresistance effect demonstrates the efficient spin-polarizing nature of magnetic semiconductors on silicon and its prospective functionality as spin injectors/detectors in hybrid semiconductor devices. © 2011 American Institute of Physics.
    view abstract10.1063/1.3572016
  • Origin of the planar Hall effect in nanocrystalline Co60Fe 20B20
    Seemann, K.M. and Freimuth, F. and Zhang, H. and Blügel, S. and Mokrousov, Y. and Bürgler, D.E. and Schneider, C. M.
    Physical Review Letters 107 (2011)
    An angle dependent analysis of the planar Hall effect (PHE) in nanocrystalline single-domain Co60Fe20B20 thin films is reported. In a combined experimental and theoretical study we show that the transverse resistivity of the PHE is entirely driven by anisotropic magnetoresistance (AMR). Our results for Co60Fe20B 20 obtained from first principles theory in conjunction with a Boltzmann transport model take into account the nanocrystallinity and the presence of 20 at.% boron. The ab initio AMR ratio of 0.12% agrees well with the experimental value of 0.22%. Furthermore, we experimentally demonstrate that the anomalous Hall effect contributes negligibly in the present case. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.086603
  • Probing bulk electronic structure with hard X-ray angle-resolved photoemission
    Gray, A.X. and Papp, C. and Ueda, S. and Balke, B. and Yamashita, Y. and Plucinski, L. and Minár, J. and Braun, J. and Ylvisaker, E.R. and Schneider, C.M. and Pickett, W.E. and Ebert, H. and Kobayashi, K. and Fadley, C.S.
    Nature Materials 10 (2011)
    Traditional ultraviolet/soft X-ray angle-resolved photoemission spectroscopy (ARPES) may in some cases be too strongly influenced by surface effects to be a useful probe of bulk electronic structure. Going to hard X-ray photon energies and thus larger electron inelastic mean-free paths should provide a more accurate picture of bulk electronic structure. We present experimental data for hard X-ray ARPES (HARPES) at energies of 3.2 and 6.0 keV. The systems discussed are W, as a model transition-metal system to illustrate basic principles, and GaAs, as a technologically-relevant material to illustrate the potential broad applicability of this new technique. We have investigated the effects of photon wave vector on wave vector conservation, and assessed methods for the removal of phonon-associated smearing of features and photoelectron diffraction effects. The experimental results are compared to free-electron final-state model calculations and to more precise one-step photoemission theory including matrix element effects. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nmat3089
  • Quantitative measurements of magnetic stray field dynamics of Permalloy particles in a photoemission electron microscopy
    Nepijko, S.A. and Krasyuk, A. and Oelsner, A. and Schneider, C. M. and Schönhense, G.
    Journal of Microscopy 242 (2011)
    By example of a Permalloy particle (40 × 40 μm 2 size, 30 nm thickness) we demonstrate a procedure to quantitatively investigate the dynamics of magnetic stray fields during ultrafast magnetization reversal. The measurements have been performed in a time-resolving photoemission electron microscope using the X-ray magnetic circular dichroism. In the particle under investigation, we have observed a flux-closure-dominated magnetic ground structure, minimizing the magnetic stray field outside the sample. A fast magnetic field pulse introduced changes in the micromagnetic structure accompanied with an incomplete flux closure. As a result, stray fields arise along the edges of domains, which cause a change of contrast and an image deformation of the particles geometry (curvature of its edge). The magnetic stray fields are calculated from a deformation of the X-ray magnetic circular dichroism (XMCD) images taken after the magnetic field pulse in a 1 ns interval. These measurements reveal a decrease of magnetic stray fields with time. An estimate of the lower limit of the domain wall velocity yields about 2 × 10 3 m s -1. © 2010 The Authors Journal of Microscopy © 2010 Royal Microscopical Society.
    view abstract10.1111/j.1365-2818.2010.03472.x
  • Robust surface electronic properties of topological insulators: Bi 2 Te3 films grown by molecular beam epitaxy
    Plucinski, L. and Mussler, G. and Krumrain, J. and Herdt, A. and Suga, S. and Grützmacher, D. and Schneider, C.M.
    Applied Physics Letters 98 (2011)
    The surface electronic properties of the important topological insulator Bi2 Te3 are shown to be robust under an extended surface preparation procedure, which includes exposure to atmosphere and subsequent cleaning and recrystallization by an optimized in situ sputter-anneal procedure under ultrahigh vacuum conditions. Clear Dirac-cone features are displayed in high-resolution angle-resolved photoemission spectra from the resulting samples, indicating remarkable insensitivity of the topological surface state to cleaning-induced surface roughness. © 2011 American Institute of Physics.
    view abstract10.1063/1.3595309
  • Spatially resolved observation of uniform precession modes in spin-valve systems
    Kaiser, A.M. and Wiemann, C. and Cramm, S. and Schneider, C.M.
    Journal of Applied Physics 109 (2011)
    Using time-resolved photoemission electron microscopy, the excitation of uniform precession modes in individual domains of a weakly coupled spin-valve system has been studied. A coupling dependence of the precession frequencies has been found that can be reasonably well understood on the basis of a macrospin model. By tuning the frequency of the excitation source the uniform precession modes are excited in a resonant way. © 2011 American Institute of Physics.
    view abstract10.1063/1.3535439
  • Spin transport in ferromagnetically contacted carbon nanotubes
    Meyer, C. and Morgan, C. and Schneider, C.M.
    Physica Status Solidi (B) Basic Research 248 (2011)
    We present magnetoresistance (MR) measurements on carbon nanotubes (CNTs) with different ferromagnetic leads. A sample with permalloy (Ni 80Fe 20) contacts shows the expected tunneling-type MR effect. Measurements on devices with CoPd contacts show a larger change of resistance with magnetic field. However, only minor loops are observed, which is explained with domain wall pinning. This is supported by magnetic force microscopy (MFM) measurements, which reveal a complicated bubble and stripe domain pattern. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201100131
  • Spin-dependent electronic hybridization in a rope of carbon nanotubes
    Goß, K. and Smerat, S. and Leijnse, M. and Wegewijs, M.R. and Schneider, C.M. and Meyer, C.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    We demonstrate single-electron addition to different strands of a carbon nanotube rope. Anticrossings of anomalous conductance peaks occur in quantum transport measurements through the parallel quantum dots forming on the individual strands. We determine the magnitude and the sign of the hybridization as well as the Coulomb interaction between the carbon nanotube quantum dots, finding that the bonding states dominate the transport. In a magnetic field the hybridization is shown to be selectively suppressed due to spin effects. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.201403
  • Spin-transfer torque induced vortex dynamics in Fe/Ag/Fe nanopillars
    Sluka, V. and Kákay, A. and Deac, A.M. and Bürgler, D.E. and Hertel, R. and Schneider, C.M.
    Journal of Physics D: Applied Physics 44 (2011)
    We report on the experimental and analytical work on spin-transfer torque induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. We study nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15 nm and 30 nm, respectively, separated by a 6 nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe discs. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We experimentally show that in this configuration spin-transfer torque can excite vortex dynamics and analyse their dependence on a magnetic field applied in the sample plane. The centre of gyration is continuously dislocated from the disc centre, and the potential changes its shape with field strength. The latter is reflected in the field dependence of the excitation frequency. In the second part we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We analytically show that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/44/38/384002
  • Time-resolved measurements of Ni80Fe20/MgO/Co trilayers in the extreme ultraviolet range
    Adam, R. and Grychtol, P. and Cramm, S. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 184 (2011)
    We performed an element-selective magneto-optic characterization of Ni 80Fe20/MgO/Co magnetic trilayers employing the resonant magnetic reflectivity of extreme ultraviolet (XUV) radiation tuned to the M absorption edges of cobalt (60.2 eV) and nickel (67.5 eV). Static reflectivity shows a large magnetic contrast of up to 80% for the top Co and 20% for the buried Ni80Fe20 layers. The magneto-dynamic response of the trilayers to the ultrashort field pulse exhibits oscillations in a frequency range of up to 6.5 GHz associated exclusively with magnetization dynamics of the top Co layer. The presented results demonstrate the feasibility of element-specific magneto-dynamic studies of magnetic multilayers employing resonant XUV reflectivity at the M absorption edges. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2010.12.017
  • A hemispherical photoelectron spectrometer with 2-dimensional delay-line detector and integrated spin-polarization analysis
    Plucinski, L. and Oelsner, A. and Matthes, F. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 181 (2010)
    Photoelectron spectrometers usually allow detection of either spin-resolved energy-distribution curves (EDCs) at single emission angle, or 2D angle-vs.-energy images without spin-resolution. We have combined the two detection schemes into one spectrometer system which permits simultaneous detection of a 1D spin-resolved EDC and a 2D angular map. A state-of-the-art hemispherical analyzer is used as an energy filter. Its original scintillator detector has been replaced by a delay-line-detector (DLD), and part of the electron beam is allowed to pass through to reach the spin-polarized low energy electron diffraction (SPLEED) spin-detector mounted subsequently. The electron-optics between DLD and SPLEED contains a 90° deflector to feature simultaneous detection of in-plane and out-of-plane spin components. These electron-optics have been optimized for high transmission to reduce acquisition times in the spin-resolved mode. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2010.05.005
  • Electronic structure and magnetic properties of epitaxial FeRh(001) ultrathin films on W(100)
    Lee, J.-S. and Vescovo, E. and Plucinski, L. and Schneider, C.M. and Kao, C.-C.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Epitaxial FeRh(100) films (CsCl structure, ∼10 ML thick), prepared in situ on a W(100) single-crystal substrate, have been investigated via valence band and core-level photoemission. The presence of the temperature-induced, first-order, antiferromagnetic to ferromagnetic (AF→FM) transition in these films has been verified via linear dichroism in photoemission from the Fe3p levels. Core-level spectra indicate a large moment on the Fe atom, practically unchanged in the FM and AF phases. Judging from the valence band spectra, the metamagnetic transition takes place without substantial modification of the electronic structure. In the FM phase, the spin-resolved spectra compare satisfactorily to the calculated spin-polarized bulk band structure. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.224410
  • Extrinsic screening of ferroelectric domains in Pb (Zr0.48 Ti0.52) O3
    Krug, I. and Barrett, N. and Petraru, A. and Locatelli, A. and Mentes, T.O. and Niño, M.A. and Rahmanizadeh, K. and Bihlmayer, G. and Schneider, C.M.
    Applied Physics Letters 97 (2010)
    The variation in the surface potential as a function of the ferroelectric polarization of micron scale domains in a thin epitaxial film of Pb (Zr 0.48 Ti0.52) O3 is measured using mirror electron microscopy. Domains were written using piezoforce microscopy. The surface potential for each polarization was deduced from the mirror to low energy electron microscopy transition in the local reflectivity curve. The effect of extrinsic screening of the fixed polarization charge at the ferroelectric surface is demonstrated. The results are compared with density functional theory calculations. © 2010 American Institute of Physics.
    view abstract10.1063/1.3523359
  • High resolution, low hν photoelectron spectroscopy with the use of a microwave excited rare gas lamp and ionic crystal filters
    Suga, S. and Sekiyama, A. and Funabashi, G. and Yamaguchi, J. and Kimura, M. and Tsujibayashi, M. and Uyama, T. and Sugiyama, H. and Tomida, Y. and Kuwahara, G. and Kitayama, S. and Fukushima, K. and Kimura, K. and Yokoi, T. and Murakami, K. and Fujiwara, H. and Saitoh, Y. and Plucinski, L. and Schneider, C.M.
    Review of Scientific Instruments 81 (2010)
    The need for not only bulk sensitive but also extremely high resolution photoelectron spectroscopy for studying detailed electronic structures of strongly correlated electron systems is growing rapidly. Moreover, easy access to such a capability in one's own laboratory is desirable. Demonstrated here is the performance of a microwave excited rare gas (Xe, Kr, and Ar) lamp combined with ionic crystal filters (sapphire, CaF2, and LiF), which can supply three strong lines near the photon energy of hnyu hν=8.4, 10.0, and 11.6 eV, with the hν resolution of better than 600 μeV for photoelectron spectroscopy. Its performance is demonstrated on some materials by means of both angle-integrated and angle-resolved measurements. © 2010 American Institute of Physics.
    view abstract10.1063/1.3488367
  • Injection locking of the gyrotropic vortex motion in a nanopillar
    Lehndorff, R. and Bürgler, D.E. and Schneider, C.M. and Celinski, Z.
    Applied Physics Letters 97 (2010)
    Spin-torque oscillators (STOs) are a promising application for the spin-transfer torque effect. The major challenge lies in pushing the STO's microwave output power to useful levels, e.g., by operating an array of STOs in a synchronized, phase-locked mode. Our experiment on metallic, giant magnetoresistance-type nanopillars focuses on the influence of external high-frequency signals on the current-driven vortex dynamics and demonstrates the injection locking of the gyrotropic mode. We find a gap of about three orders of magnitude between the high-frequency power emitted by one oscillator and the power needed for phase-locking. © 2010 American Institute of Physics.
    view abstract10.1063/1.3498009
  • Observation of breathing-like modes in an individual multiwalled carbon nanotube
    Spudat, C. and Müller, M. and Houben, L. and Maultzsch, J. and Goss, K. and Thomsen, C. and Schneider, C.M. and Meyer, C.
    Nano Letters 10 (2010)
    We study collective vibrational breathing modes in the Raman spectrum of a multiwalled carbon nanotube. In correlation with high-resolution transmission electron microscopy, we find that these modes have energies differing by more than 23% from the radial breathing modes of the corresponding single-walled nanotubes. This shift in energy is explained with intershell interactions using a model of coupled harmonic oscillators. The strength of this interaction is related to the coupling strength expected for few-layer graphene. © 2010 American Chemical Society.
    view abstract10.1021/nl102305a
  • Photoemission microscopy study of picosecond magnetodynamics in spin-valve-type thin film elements
    Schneider, C.M. and Kaiser, A. and Wiemann, C. and Tieg, C. and Cramm, S.
    Journal of Electron Spectroscopy and Related Phenomena 181 (2010)
    Exploring ultimate time scales of magnetic switching processes is an important issue in spin electronics. In spin valves or magnetic tunnelling junctions magnetic anisotropies and coupling phenomena alter the magnetodynamic response of the entire system. Understanding the role of these interactions is a key to the design of optimized devices. We have employed time-resolved X-ray photoemission microscopy to address the magnetodynamics in spin-valve-type model systems in the ns- and ps-regime. In Co/Cr/Fe(0 0 1) single crystal elements we find a strong influence of the magnetocrystalline anisotropy, which tends to suppress rotation processes. In addition, we observe a dynamic "decoupling" of the layers. In low-anisotropy FeNi/Cr/FeCo trilayers, the interlayer coupling character determines the dynamic response. Particularly, rotational processes in the FeNi and FeCo layers are temporarily shifted to each other, which can be related to different coercivities of the individual layers. By contrast, the domain wall motion in both layers closely agrees, caused by an enhanced coupling due to the domain wall stray fields. Our examples demonstrate that the detailed magnetodynamics in coupled magnetic layers is quite complex and depends strongly on the timescale under consideration. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2010.01.005
  • Photoluminescence studies on structural defects and room temperature ferromagnetism in Ni and Ni-H doped ZnO nanoparticles
    Tong, L.-N. and Cheng, T. and Han, H.-B. and Hu, J.-L. and He, X.-M. and Tong, Y. and Schneider, C.M.
    Journal of Applied Physics 108 (2010)
    We explore the effects of hydrogenated annealing on the crystal structure, room temperature ferromagnetism (RT-FM) and photoluminescence (PL) properties of Ni-doped ZnO (Zn1-xNixO, x=0.0 to 0.2) nanoparticles prepared by a sol-gel method. The x-ray photoelectron spectra and x-ray diffraction data provide evidence that Ni has been incorporated into the wurtzite ZnO lattice as Ni2+ ions substituting for Zn2+ ions at x0.05. A secondary phase of NiO type begins to form inside ZnO when x≤0.05 and segregates from ZnO host lattice at x=0.2, leading to a large variation in the lattice constants of ZnO. The magnetization measurements show that the saturation magnetization (Ms) increases with increasing Ni concentration in the single-phase Zn1-xNixO (x≤0.05) nanoparticles. The secondary phase formation reduces the magnetization of Zn1-xNixO (x=0.1 and 0.15), while the segregation of NiO from the ZnO lattice at x=0.2 is accompanied by a large increase in M s again. The PL measurements show that the UV emission intensity of single-phase Zn1-xNixO (x≤0.05) nanoparticles increases with a blueshift in the UV emission line when the Ni concentration increases, while the dominant green emission intensity decreases with increasing Ni dopant. The PL data strongly suggest that the FM in single-phase Zn 1-xNixO (x≤0.05) nanoparticles is intrinsically correlated with a doping induced increase in the electron concentration in the conduction band of Ni-doped ZnO. After H2 -annealing, the single-phase Zn1-x NixO:H (x≤0.05) nanoparticles show increases in both coercivity and saturation magnetization. The PL and diffuse reflectance spectra suggest that hydrogen-related shallow donors and an improved sample quality may be responsible for the H2-annealing induced enhancement of the RT-FM. The obvious correlation between FM and carrier concentration in Ni and Ni-H doped ZnO points towards a mechanism of carrier-mediated FM for Ni-doped ZnO diluted magnetic semiconductors. © 2010 American Institute of Physics.
    view abstract10.1063/1.3460644
  • Resonant magnetic reflectivity in the extreme ultraviolet spectral range: Interlayer-coupled Co/Si/Ni/Fe multilayer system
    Grychtol, P. and Adam, R. and Valencia, S. and Cramm, S. and Bürgler, D.E. and Schneider, C.M.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    A polycrystalline test structure comprising a 5 nm cobalt and a 10 nm nickel/iron layer separated by a silicon layer ranging from 1.5 to 4 nm prepared by thermal evaporation has been investigated by resonant magnetic reflectivity measurements of horizontally polarized light in the extreme ultraviolet spectral range. By exploiting the transversal magneto-optical Kerr effect at the M absorption edges of cobalt and nickel (59.5 eV and 66.5 eV) a magnetic contrast as large as 80% for cobalt and 25% for nickel can be obtained near a Brewster angle of about 45°. Angle- and energy-dependent scans of the magnetic asymmetry as well as element-selective, magneto-optical loops of the hysteresis were recorded against the thickness of the interlayer, reflecting the switching behavior of the individual ferromagnetic layers as a function of the interlayer coupling. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.054433
  • Spin wave resonance excitation in ferromagnetic films using planar waveguide structures
    Khivintsev, Y.V. and Reisman, L. and Lovejoy, J. and Adam, R. and Schneider, C.M. and Camley, R.E. and Celinski, Z.J.
    Journal of Applied Physics 108 (2010)
    We explore a new geometry allowing effective excitation of the lowest antisymmetric standing spin wave mode in ferromagnetic metallic films with symmetrical boundary conditions. The approach is based on the use of a coplanar waveguide with the ferromagnetic film, Permalloy (Py), playing the role of the signal line. In addition, we study a signal line which is a sandwich of Py inside two nonmagnetic metallic films. We find that the thickness and conductivity of the metal films can significantly alter the amount of absorption, at ferromagnetic resonance, between the symmetric and antisymmetric spin wave modes. The experimental results are supported by numerical calculations indicating the origin of the strength of the absorption. © 2010 American Institute of Physics.
    view abstract10.1063/1.3435318
  • Standing-wave excited soft x-ray photoemission microscopy: Application to Co microdot magnetic arrays
    Gray, A.X. and Kronast, F. and Papp, C. and Yang, S.-H. and Cramm, S. and Krug, I.P. and Salmassi, F. and Gullikson, E.M. and Hilken, D.L. and Anderson, E.H. and Fischer, P. and Dürr, H.A. and Schneider, C.M. and Fadley, C.S.
    Applied Physics Letters 97 (2010)
    We demonstrate the addition of depth resolution to the usual two-dimensional images in photoelectron emission microscopy (PEEM), with application to a square array of circular magnetic Co microdots. The method is based on excitation with soft x-ray standing-waves generated by Bragg reflection from a multilayer mirror substrate. Standing wave is moved vertically through sample simply by varying the photon energy around the Bragg condition. Depth-resolved PEEM images were obtained for all of the observed elements. Photoemission intensities as functions of photon energy were compared to x-ray optical calculations in order to quantitatively derive the depth-resolved film structure of the sample. © 2010 American Institute of Physics.
    view abstract10.1063/1.3478215
  • Theory of relativistic photoemission for correlated magnetic alloys: LSDA+DMFT study of the electronic structure of Nix Pd1-x
    Braun, J. and Minár, J. and Matthes, F. and Schneider, C.M. and Ebert, H.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Electronic correlations play an important role in determining the properties of solid state systems, in particular, in the presence of narrow bands. In intermetallic alloys the strength of correlation effects and, thus, the details of the electronic structure depend on the concentration of the constituents, their interactions, and the degree of chemical order. Although the electronic structure of such a system can be conveniently studied by photoelectron spectroscopy, the interpretation of the spectra is nontrivial. To enable a quantitative analysis of chemically disordered systems showing correlation effects in photoemission spectroscopy we therefore incorporate dynamical mean-field theory in the fully relativistic version of layer-Korringa-Kohn-Rostoker theory and treat the results within the relativistic one-step model of photoemission generalized to the magnetic alloy case. Our ansatz allows the study of complex layered structures like thin films and multilayers and an almost naturally incorporation of a realistic surface barrier potential. We apply our theory to photoemission data of the magnetic alloy system Nix Pd1-x (001) and demonstrating that state-of-the-art photoemission theory is required to deal with this complex system. The comparison over a large alloy concentration range provides us with a means to disentangle the influence of alloying and correlation effects. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.024411
  • carbon nanotubes

  • electronic structure

  • ferromagnetic materials

  • magnetism

  • photoemission electron microscopy

  • properties of solids

  • spin dynamics

  • spintronics

  • synchrotron-radiation based analytics

  • thin films

  • transport

  • x-ray diffraction

  • x-ray photoelectron spectroscopy

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