Prof. Dr. Mirko Cinchetti

Experimental Physics - Coherent Spin Phenomena in Solids
TU Dortmund University

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  • 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
  • Momentum and energy dissipation of hot electrons in a Pb/Ag(111) quantum well system
    Haag, F. and Eul, T. and Grad, L. and Haag, N. and Knippertz, J. and Mathias, S. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 104 (2021)
    The band structure of multilayer systems plays a crucial role for the ultrafast hot carrier dynamics at interfaces. Here, we study the energy- and momentum-dependent quasiparticle lifetimes of excited electrons in a highly ordered Pb monolayer film on Ag(111) prior and after the adsorption of a monolayer of 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). Using time-resolved two-photon momentum microscopy with femtosecond visible light pulses, we show that the electron dynamics of the Pb/Ag(111) quantum well system is largely dominated by two types of scattering processes: (i) isotropic intraband scattering processes within the quantum well state (QWS) and (ii) isotropic interband scattering processes from the -like QWS into the Pb band. In the latter case, the Pb QWS acts as an electron source for the momentum space refilling process of the Pb band. This conclusion is confirmed by the modification of the band structure and the quasiparticle dynamics of the Pb/Ag(111) bilayer film after the adsorption of PTCDA. We find both an adsorption-induced suppression of the QWS itself as well as of the refilling process into the Pb band. Our study hence demonstrates the isotropic nature of the momentum-dependent scattering processes of metallic bilayer systems and uncovers a new possibility to selectively tune and control scattering processes occurring in quantum (well) materials by the adsorption of organic molecules. ©2021 American Physical Society
    view abstract10.1103/PhysRevB.104.104308
  • Nonlinear Bicolor Holography Using Plasmonic Metasurfaces
    Frese, D. and Wei, Q. and Wang, Y. and Cinchetti, M. and Huang, L. and Zentgraf, T.
    ACS Photonics 8 (2021)
    Nonlinear metasurface holography shows the great potential of metasurfaces to control the phase, amplitude, and polarization of light while simultaneously converting the frequency of the light. The possibility of tailoring the scattering properties of a coherent beam, as well as the scattering properties of nonlinear signals originating from the meta-atoms, facilitates a huge degree of freedom in beam shaping application. Recently, several approaches showed that virtual objects or any kind of optical information can be generated at a wavelength different from the laser input beam. Here, we demonstrate a single-layer nonlinear geometric-phase metasurface made of plasmonic nanostructures for a simultaneous second- and third-harmonic generation. Different from previous works, we demonstrate a two-color hologram with dissimilar types of nanostructures that generate the color information by different nonlinear optical processes. The amplitude ratio of both harmonic signals can be adapted depending on the nanostructures' resonance as well as the power and the wavelength of the incident laser beam. The two-color holographic image is reconstructed in the Fourier space at visible wavelengths with equal amplitudes using a single near-infrared wavelength. Nonlinear holography using multiple nonlinear processes simultaneously provides an alternative path to holographic color display applications, enhanced optical encryption schemes, and multiplexed optical data storage. © 2021 The Authors. Published by American Chemical Society.
    view abstract10.1021/acsphotonics.1c00028
  • Positive magnetoresistance and chiral anomaly in exfoliated type-ii weyl semimetal td-wte2
    Adhikari, R. and Adhikari, S. and Faina, B. and Terschanski, M. and Bork, S. and Leimhofer, C. and Cinchetti, M. and Bonanni, A.
    Nanomaterials 11 (2021)
    Layered van der Waals semimetallic Td-WTe2, exhibiting intriguing properties which include non-saturating extreme positive magnetoresistance (MR) and tunable chiral anomaly, has emerged as a model topological type-II Weyl semimetal system. Here, ∼45 nm thick mechanically exfoliated flakes of Td-WTe2 are studied via atomic force microscopy, Raman spectroscopy, low-T/high-µ0 H magnetotransport measurements and optical reflectivity. The contribution of anisotropy of the Fermi liquid state to the origin of the large positive transverse MR⊥ and the signature of chiral anomaly of the type-II Weyl Fermions are reported. The samples are found to be stable in air and no oxidation or degradation of the electronic properties is observed. A transverse MR⊥ ∼1200 % and an average carrier mobility of 5000 cm2 V−1 s−1 at T = 5 K for an applied perpendicular field µ0 H⊥ = 7 T are established. The system follows a Fermi liquid model for T ≤ 50 K and the anisotropy of the Fermi surface is concluded to be at the origin of the observed positive MR. Optical reflectivity measurements confirm the anisotropy of the electronic behaviour. The relative orientation of the crystal axes and of the applied electric and magnetic fields is proven to determine the observed chiral anomaly in the in-plane magnetotransport. The observed chiral anomaly in the WTe2 flakes is found to persist up to T = 120 K, a temperature at least four times higher than the ones reported to date. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano11102755
  • 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
  • Temperature dependence of the picosecond spin Seebeck effect
    Kholid, F.N. and Hamara, D. and Terschanski, M. and Mertens, F. and Bossini, D. and Cinchetti, M. and McKenzie-Sell, L. and Patchett, J. and Petit, D. and Cowburn, R. and Robinson, J. and Barker, J. and Ciccarelli, C.
    Applied Physics Letters 119 (2021)
    We performed temperature-dependent optical pump-THz emission measurements in Y3Fe5O12 (YIG)|Pt from 5 K to room temperature in the presence of an externally applied magnetic field. We study the temperature dependence of the spin Seebeck effect and observe a continuous increase as temperature is decreased, opposite to what is observed in electrical measurements, where the spin Seebeck effect is suppressed as 0 K is approached. By quantitatively analyzing the different contributions, we isolate the temperature dependence of the spin-mixing conductance and observe features that are correlated with the bands of magnon spectrum in YIG. © 2021 Author(s).
    view abstract10.1063/5.0050205
  • Ultrafast Amplification and Nonlinear Magnetoelastic Coupling of Coherent Magnon Modes in an Antiferromagnet
    Bossini, D. and Pancaldi, M. and Soumah, L. and Basini, M. and Mertens, F. and Cinchetti, M. and Satoh, T. and Gomonay, O. and Bonetti, S.
    Physical Review Letters 127 (2021)
    We investigate the role of domain walls in the ultrafast magnon dynamics of an antiferromagnetic NiO single crystal in a pump-probe experiment with variable pump photon energy. Analyzing the amplitude of the energy-dependent photoinduced ultrafast spin dynamics, we detect a yet unreported coupling between the material's characteristic terahertz- and gigahertz-magnon modes. We explain this unexpected coupling between two orthogonal eigenstates of the corresponding Hamiltonian by modeling the magnetoelastic interaction between spins in different domains. We find that such interaction, in the nonlinear regime, couples the two different magnon modes via the domain walls and it can be optically exploited via the exciton-magnon resonance. © 2021 American Physical Society.
    view abstract10.1103/PhysRevLett.127.077202
  • Ultrafast charge carrier dynamics in potassium-doped endohedral metallofullerene Sc3N@C80 thin films
    Emmerich, S. and Hedwig, S. and Cinchetti, M. and Stadtmüller, B. and Aeschlimann, M.
    Journal of Electron Spectroscopy and Related Phenomena 252 (2021)
    Molecular materials have emerged as highly flexible platform for photovoltaic and light-harvesting applications. One of the most important challenges for this class of materials is the trapping of charge carriers in bound electron–hole pairs, which severely limits the free charge carrier generation. Here, we demonstrate a significant modification of the exciton dynamics in thin films of endohedral metallofullerene complexes upon alkali metal doping. For the exemplary case of Sc3N@C80 thin films, we show that potassium doping results in an additional relaxation channel for the optically excited charge-transfer excitons that prevents the trapping of excitons in a long-lived Frenkel exciton-like state. Instead, potassium doping leads to an ultrafast exciton dissociation and most likely to the generation of free charge carriers. In this way, we propose alkali metal doping of molecular films as a novel approach to enhance the light-to-charge carrier conversion efficiency in photovoltaic molecular materials. © 2021 The Authors
    view abstract10.1016/j.elspec.2021.147110
  • 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
  • Exchange-mediated magnetic blue-shift of the band-gap energy in the antiferromagnetic semiconductor MnTe
    Bossini, D. and Terschanski, M. and Mertens, F. and Springholz, G. and Bonanni, A. and Uhrig, G.S. and Cinchetti, M.
    New Journal of Physics 22 (2020)
    In magnetic semiconductors the optical spectrum and, in particular, the absorption edge representing the band-gap are strongly affected by the onset of the magnetic order. This contribution to the band-gap energy has hitherto been described theoretically in terms of a Heisenberg Hamiltonian, in which a delocalized conduction carrier is coupled to the localized magnetic moments by the exchange interaction. Such models, however, do not take into account the strong correlations displayed in a wide variety of magnetic semiconductors, which are responsible for the formation of the local moments. In particular, the itinerant carrier itself contributes to the spin moment. Here, we overcome this simplification in a combined experimental and theoretical study of the antiferromagnetic semiconductor α-MnTe. First, we present a spectroscopic optical investigation as a function of temperature, from which we extract the magnetic contribution to the blue-shift of the band-gap. Second, we formulate a minimal model based on a Hubbard-Kondo Hamiltonian. In this model, the itinerant charge is one of the electrons forming the localized magnetic moment, which properly captures correlation effects in the material. Our theory reproduces the experimental findings with excellent quantitative agreement, demonstrating that the magnetic contribution to the band-gap energy of α-MnTe is mediated solely by the exchange interaction. These results describe an intrinsic property of the material, independent of the thickness, substrate and capping layer of the specimen. One of the key findings of the model is that the basic effect, namely a blue-shift of the band-gap due to the establishment of the magnetic order, is a general phenomenon in charge-transfer insulators. The identification of the relevant magnetic interaction discloses the possibility to exploit the effect here discussed to induce a novel regime of coherent spin dynamics, in which spin oscillations on a characteristic time-scale of 100 fs are triggered and are intrinsically coupled to charges. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/aba0e7
  • 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
  • Signatures of an atomic crystal in the band structure of a C60 thin film
    Haag, N. and Lüftner, D. and Haag, F. and Seidel, J. and Kelly, L.L. and Zamborlini, G. and Jugovac, M. and Feyer, V. and Aeschlimann, M. and Puschnig, P. and Cinchetti, M. and Stadtmüller, B.
    Physical Review B 101 (2020)
    Transport phenomena in molecular materials are intrinsically linked to the orbital character and the degree of localization of the valence states. Here we combine angle-resolved photoemission with photoemission tomography to determine the spatial distribution of all molecular states of the valence band structure of a C60 thin film. While the two most frontier valence states exhibit a strong band dispersion, the states at larger binding energies are characterized by distinct emission patterns in energy and momentum space. Our findings demonstrate the formation of an atomic crystal-like band structure in a molecular solid with delocalized π-like valence states and strongly localized σ states at larger binding energies. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.165422
  • Ultrafast charge-transfer exciton dynamics in C60 thin films
    Emmerich, S. and Hedwig, S. and Arnoldi, B. and Stöckl, J. and Haag, F. and Hemm, R. and Cinchetti, M. and Mathias, S. and Stadtmüller, B. and Aeschlimann, M.
    Journal of Physical Chemistry C 124 (2020)
    The high flexibility of organic molecules offers great potential for designing the optical properties of optically active materials for the next generation of optoelectronic and photonic applications. However, despite successful implementations of molecular materials in today's display and photovoltaic technology, many fundamental aspects of the light-to-charge conversion in molecular materials have still to be uncovered. Here, we focus on the ultrafast dynamics of optically excited excitons in C60 thin films depending on the molecular coverage and the light polarization of the optical excitation. Using time- and momentum-resolved photoemission with femtosecond extreme ultraviolet (fs-XUV) radiation, we follow the exciton dynamics in the excited states while simultaneously monitoring the signatures of the excitonic charge character in the renormalization of the molecular valence band structure. Optical excitation with visible light results in the instantaneous formation of charge-transfer (CT) excitons, which transform stepwise into Frenkel-like excitons at lower energies. The number and energetic position of the CT and Frenkel-like excitons within this cascade process are independent of the molecular coverage and the light polarization of the optical excitation. In contrast, the depopulation times of the CT and Frenkel-like excitons depend on the molecular coverage, while the excitation efficiency of CT excitons is determined by the light polarization. Our comprehensive study reveals the crucial role of CT excitons for the excited-state dynamics of homomolecular fullerene materials and thin films. © 2020 American Chemical Society
    view abstract10.1021/acs.jpcc.0c08011
  • Ultrafast optically induced spin transfer in ferromagnetic alloys
    Hofherr, M. and Häuser, S. and Dewhurst, J.K. and Tengdin, P. and Sakshath, S. and Nembach, H.T. and Weber, S.T. and Shaw, J.M. and Silva, T.J. and Kapteyn, H.C. and Cinchetti, M. and Rethfeld, B. and Murnane, M.M. and Steil, D. and Stadtmüller, B. and Sharma, S. and Aeschlimann, M. and Mathias, S.
    Science Advances 6 (2020)
    The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism. Copyright © 2020 The Authors,
    view abstract10.1126/sciadv.aay8717
  • Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy
    Knippertz, J. and Kelly, L.L. and Franke, M. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 102 (2020)
    Molecular materials enable a vast variety of functionalities for novel electronic and spintronic devices. The unique possibility to alter organic molecules or metallic substrates offers the opportunity to optimize interfacial properties for almost any desired field of application. For this reason, we extend the successful approach to control metal-organic interfaces by surface alloying. We present a comprehensive characterization of the structural and electronic properties of the interface formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown on an Ag(111) single crystal surface. We monitor the changes of adsorption height of the surface alloy atoms and electronic valence band structure upon adsorption of one layer of PTCDA using the normal incidence X-ray standing wave technique in combination with momentum-resolved photoelectron spectroscopy. We find that the vertical buckling and the surface band structure of the SnAg2 surface alloy is not altered by the adsorption of one layer of PTCDA, in contrast to our recent study of PTCDA on a PbAg2 surface alloy [B. Stadtmüller, Phys. Rev. Lett. 117, 096805 (2016)PRLTAO0031-900710.1103/PhysRevLett.117.096805]. In addition, the vertical adsorption geometry of PTCDA and the interfacial energy level alignment indicate the absence of any chemical interaction between the molecule and the surface alloy. We attribute the different interactions at these PTCDA/surface alloy interfaces to the presence or absence of local σ-bonds between the PTCDA oxygen atoms and the surface atoms. Combining our findings with results from literature, we are able to propose an empiric rule for engineering the surface band structure of alloys by adsorption of organic molecules. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.075447
  • 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
  • Wide spectral range ultrafast pump-probe magneto-optical spectrometer at low temperature, high-magnetic and electric fields
    Mertens, F. and Terschanski, M. and Mönkebüscher, D. and Ponzoni, S. and Bossini, D. and Cinchetti, M.
    Review of Scientific Instruments 91 (2020)
    We developed a table-top setup to perform magneto-optical pump-probe measurements with the possibility to independently tune the photon-energy of both pump and probe beams in the 0.5 eV-3.5 eV range. Our apparatus relies on a commercial turn-key amplified laser system, able to generate light pulses with duration shorter than or comparable to 100 fs throughout the whole spectral range. The repetition rate of the source can be modified via the computer in the 1 kHz to 1 MHz range. A commercial balanced detector is connected to a high-frequency digitizer, allowing for a highly-sensitive detection scheme: rotations of the probe polarization as small as 70 μdeg can be measured. Additionally, a DC magnetic field as high as 9 T and voltages in the kV regime can be applied on the sample. A cryostat allows us to precisely set the temperature of the specimen in the 4 K-420 K interval. We prove the performance of our setup by measuring the ultrafast demagnetization of a cobalt crystal as a function of a wide variety of experimental parameters. © 2020 Author(s).
    view abstract10.1063/5.0024449
  • A case study for the formation of stanene on a metal surface
    Maniraj, M. and Stadtmüller, B. and Jungkenn, D. and Düvel, M. and Emmerich, S. and Shi, W. and Stöckl, J. and Lyu, L. and Kollamana, J. and Wei, Z. and Jurenkow, A. and Jakobs, S. and Yan, B. and Steil, S. and Cinchetti, M. and Mathias, S. and Aeschlimann, M.
    Communications Physics 2 (2019)
    The discovery and realization of graphene as an ideal two-dimensional (2D) material has triggered extensive efforts to create similar 2D materials with exciting spin-dependent properties. Here, we report on a novel Sn 2D superstructure on Au(111) that shows similarities and differences to the expected electronic features of ideal stanene. Using spin- and angle-resolved photoemission spectroscopy, we find that a particular Sn/Au superstructure reveals a linearly dispersing band centered at the Γ ¯ -point and below the Fermi level with anti-parallel spin polarization and a Fermi velocity of vF ≈ 1×106 m/s, the same value as for graphene. We attribute the origin of the band structure to the hybridization between the Sn and the Au orbitals at the 2D Sn-Au interface. Considering that free-standing stanene simply cannot exist, our investigated structure is an important step towards the search of useful stanene-like overstructures for future technological applications. © 2019, The Author(s).
    view abstract10.1038/s42005-019-0111-2
  • Enhancing Light Emission in Interface Engineered Spin-OLEDs through Spin-Polarized Injection at High Voltages
    Prieto-Ruiz, J.P. and Miralles, S.G. and Prima-García, H. and López-Muñoz, A. and Riminucci, A. and Graziosi, P. and Aeschlimann, M. and Cinchetti, M. and Dediu, V.A. and Coronado, E.
    Advanced Materials 31 (2019)
    The quest for a spin-polarized organic light-emitting diode (spin-OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin-OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light-emitting device showing spin-valve effects at high voltages (up to 14 V). This allows the detection of a magneto-electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long-standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adma.201806817
  • Modification of Pb quantum well states by the adsorption of organic molecules
    Stadtmüller, B. and Grad, L. and Seidel, J. and Haag, F. and Haag, N. and Cinchetti, M. and Aeschlimann, M.
    Journal of physics. Condensed matter : an Institute of Physics journal 31 (2019)
    The successful implementation of nanoscale materials in next generation optoelectronic devices crucially depends on our ability to functionalize and design low dimensional materials according to the desired field of application. Recently, organic adsorbates have revealed an enormous potential to alter the occupied surface band structure of tunable materials by the formation of tailored molecule-surface bonds. Here, we extend this concept of adsorption-induced surface band structure engineering to the unoccupied part of the surface band structure. This is achieved by our comprehensive investigation of the unoccupied band structure of a lead (Pb) monolayer film on the Ag(1 1 1) surface prior and after the adsorption of one monolayer of the aromatic molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA). Using two-photon momentum microscopy, we show that the unoccupied states of the Pb/Ag(1 1 1) bilayer system are dominated by a parabolic quantum well state (QWS) in the center of the surface Brillouin zone with Pb p[Formula: see text] orbital character and a side band with almost linear dispersion showing Pb p[Formula: see text] orbital character. After the adsorption of PTCDA, the Pb side band remains completely unaffected while the signal of the Pb QWS is fully suppressed. This adsorption induced change in the unoccupied Pb band structure coincides with an interfacial charge transfer from the Pb layer into the PTCDA molecule. We propose that this charge transfer and the correspondingly vertical (partially chemical) interaction across the PTCDA/Pb interface suppresses the existence of the QWS in the Pb layer. Our results hence unveil a new possibility to orbital selectively tune and control the entire surface band structure of low dimensional systems by the adsorption of organic molecules.
    view abstract10.1088/1361-648X/aafcf5
  • Molecular spectroscopy in a solid-state device
    Atxabal, A. and Arnold, T. and Parui, S. and Zuccatti, E. and Cinchetti, M. and Casanova, F. and Ortmann, F. and Hueso, L.E.
    Materials Horizons 6 (2019)
    The quantification of the electronic transport energy gap of a molecular semiconductor is essential for pursuing any challenge in molecular optoelectronics. However, this remains largely elusive because of the difficulties in its determination by conventional spectroscopic methods. This communication presents an in-device molecular spectroscopy (i-MOS) technique, which permits measuring this gap seamlessly, in real device operative conditions, at room temperature and without any previous knowledge of the material's parameters. This result is achieved by determining the occupied and unoccupied molecular orbitals of an organic semiconductor thin-film by using a single three terminal solid-state device. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9mh00218a
  • Strong modification of the transport level alignment in organic materials after optical excitation
    Stadtmüller, B. and Emmerich, S. and Jungkenn, D. and Haag, N. and Rollinger, M. and Eich, S. and Maniraj, M. and Aeschlimann, M. and Cinchetti, M. and Mathias, S.
    Nature Communications 10 (2019)
    Organic photovoltaic devices operate by absorbing light and generating current. These two processes are governed by the optical and transport properties of the organic semiconductor. Despite their common microscopic origin—the electronic structure—disclosing their dynamical interplay is far from trivial. Here we address this issue by time-resolved photoemission to directly investigate the correlation between the optical and transport response in organic materials. We reveal that optical generation of non-interacting excitons in a fullerene film results in a substantial redistribution of all transport levels (within 0.4 eV) of the non-excited molecules. As all observed dynamics evolve on identical timescales, we conclude that optical and transport properties are completely interlinked. This finding paves the way for developing novel concepts for transport level engineering on ultrafast time scales that could lead to novel functional optoelectronic devices. © 2019, The Author(s).
    view abstract10.1038/s41467-019-09136-7
  • Tuning the charge flow between Marcus regimes in an organic thin-film device
    Atxabal, A. and Arnold, T. and Parui, S. and Hutsch, S. and Zuccatti, E. and Llopis, R. and Cinchetti, M. and Casanova, F. and Ortmann, F. and Hueso, L.E.
    Nature Communications 10 (2019)
    Marcus’s theory of electron transfer, initially formulated six decades ago for redox reactions in solution, is now of great importance for very diverse scientific communities. The molecular scale tunability of electronic properties renders organic semiconductor materials in principle an ideal platform to test this theory. However, the demonstration of charge transfer in different Marcus regions requires a precise control over the driving force acting on the charge carriers. Here, we make use of a three-terminal hot-electron molecular transistor, which lets us access unconventional transport regimes. Thanks to the control of the injection energy of hot carriers in the molecular thin film we induce an effective negative differential resistance state that is a direct consequence of the Marcus Inverted Region. © 2019, The Author(s).
    view abstract10.1038/s41467-019-10114-2
  • Adsorption-induced pyramidal distortion of the trimetallic nitride core inside the endohedral fullerene Sc3 N@ C80 on the Ag(111) surface
    Seidel, J. and Kelly, L.L. and Franke, M. and Van Straaten, G. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M. and Stadtmüller, B.
    Physical Review B 98 (2018)
    Our ability to understand and tailor metal-organic interfaces is mandatory to functionalize organic complexes for next generation electronic and spintronic devices. For magnetic data storage applications, metal-carrying organic molecules, the so-called single molecular magnets (SMM) are of particular interest as they yield the possibility to store information on the molecular scale. In this work, we focus on the adsorption properties of the prototypical SMM Sc3N@C80 grown in a monolayer film on the Ag(111) substrate. We provide clear evidence of a pyramidal distortion of the otherwise planar Sc3N core inside the carbon cage upon the adsorption on the Ag(111) surface. This adsorption-induced structural change of the Sc3N@C80 molecule can be correlated to a charge transfer from the substrate into the lowest unoccupied molecular orbital of Sc3N@C80, which significantly alters the charge density of the fullerene core. Our comprehensive characterization of the Sc3N@C80-Ag(111) interface hence reveals an indirect coupling mechanism between the Sc3N core of the fullerene molecule and the noble metal surface mediated via an interfacial charge transfer. Our work shows that such an indirect coupling between the encapsulated metal centers of SMM and metal surfaces can strongly affect the geometric structure of the metallic centers and thereby potentially also alters the magnetic properties of SMMs on surfaces. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.085434
  • Control of Cooperativity through a Reversible Structural Phase Transition in MoMo-Methyl/Cu(111)
    Kollamana, J. and Wei, Z. and Lyu, L. and Zimmer, M. and Dietrich, F. and Eul, T. and Stöckl, J. and Maniraj, M. and Ponzoni, S. and Cinchetti, M. and Stadtmüller, B. and Gerhards, M. and Aeschlimann, M.
    Advanced Functional Materials 28 (2018)
    The bimetallic molecular compound Dimolybdenum tetraacetate (MoMo-Methyl) is grown on a Cu(111) surface with submonolayer coverage. Scanning tunneling microscopy experiments reveal that the compound forms two different structural phases on the Cu surface, whose ratio can be reversibly controlled by changing the sample temperature. The so-called chain-phase is characterized by tilted MoMo dimers bonded to the Cu surface via the methyl groups. In the so-called mesh-phase, on the other hand, the molecules adsorb in a flat lying adsorption configuration with one of the Mo-atoms in direct contact with the Cu surface. Crucially, the different structural properties of the two phases reflect the different inter- and intramolecular interactions between the Mo metal centers, as well as the different interactions between Mo and the Cu surface atoms. In this way, the structural changes result in a modification of the cooperative effects in the system. Therefore, it is proposed that the observed reversible structural phase transition could be used to control the strength of cooperative effects in MoMo-Methyl on Cu(111). © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/adfm.201703544
  • Induced versus intrinsic magnetic moments in ultrafast magnetization dynamics
    Hofherr, M. and Moretti, S. and Shim, J. and Häuser, S. and Safonova, N.Y. and Stiehl, M. and Ali, A. and Sakshath, S. and Kim, J.W. and Kim, D.H. and Kim, H.J. and Hong, J.I. and Kapteyn, H.C. and Murnane, M.M. and Cinchetti, M. and Steil, D. and Mathias, S. and Stadtmüller, B. and Albrecht, M. and Kim, D.E. and Nowak, U. and Aeschlimann, M.
    Physical Review B 98 (2018)
    Ferromagnetic metal alloys are today commonly used in spintronic and magnetic data storage devices. These multicompound structures consist of several magnetic sublattices exhibiting both intrinsic and induced magnetic moments. Here, we study the response of the element-specific magnetization dynamics for thin film systems based on purely intrinsic (CoFeB) and partially induced (FePt) magnetic moments using extreme ultraviolet pulses from high-harmonic generation (HHG) as an element-sensitive probe. In FePt, on the one hand, we observe an identical normalized transient magnetization for Fe and Pt throughout both the ultrafast demagnetization and the subsequent remagnetization. On the other hand, Co and Fe show a clear difference in the asymptotic limit of the remagnetization process in CoFeB, which is supported by calculations for the temperature-dependent behavior of the equilibrium magnetization using a dynamic spin model. Thus, in this work, we provide a vital step toward a comprehensive understanding of ultrafast light-induced magnetization dynamics in ferromagnetic alloys with sublattices of intrinsic and induced magnetic moments. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.174419
  • Spin- and Angle-Resolved Photoemission Study of the Alq3/Co Interface
    Stöckl, J. and Jurenkow, A. and Großmann, N. and Cinchetti, M. and Stadtmüller, B. and Aeschlimann, M.
    Journal of Physical Chemistry C 122 (2018)
    Controlling the interaction between organic semiconductors and ferromagnetic surfaces is one of the key issues for designing metal-organic hybrid interfaces for spintronic applications. The strong chemical interaction across such hybrid interfaces results in the formation of new spin-polarized hybrid interface states which determine all device-relevant properties. Here, we revisit the hybrid interface formed between the prototypical molecule Alq3 and the Co surface using spin- and angle-resolved photoemission. We reveal a significant change of the spectroscopic lineshape of the cobalt 3d bands by the adsorption of Alq3. The hole-like minority and the electron-like majority bands of the bare Co surface are replaced by an energetically very broad band with neglectable band dispersion along the Γ -X direction. Moreover, the magnitude and shape of the spin polarization of the Alq3/Co valence band structure are also significantly modified by the adsorption of Alq3 and become completely independent of the momentum space positions along the Γ -X direction. Our findings are attributed to an elastic scattering of the Co photoelectrons at the disordered Alq3 overlayer, leading to a redistribution of the spin-dependent spectral intensity in momentum space. A careful analysis of our data shows that such elastic scattering takes place without significant spin-flip scattering processes and that the spectral feature of the highest occupied molecular orbital of Alq3 is fully unpolarized. © 2018 American Chemical Society.
    view abstract10.1021/acs.jpcc.7b10825
  • Structure and electronic properties of the (3 × 3)R30 SnA u2/Au(111) surface alloy
    Maniraj, M. and Jungkenn, D. and Shi, W. and Emmerich, S. and Lyu, L. and Kollamana, J. and Wei, Z. and Yan, B. and Cinchetti, M. and Mathias, S. and Stadtmüller, B. and Aeschlimann, M.
    Physical Review B 98 (2018)
    We have investigated the atomic and electronic structure of the (3×3)R30SnAu2/Au(111) surface alloy. Low-energy electron diffraction and scanning tunneling microscopy measurements show that the native herringbone reconstruction of bare Au(111) surface remains intact after formation of a long-range ordered (3×3)R30SnAu2/Au(111) surface alloy. Angle-resolved photoemission and two-photon photoemission spectroscopy techniques reveal Rashba-type spin-split bands in the occupied valence band with comparable momentum space splitting as observed for the Au(111) surface state, but with a hole-like parabolic dispersion. Our experimental findings are compared with density functional theory (DFT) calculation that fully support our experimental findings. Taking advantage of the good agreement between our DFT calculations and the experimental results, we are able to extract that the occupied Sn-Au hybrid band is of (s,d)-orbital character, while the unoccupied Sn-Au hybrid bands are of (p,d)-orbital character. Hence we can conclude that the Rashba-type spin splitting of the hole-like Sn-Au hybrid surface state is caused by the significant mixing of Au d with Sn s states in conjunction with the strong atomic spin-orbit coupling of Au, i.e., of the substrate. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.205419
  • Activating the molecular spinterface
    Cinchetti, M. and Dediu, V.A. and Hueso, L.E.
    16 (2017)
    The miniaturization trend in the semiconductor industry has led to the understanding that interfacial properties are crucial for device behaviour. Spintronics has not been alien to this trend, and phenomena such as preferential spin tunnelling, the spin-to-charge conversion due to the Rashba-Edelstein effect and the spin-momentum locking at the surface of topological insulators have arisen mainly from emergent interfacial properties, rather than the bulk of the constituent materials. In this Perspective we explore inorganic/molecular interfaces by looking closely at both sides of the interface. We describe recent developments and discuss the interface as an ideal platform for creating new spin effects. Finally, we outline possible technologies that can be generated thanks to the unique active tunability of molecular spinterfaces. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
    view abstract10.1038/nmat4902
  • 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
  • Cavity-assisted ultrafast long-range periodic energy transfer between plasmonic nanoantennas
    Aeschlimann, M. and Brixner, T. and Cinchetti, M. and Frisch, B. and Hecht, B. and Hensen, M. and Huber, B. and Kramer, C. and Krauss, E. and Loeber, T.H. and Pfeiffer, W. and Piecuch, M. and Thielen, P.
    Light: Science and Applications 6 (2017)
    view abstract10.1038/lsa.2017.111
  • Design of Molecular Spintronics Devices Containing Molybdenum Oxide as Hole Injection Layer
    Prieto-Ruiz, J.P. and Miralles, S.G. and Großmann, N. and Aeschlimann, M. and Cinchetti, M. and Prima-García, H. and Coronado, E.
    3 (2017)
    view abstract10.1002/aelm.201600366
  • Speed and efficiency of femtosecond spin current injection into a nonmagnetic material
    Hofherr, M. and Maldonado, P. and Schmitt, O. and Berritta, M. and Bierbrauer, U. and Sadashivaiah, S. and Schellekens, A.J. and Koopmans, B. and Steil, D. and Cinchetti, M. and Stadtmüller, B. and Oppeneer, P.M. and Mathias, S. and Aeschlimann, M.
    Physical Review B 96 (2017)
    We investigate femtosecond spin injection from an optically excited Ni top layer into an Au bottom layer using time-resolved complex magneto-optical Kerr effect (C-MOKE) measurements. Employing the C-MOKE formalism, we are able to follow layer-resolved demagnetization in Ni and the simultaneous spin injection into the adjacent Au film, both occurring within ∼40fs. We confirm the ballistic to diffusive propagation of the spin transfer process with ab initio theory and superdiffusive transport calculations. In particular, our combined experimental-theoretical effort does allow us to quantify the so far elusive amount of spin injection, and therefore the spin injection efficiency at the interface. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.100403
  • Adsorption heights and bonding strength of organic molecules on a Pb-Ag surface alloy
    Stadtmüller, B. and Haag, N. and Seidel, J. and Van Straaten, G. and Franke, M. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M.
    (2016)
    The understanding of the fundamental geometric and electronic properties of metal-organic hybrid interfaces is a key issue on the way to improving the performance of organic electronic and spintronic devices. Here, we studied the adsorption heights of copper-II-phthalocyanine (CuPc) and 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on a Pb1Ag2 surface alloy on Ag(111) using the normal-incidence x-ray standing waves technique. We find a significantly larger adsorption height of both molecules on the Pb-Ag surface alloy compared to the bare Ag(111) surface which is caused by the larger size of Pb. This increased adsorption height suppresses the partial chemical interaction of both molecules with Ag surface atoms. Instead, CuPc and PTCDA molecules bond only to the Pb atoms with different interaction strength ranging from a van der Waals-like interaction for CuPc to a weak chemical interaction with additional local bonds for PTCDA. The different adsorption heights for CuPc and PTCDA on Pb1Ag2 are the result of local site-specific molecule-surface bonds mediated by functional molecular groups and the different charge donating and accepting character of CuPc and PTCDA. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.235436
  • Controlled manipulation of the Co-Alq3 interface by rational design of Alq3 derivatives
    Großmann, N. and Magri, A. and Laux, M. and Stadtmüller, B. and Thielen, P. and Schäfer, B. and Fuhr, O. and Ruben, M. and Cinchetti, M. and Aeschlimann, M.
    45 (2016)
    Recently, research has revealed that molecules can be used to steer the local spin properties of ferromagnetic surfaces. One possibility to manipulate ferromagnetic-metal-molecule interfaces in a controlled way is to synthesize specific, non-magnetic molecules to obtain a desired interaction with the ferromagnetic substrate. Here, we have synthesized derivatives of the well-known semiconductor Alq3 (with q = 8-hydroxyquinolinate), in which the 8-hydroxyquinolinate ligands are partially or completely replaced by similar ligands bearing O- or N-donor sets. The goal of this study was to investigate how the presence of (i) different donor atom sets and (ii) aromaticity in different conjugated π-systems influences the spin properties of the metal-molecule interface formed with a Co(100) surface. The spin-dependent metal-molecule-interface properties have been measured by spin-resolved photoemission spectroscopy, backed up by DFT calculations. Overall, our results show that, in the case of the Co-molecule interface, chemical synthesis of organic ligands leads to specific electronic properties of the interface, such as exciton formation or highly spin-polarized interface states. We find that these properties are even additive, i.e. they can be engineered into one single molecular system that incorporates all the relevant ligands. © The Royal Society of Chemistry.
    view abstract10.1039/c6dt03183h
  • Dynamic spin filtering at the Co/Alq3 interface mediated by weakly coupled second layer molecules
    Droghetti, A. and Thielen, P. and Rungger, I. and Haag, N. and Großmann, N. and Stöckl, J. and Stadtmüller, B. and Aeschlimann, M. and Sanvito, S. and Cinchetti, M.
    7 (2016)
    Spin filtering at organic-metal interfaces is often determined by the details of the interaction between the organic molecules and the inorganic magnets used as electrodes. Here we demonstrate a spin-filtering mechanism based on the dynamical spin relaxation of the long-living interface states formed by the magnet and weakly physisorbed molecules. We investigate the case of Alq3 on Co and, by combining two-photon photoemission experiments with electronic structure theory, show that the observed long-time spin-dependent electron dynamics is driven by molecules in the second organic layer. The interface states formed by physisorbed molecules are not spin-split, but acquire a spin-dependent lifetime, that is the result of dynamical spin-relaxation driven by the interaction with the Co substrate. Such spin-filtering mechanism has an important role in the injection of spin-polarized carriers across the interface and their successive hopping diffusion into successive molecular layers of molecular spintronics devices. © 2016 The Author(s).
    view abstract10.1038/ncomms12668
  • Epitaxial growth of thermally stable cobalt films on Au(111)
    Haag, N. and Laux, M. and Stöckl, J. and Kollamana, J. and Seidel, J. and Großmann, N. and Fetzer, R. and Kelly, L.L. and Wei, Z. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M.
    18 (2016)
    Ferromagnetic thin films play a fundamental role in spintronic applications as a source for spin polarized carriers and in fundamental studies as ferromagnetic substrates. However, it is challenging to produce such metallic films with high structural quality and chemical purity on single crystalline substrates since the diffusion barrier across the metal-metal interface is usually smaller than the thermal activation energy necessary for smooth surface morphologies. Here, we introduce epitaxial thin Co films grown on an Au(111) single crystal surface as a thermally stable ferromagnetic thin film. Our structural investigations reveal an identical growth of thin Co/Au(111) films compared to Co bulk single crystals with large monoatomic Co terraces with an average width of 500 Å, formed after thermal annealing at 575 K. Combining our results from photoemission and Auger electron spectroscopy, we provide evidence that no significant diffusion of Au into the near surface region of the Co film takes place for this temperature and that no Au capping layer is formed on top of Co films. Furthermore, we show that the electronic valence band is dominated by a strong spectral contribution from a Co 3d band and a Co derived surface resonance in the minority band. Both states lead to an overall negative spin polarization at the Fermi energy. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/18/10/103054
  • Impact of CoFe buffer layers on the structural and electronic properties of the Co2MnSi/MgO interface
    Fetzer, R. and Liu, H.-X. and Stadtmüller, B. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    49 (2016)
    The latest improvement of MgO-based magnetic tunnel junctions has been achieved by the combination of CoFe buffer layers and potentially half-metallic ultrathin Co2MnSi electrodes. By this, tunnel magnetoresistance ratios of almost 2000% could be obtained. However, a complete understanding of the underlying processes leading to this enhancement is not yet given. We present a comprehensive study regarding the structural and electronic spin properties of the CoFe(30 nm)-buffered Co2MnSi(3 nm)/MgO(2 nm) buried interface identical to the one formed in actual devices. Low energy electron diffraction experiments show that the ultrathin Co2MnSi layer adopts the lattice constant of the underlying CoFe buffer layer, leading to improved structural conditions at the interface to MgO. In contrast, the Co2MnSi/MgO interface spin polarization at the Fermi level is not affected by the magnetic CoFe buffer layer, as found by interface-sensitive spin-resolved extremely low energy photoemission spectroscopy. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/49/19/195002
  • Light Localization and Magneto-Optic Enhancement in Ni Antidot Arrays
    Rollinger, M. and Thielen, P. and Melander, E. and Östman, E. and Kapaklis, V. and Obry, B. and Cinchetti, M. and García-Martín, A. and Aeschlimann, M. and Papaioannou, E.Th.
    16 (2016)
    We reveal an explicit strategy to design the magneto-optic response of a magneto-plasmonic crystal by correlating near- and far-fields effects. We use photoemission electron microscopy to map the spatial distribution of the electric near-field on a nanopatterned magnetic surface that supports plasmon polaritons. By using different photon energies and polarization states of the incident light we reveal that the electric near-field is either concentrated in spots forming a hexagonal lattice with the same symmetry as the Ni nanopattern or in stripes oriented along the Γ-K direction of the lattice and perpendicular to the polarization direction. We show that the polarization-dependent near-field enhancement on the patterned surface is directly correlated to both the excitation of surface plasmon polaritons on the patterned surface as well as the enhancement of the polar magneto-optical Kerr effect. We obtain a relationship between the size of the enhanced magneto-optical behavior and the polarization and wavelength of optical excitation. The engineering of the magneto-optic response based on the plasmon-induced modification of the optical properties introduces the concept of a magneto-plasmonic meta-structure. © 2016 American Chemical Society.
    view abstract10.1021/acs.nanolett.5b05279
  • Modifying the Surface of a Rashba-Split Pb-Ag Alloy Using Tailored Metal-Organic Bonds
    Stadtmüller, B. and Seidel, J. and Haag, N. and Grad, L. and Tusche, C. and Van Straaten, G. and Franke, M. and Kirschner, J. and Kumpf, C. and Cinchetti, M. and Aeschlimann, M.
    117 (2016)
    Hybridization-related modifications of the first metal layer of a metal-organic interface are difficult to access experimentally and have been largely neglected so far. Here, we study the influence of specific chemical bonds (as formed by the organic molecules CuPc and PTCDA) on a Pb-Ag surface alloy. We find that delocalized van der Waals or weak chemical π-type bonds are not strong enough to alter the alloy, while localized σ-type bonds lead to a vertical displacement of the Pb surface atoms and to changes in the alloy's surface band structure. Our results provide an exciting platform for tuning the Rashba-type spin texture of surface alloys using organic molecules. © 2016 American Physical Society.
    view abstract10.1103/PhysRevLett.117.096805
  • Scanning Tunneling Microscopy Study of Ordered C60 Submonolayer Films on Co/Au(111)
    Kollamana, J. and Wei, Z. and Laux, M. and Stöckl, J. and Stadtmüller, B. and Cinchetti, M. and Aeschlimann, M.
    120 (2016)
    We demonstrate the successful preparation of ordered C60 films on Co/Au(111) by scanning tunneling microscopy (STM). In particular, we show that the C60 molecules arrange in a close-packed hexagonal pattern after postdeposition annealing at T = 350 °C. From STM measurements with intramolecular resolution we find domains with different orientational ordering, that is ultimately a result of the lateral arrangement of the C60 molecules on the Co surface. The local ordering of the various domains is also clearly reflected in the measured differential conductance spectra. In particular, we find an energetic shift of the LUMO + 1 due to the reconstruction of the cobalt substrate. Our study opens the way for the introduction of ordered "spinterfaces" in molecular spintronics. © 2016 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b12365
  • Spin-resolved photoemission spectroscopy of the Heusler compound co2MnSi
    Fetzer, R. and Aeschlimann, M. and Cinchetti, M.
    222 (2016)
    The Heusler compound Co2MnSi (CMS) is probably the most extensively studied half-metallic material. Its peculiar spin-dependent electronic properties have been addressed both theoretically and experimentally; they also already found wide application in prototypical spintronics devices. In this chapter we review our spinresolved photoemission studies conducted on the free surface of both stoichiometric and off-stoichiometric Co2MnαSi samples. Our findings shed light on the peculiar dependence observed in the performance of CMS-based magnetic tunnel junctions on theMn composition α.We also reviewour studies on the buried CMS/MgO interface, that could be directly investigated by spin-resolved low-energy photoemission spectroscopy. With this method we can then access the spin and symmetry properties of the interface electronic wave functions, which are ultimately responsible for the performance of CMS-based spintronic devices. In particular, the collected experimental data led us conclude that non-collinear spin moments at the CMS/MgO interface might be the main mechanism behind the controversially discussed temperaturedependent performance loss of state-of-the-art CMS-based magnetic tunnel junctions. © Springer International Publishing Switzerland 2016.
    view abstract10.1007/978-3-319-21449-8_3
  • All-optical switching in CoTb alloys: Composition and thickness dependent studies
    Bierbrauer, U. and Alebrand, S. and Hehn, M. and Gottwald, M. and Steil, D. and Lacour, D. and Fullerton, E.E. and Mangin, S. and Cinchetti, M. and Aeschlimann, M.
    159 (2015)
    We investigate all-optical switching (AOS) in TbCo alloys, demonstrating that AOS occurs in a limited Tb concentration range and above a threshold thickness of the magnetic layer. Overall, we find that the trend of the minimum laser fluence needed for light-induced domain formation depends on the sample composition. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_76
  • Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules
    Jakobs, S. and Narayan, A. and Stadtmüller, B. and Droghetti, A. and Rungger, I. and Hor, Y.S. and Klyatskaya, S. and Jungkenn, D. and Stöckl, J. and Laux, M. and Monti, O.L.A. and Aeschlimann, M. and Cava, R.J. and Ruben, M. and Mathias, S. and Sanvito, S. and Cinchetti, M.
    15 (2015)
    We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi<inf>2</inf>Se<inf>3</inf>. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi<inf>2</inf>Se<inf>3</inf>. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that, by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface properties-passivation, spin-texture tuning, and creation of hybrid interface states-lay a solid foundation for interface-assisted molecular spintronics in spin-textured materials. © 2015 American Chemical Society.
    view abstract10.1021/acs.nanolett.5b02213
  • Electron lifetimes in a 2D electron-gas with rashba SO-coupling: Screening properties
    Vollmar, S. and Ruffing, A. and Jakobs, S. and Baral, A. and Kaltenborn, S. and Cinchetti, M. and Aeschlimann, M. and Mathias, S. and Schneider, H.C.
    159 (2015)
    We calculate lifetimes due to electron-electron scattering in a 2D Rashba band structure and study the influence of the substrate screening. A comparison with measurements on the quantum-well system Bi/Cu(111) is presented. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_56
  • Electronic scattering dynamics and ultrafast magnetization dynamics
    Aeschlimann, M. and Steil, D. and Cinchetti, M. and Schneider, H.C.
    159 (2015)
    A dynamical model for Elliott-Yafet type scattering of carriers and its importance for the demagnetization dynamics after ultrafast optical excitation is reviewed. It is pointed out that the demagnetization in 3d ferromagnets as well as recent experimental results on “novel” materials are still in need of a microscopic explanation. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_9
  • Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and Rodan, S. and Blum, C.G.F. and Balke, B. and Wurmehl, S. and Aeschlimann, M. and Cinchetti, M.
    48 (2015)
    Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics. Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 - xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/16/164016
  • Influence of alkylphosphonic acid grafting on the electronic and magnetic properties of La2/3Sr1/3MnO3 surfaces
    Galbiati, M. and Tatay, S. and Delprat, S. and Barraud, C. and Cros, V. and Jacquet, E. and Coloma, F. and Choueikani, F. and Otero, E. and Ohresser, P. and Haag, N. and Cinchetti, M. and Aeschlimann, M. and Seneor, P. and Mattana, R. and Petroff, F.
    353 (2015)
    Self-Assembled monolayers (SAMs) are highly promising materials for molecular engineering of electronic and spintronics devices thanks to their surface functionalization properties. In this direction, alkylphosphonic acids have been used to functionalize the most common ferromagnetic electrode in organic spintronics: La<inf>2/3</inf>Sr<inf>1/3</inf>MnO<inf>3</inf> (LSMO). However, a study on the influence of SAMs grafting on LSMO electronic and magnetic properties is still missing. In this letter, we probe the influence of alkylphosphonic acids-based SAMs on the electronic and magnetic properties of the LSMO surface using different spectroscopies. We observe by X-ray photoemission and X-ray absorption that the grafting of the molecules on the LSMO surface induces a reduction of the Mn oxidation state. Ultraviolet photoelectron spectroscopy measurements also show that the LSMO work function can be modified by surface dipoles opening the door to both tune the charge and spin injection efficiencies in organic devices such as organic light-emitting diodes. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.apsusc.2015.06.051
  • Relaxation dynamics of majority and minority electrons after ultrashort laser excitation
    Mueller, B.Y. and Cinchetti, M. and Aeschlimann, M. and Schneider, H.C. and Rethfeld, B.
    159 (2015)
    We analyze the ultrafast demagnetization after ultrashort laser excitation by a kinetic model based on Elliott-Yafet scattering processes. By applying complete Boltzmann scattering integrals we trace the majority and minority electrons as well as phonons in the sample. Additionally, we allow for dynamical changes in the exchange splitting between majority and minority electrons. We find that our model has the potential to describe the magnetization dynamics and provides insights in the relaxation dynamics of the non-equilibrium electron system. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/978-3-319-07743-7_37
  • Spin optics: Between two spins
    Cinchetti, M.
    9 (2015)
    view abstract10.1038/nphoton.2015.135
  • Spin structure of Rashba-split electronic states of Bi overlayers on Cu(1 1 1)
    Jakobs, S. and Ruffing, A. and Jungkenn, D. and Cinchetti, M. and Mathias, S. and Aeschlimann, M.
    201 (2015)
    We investigate the unoccupied Rashba-type spin-orbit split band structure of the commensurate and incommensurate Bi monolayer on Cu(1 1 1) with spin- and angle-resolved two-photon-photoemission spectroscopy. Because of the unique geometrical structure of these Bi monolayers on Cu(1 1 1), it can be expected that both in-plane and out-of-plane potential gradients play an important role for the Rashba-type spin-structure in these systems. Our spin-resolved data of spin-split states in Bi/Cu(1 1 1) confirm the expected Rashba behavior of the in-plane spin-components that is caused by the out-of-plane potential gradient. But in addition, we indeed find out-of-plane spin components with different magnitudes in both monolayer Bi/Cu(1 1 1) systems, which we therefore attribute to the structurally induced in-plane potential gradients. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2014.09.003
  • Spin-resolved low-energy and hard x-ray photoelectron spectroscopy of off-stoichiometric Co2MnSi Heusler thin films exhibiting a record TMR
    Fetzer, R. and Ouardi, S. and Honda, Y. and Liu, H.-X. and Chadov, S. and Balke, B. and Ueda, S. and Suzuki, M. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M. and Fecher, G.H. and Felser, C.
    48 (2015)
    Half-metallic Co<inf>2</inf>MnSi-based Heusler compounds have attracted attention because they yield very high tunnelling magnetoresistance (TMR) ratios. Record TMR ratios of 1995% (at 4.2 K) are obtained from off-stoichiometric Co<inf>2</inf>MnSi-based magnetic tunnel junctions. This work reports on a combination of band structure calculations and spin-resolved and photon-polarisation-dependent photoelectron spectroscopy for off-stoichiometric Heusler thin films with the composition Co<inf>2</inf>Mn<inf>1.30</inf>Si<inf>0.84</inf>. Co and Mn are probed by magnetic dichroism in angle-resolved photoelectron spectroscopy at the 2p core level. In contrast to the delocalised Co 3d states, a pronounced localisation of the Mn 3d states is deduced from the corresponding 2p core level spectra. The valence states are investigated by linear dichroism using both hard x-ray and very-low-photon-energy excitation. When a very low photon energy is used for excitation, the valence bands exhibit a spin polarisation of about 30% at the Fermi energy. First principles calculations reveal that the low spin polarisation might be caused by a spin-flip process in the photoelectron final states. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/16/164002
  • Topological states on the gold surface
    Yan, B. and Stadtmüller, B. and Haag, N. and Jakobs, S. and Seidel, J. and Jungkenn, D. and Mathias, S. and Cinchetti, M. and Aeschlimann, M. and Felser, C.
    6 (2015)
    Gold surfaces host special electronic states that have been understood as a prototype of Shockley surface states. These surface states are commonly employed to benchmark the capability of angle-resolved photoemission spectroscopy (ARPES) and scanning tunnelling spectroscopy. Here we show that these Shockley surface states can be reinterpreted as topologically derived surface states (TDSSs) of a topological insulator (TI), a recently discovered quantum state. Based on band structure calculations, the Z2-type invariants of gold can be well-defined to characterize a TI. Further, our ARPES measurement validates TDSSs by detecting the dispersion of unoccupied surface states. The same TDSSs are also recognized on surfaces of other well-known noble metals (for example, silver, copper, platinum and palladium), which shines a new light on these long-known surface states.
    view abstract10.1038/ncomms10167
  • Vibron-assisted spin relaxation at a metal/organic interface
    Droghetti, A. and Rungger, I. and Cinchetti, M. and Sanvito, S.
    91 (2015)
    Inspired by recent experiments for hybrid organic-ferromagnet interfaces, we propose a spin-relaxation mechanism which does not depend on either the spin-orbit or the hyperfine interaction. This takes place when a molecule with initial spin imbalance is weakly coupled to a metal surface and can be excited in various vibrational states. In such a situation the electron-vibron interaction promotes the exchange of spin-polarized electrons between the molecule and the surface, serving as an energy and angular momentum reservoir. This process leads to an effective spin relaxation of the electron population in the molecule. We suggest that this nonequilibrium mechanism can be investigated through time-resolved spin-polarized scanning tunneling microscopy experiments. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.224427
  • All-optical control of ferromagnetic thin films and nanostructures
    Lambert, C.-H. and Mangin, S. and Varaprasad, B.S.D.Ch.S. and Takahashi, Y.K. and Hehn, M. and Cinchetti, M. and Malinowski, G. and Hono, K. and Fainman, Y. and Aeschlimann, M. and Fullerton, E.E.
    345 (2014)
    The interplay of light and magnetism allowed light to be used as a probe of magnetic materials. Now the focus has shifted to use polarized light to alter or manipulate magnetism. Here, we demonstrate optical control of ferromagnetic materials ranging from magnetic thin films to multilayers and even granular films being explored for ultra-high-density magnetic recording. Our finding shows that optical control of magnetic materials is a much more general phenomenon than previously assumed and may have a major impact on data memory and storage industries through the integration of optical control of ferromagnetic bits.
    view abstract10.1126/science.1253493
  • Electronic and magnetic properties of the interface between metal-quinoline molecules and cobalt
    Droghetti, A. and Steil, S. and Großmann, N. and Haag, N. and Zhang, H. and Willis, M. and Gillin, W.P. and Drew, A.J. and Aeschlimann, M. and Sanvito, S. and Cinchetti, M.
    89 (2014)
    It was recently established that spin injection from a ferromagnetic metal into an organic semiconductor depends largely on the formation of hybrid interface states. Here we investigate whether the magnetic properties of the interface between cobalt and tris(8-hydroxyquinolinato)-Al(III) (Alq3), the most prominent molecular candidate for organic spin-valve devices, can be modified by substituting the aluminum atom with either gallium or indium. The electronic structure of Alq3, Gaq3, and Inq3 and the properties of their interfaces with ferromagnetic cobalt are probed experimentally, by using different photoemission spectroscopy methods, and theoretically, through density functional theory calculations. For all cases, the results highlight the presence of spin-polarized interface states. However no striking difference between the properties of the various molecules and interfaces is observed. This is a consequence of the fact that the molecules frontier orbitals are mainly localized on the ligands and they show only a negligible contribution coming from the metal ion. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.094412
  • Electronic structure of metal quinoline molecules from G0W0 calculations
    Droghetti, A. and Cinchetti, M. and Sanvito, S.
    89 (2014)
    The photoemission spectra of four different metal quinoline molecules, namely, the prototypical tris(8-hydroxyquinolinato)-aluminum(III) (Alq3) complex and the related compounds Gaq3,Inq3, and tris-(9-hydroxyphenalen-1oate)- aluminum(III) [Al(OP)3] are compared to the electronic structure computed with different first-principles methods. In general, we found that, for Alq3,Gaq3, and Inq3, the molecular orbitals obtained with density functional theory and hybrid functionals represent a good approximation to the quasiparticle states. The same conclusion can be partially extended to the interesting case of Al(OP)3, although a direct comparison between theoretical and experimental results appears rather difficult for states, which are lower in energy than the first ten highest occupied molecular orbitals. Taking our results as a starting point we critically discuss the different available experimental data concerning the charge transport gap of Alq3. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.245137
  • Engineered materials for all-optical helicity-dependent magnetic switching
    Mangin, S. and Gottwald, M. and Lambert, C.-H. and Steil, D. and Uhlíř, V. and Pang, L. and Hehn, M. and Alebrand, S. and Cinchetti, M. and Malinowski, G. and Fainman, Y. and Aeschlimann, M. and Fullerton, E.E.
    13 (2014)
    The possibility of manipulating magnetic systems without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of the magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization in engineered magnetic materials. We demonstrate that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. These results challenge present theories of AO-HDS and provide a pathway to engineering materials for future applications based on all-optical control of magnetic order. © 2014 Macmillan Publishers Limited.
    view abstract10.1038/nmat3864
  • Kerr and Faraday microscope for space- and time-resolved studies
    Schmitt, O. and Steil, D. and Alebrand, S. and Ganss, F. and Hehn, M. and Mangin, S. and Albrecht, M. and Mathias, S. and Cinchetti, M. and Aeschlimann, M.
    87 (2014)
    We present a multi-purpose scanning magneto-optical microscope for the investigation of magnetic thin films. The setup can be used for both static and time-resolved (pump-probe) measurements. It is moreover compatible with samples with arbitrary magnetic anisotropy, as it allows Kerr measurements in polar and longitudinal geometry as well as in transmission (Faraday geometry). We demonstrate that the microscope can be used in the following modi: (i) static imaging mode (in polar Kerr and Faraday geometry) with a spatial resolution of 1.7 μm; (ii) time-resolved mode (polar Kerr geometry) with a temporal resolution of 300 femtoseconds. © 2014, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstract10.1140/epjb/e2014-50257-3
  • Molecular spintronics: Topology communicates
    Cinchetti, M.
    9 (2014)
    view abstract10.1038/nnano.2014.284
  • Probing the electronic and spintronic properties of buried interfaces by extremely low energy photoemission spectroscopy
    Fetzer, R. and Stadtmüller, B. and Ohdaira, Y. and Naganuma, H. and Oogane, M. and Ando, Y. and Taira, T. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    5 (2014)
    Ultraviolet photoemission spectroscopy (UPS) is a powerful tool to study the electronic spin and symmetry features at both surfaces and interfaces to ultrathin top layers. However, the very low mean free path of the photoelectrons usually prevents a direct access to the properties of buried interfaces. The latter are of particular interest since they crucially influence the performance of spintronic devices like magnetic tunnel junctions (MTJs). Here, we introduce spin-resolved extremely low energy photoemission spectroscopy (ELEPS) to provide a powerful way for overcoming this limitation. We apply ELEPS to the interface formed between the half-metallic Heusler compound Co2MnSi and the insulator MgO, prepared as in state-of-the-art Co2MnSi/MgO-based MTJs. The high accordance between the spintronic fingerprint of the free Co2MnSi surface and the Co2MnSi/MgO interface buried below up to 4 nm MgO provides clear evidence for the high interface sensitivity of ELEPS to buried interfaces. Although the absolute values of the interface spin polarization are well below 100%, the now accessible spin- and symmetry-resolved wave functions are in line with the predicted existence of non-collinear spin moments at the Co2MnSi/MgO interface, one of the mechanisms evoked to explain the controversially discussed performance loss of Heusler-based MTJs at room temperature. © 2014 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/srep08537
  • Spin-orbit enhanced demagnetization rate in Co/Pt-multilayers
    Kuiper, K.C. and Roth, T. and Schellekens, A.J. and Schmitt, O. and Koopmans, B. and Cinchetti, M. and Aeschlimann, M.
    105 (2014)
    In order to explore the role of enhanced spin-orbit interactions on the laser-induced ultrafast magnetization dynamics, we performed a comparative study on cobalt thin films and Co/Pt multilayers. We show that the presence of the Co/Pt interfaces gives rise to a three-fold faster demagnetization upon femtosecond laser heating. Experimental data for a wide range of laser fluences are analyzed using the Microscopic 3-Temperature Model. We find that the Elliott-Yafet spin-flip scattering in the multilayer structure is increased by at least a factor of four with respect to the elementary Co film. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4902069
  • Subpicosecond magnetization dynamics in TbCo alloys
    Alebrand, S. and Bierbrauer, U. and Hehn, M. and Gottwald, M. and Schmitt, O. and Steil, D. and Fullerton, E.E. and Mangin, S. and Cinchetti, M. and Aeschlimann, M.
    89 (2014)
    Since the discovery of all-optical magnetization switching in rare-earth transition-metal alloys the underlying magnetization dynamics of multisublattice magnets has become a hot topic of modern magnetism. We studied the ultrafast magnetization dynamics in TbCo alloys as a function of the alloy composition and the laser fluence using either 800 nm or 400 nm probe pulses. Direct comparison between TbCo samples with different compositions for equal excitation conditions demonstrates that the magnetization dynamics of the Co sublattice strongly depends on the Tb concentration. For Tb32Co68 the magnetization of the sublattices can even transiently be reversed on a subpicosecond time scale. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.144404
  • Ultrafast magnetization dynamics in Co-based Heusler compounds with tuned chemical ordering
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and K Suszka, A. and Idigoras, O. and Wolf, G. and Hillebrands, B. and Berger, A. and Aeschlimann, M. and Cinchetti, M.
    16 (2014)
    We have studied thin film samples of Co2FeSi and Co 2MnSi with different degrees of chemical ordering using the time-resolved magneto-optical Kerr effect to elucidate the influence of defects in the crystal structure on magnetization dynamics. Surprisingly, we find that the presence of defects does not influence the optically induced magnetization dynamics on the ultrashort timescale (some 100 fs). However, we observe a second demagnetization stage with a timescale of tens of picoseconds in Co 2MnSi for low chemical ordering; that is, a large number of defects. We interpret this second demagnetization step as originating from scattering of mostly thermalized majority electrons into unoccupied minority defect states. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/16/6/063068
  • Characterization of the surface electronic properties of Co2Cr1−xFexAl
    Wüstenberg, J.-P. and Aeschlimann, M. and Cinchetti, M.
    (2013)
    Due to their half metallic properties and high Curie temperatures Heusler alloys are potential key materials for future application in spin-based devices. One crucial aspect for device performance is the electron spin polarization at surfaces and interfaces, which in general deviates from bulk values due to the different bonding environment. Here, we present spin-resolved photoemission data on surfaces of Co2Cr1−xFexAl thin films. The influence of bulk and surface contributions on the spin-resolved spectra is discussed for x=0, x=0.4 and x=1, yielding information on disorder, surface states and calculation schemes that are consistent with our spin-resolved photoemission experiments. © Springer Science+Business Media Dordrecht 2013.
    view abstract10.1007/978-90-481-3832-6_12
  • Energy-resolved magnetic domain imaging in TbCo alloys by valence band photoemission magnetic circular dichroism
    Melchior, P. and Rollinger, M. and Thielen, P. and Alebrand, S. and Bierbrauer, U. and Schneider, C. and Gottwald, M. and Hehn, M. and Mangin, S. and Cinchetti, M. and Aeschlimann, M.
    88 (2013)
    We report magnetic domain imaging of a terbium cobalt (TbCo) alloy thin film with a high perpendicular magnetic anisotropy in one- and two-photon photoemission electron microscopy (PEEM). Both photoemission processes deliver a clear magnetic circular dichroism (MCD) whose strength is strongly energy dependent. Comparing the energy dependence of the MCD signal in one- and two-photon photoemission, we conclude that the magnetic contrast is mainly an initial state effect. Our results ultimately show that MCD contrast can be obtained in PEEM in valence band photoemission from a material supporting all-optical magnetization switching. This opens the way for the investigation of the all-optical switching process with simultaneous ultrahigh temporal and spatial resolution. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.104415
  • Feedback effect during ultrafast demagnetization dynamics in ferromagnets
    Mueller, B.Y. and Baral, A. and Vollmar, S. and Cinchetti, M. and Aeschlimann, M. and Schneider, H.C. and Rethfeld, B.
    111 (2013)
    Motivated by the recent controversy about the importance of spin-flip scattering for ultrafast demagnetization in ferromagnets, we study the spin-dependent electron dynamics based on a dynamical Elliott-Yafet mechanism. The key improvement to earlier approaches is the use of a modified Stoner model with a dynamic exchange splitting between majority and minority bands. In the framework of our microscopic model, we find a novel feedback effect between the time-dependent exchange splitting and the spin-flip scattering. This feedback effect allows us to reproduce important properties of the demagnetization dynamics quantitatively. Our results demonstrate that in general Elliott-Yafet spin-flip scattering needs to be taken into account to obtain a microscopic picture of demagnetization dynamics. © 2013 American Physical Society.
    view abstract10.1103/PhysRevLett.111.167204
  • Orbital angular momentum structure of an unoccupied spin-split quantum-well state in Pb/Cu(111)
    Jakobs, S. and Ruffing, A. and Cinchetti, M. and Mathias, S. and Aeschlimann, M.
    87 (2013)
    We performed circular dichroism angle-resolved two-photon photoemission experiments on the quantum-well system one monolayer Pb on Cu(111) to investigate the orbital angular momentum (OAM) structure of the unoccupied quantum-well state. The Pb/Cu(111) system exhibits a small Rashba-type spin-orbit splitting of the unoccupied band around the Γ̄ point and can be referred to as a material system with an intermediate spin-orbit coupling (SOC) parameter. As predicted by Kim, systems with such intermediate SOC strength should show a reversal of the OAM direction for the inner band of a spin-split system. Our results show dichroism values up to ±20% and also the expected change of the dichroism signal at a finite k â̂¥ point. The circular dichroism angle-resolved photoemission results therefore support the sensitivity of circular dichroism to OAM texture additionally to spin texture, and also that local OAM must be included to describe the experimentally observed large spin splittings in Rashba systems. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.235438
  • Spin-dependent electronic structure of the Co/Al(OP)3 interface
    Müller, S. and Steil, S. and Droghetti, A. and Großmann, N. and Meded, V. and Magri, A. and Schäfer, B. and Fuhr, O. and Sanvito, S. and Ruben, M. and Cinchetti, M. and Aeschlimann, M.
    15 (2013)
    We have studied the spin-dependent electronic properties of the interface formed between epitaxial Co thin films deposited on Cu(001) and the experimental molecule tris-(9-oxidophenalenone)-aluminum(III) (Al(OP) 3), created as a variation of the prototypical organic semiconductor Alq3 to tailor the spin filtering properties by modifying chemisorption with cobalt. The interfaces have been grown under ultra-high vacuum conditions by progressive deposition of 0.5-5 nm Al(OP)3 on the freshly prepared cobalt substrate. For every growth step we have monitored the energy level alignment at the interface as well as the spin polarization of the occupied manifold by spin-resolved photoemission spectroscopy. We identify two hybrid interface states in the energy window of 2 eV below the Fermi energy. The first is at 0.9 eV below EF and shows an 8% higher spin polarization than Co, while the second is at 1.6 eV below EF and shows a spin polarization reduced by 4%. © IOP Publishing and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/15/11/113054
  • Spin-dependent trapping of electrons at spinterfaces
    Steil, S. and Großmann, N. and Laux, M. and Ruffing, A. and Steil, D. and Wiesenmayer, M. and Mathias, S. and Monti, O.L.A. and Cinchetti, M. and Aeschlimann, M.
    9 (2013)
    Hybrid ferromagnetic metal/organic interfaces - also known as spinterfaces - can exhibit highly efficient spin-filtering properties and therefore present a promising class of materials for the future development of new spintronic devices. Advancing the field depends critically on elucidating the fundamental microscopic processes that eventually determine the spin-filtering properties in such hybrid structures. Here, we study the femtosecond spin dynamics at the prototypical interface between cobalt and the metalorganic complex tris(8-hydroxyquinolinato)aluminium. To disentangle the microscopic origin of spin filtering, we optically generate a transient spin polarization in a well-defined hybrid interface state that we follow with a spin-resolved real-time pump-probe two-photon photoemission experiment. We find that the electrons are trapped at the interface in a spin-dependent manner for a surprisingly long time of the order of 0.5-1 ps. We conclude that ferromagnetic metal/organic interfaces act as spin filters because electrons are trapped in hybrid interface states by spin-dependent confining potentials. Copyright © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/nphys2548
  • Structural, chemical, and electronic properties of the Co 2MnSi(001)/MgO interface
    Fetzer, R. and Wüstenberg, J.-P. and Taira, T. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    87 (2013)
    The performance of advanced magnetic tunnel junctions built of ferromagnetic (FM) electrodes and MgO as an insulating barrier depends decisively on the properties of the FM/insulator interface. Here we investigate interface formation between the Co-based Heusler compound Co2MnSi (CMS) and MgO by means of Auger electron spectroscopy, low-energy electron diffraction, and low-energy photoemission. The studies are performed for different annealing temperatures (TA) and MgO layer coverages (4, 6, 10, 20, and 50 ML). Thin MgO top layers (tMgO≤10 ML) show distinct surface crystalline distortions, which can only be partly healed out by annealing and, furthermore, lead to distinct adsorption of carbon species after the MgO surface is exposed to air. For tMgO&gt;10 ML, the MgO layer surface exhibits clearly improved crystalline structure and hence only marginal amounts of adsorbates. We attribute these findings to MgO misfit dislocations occurring at the interface, inducing further defects throughout the MgO layer for up to at least 10 ML. Furthermore, spin-polarized photoemission spectra of the CMS/MgO interface are obtained for MgO coverages up to 20 ML, showing a clear positive spin polarization near the Fermi energy (EF) in all cases. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.184418
  • Tailoring the energy level alignment at the Co/Alq3 interface by controlled cobalt oxidation
    Haag, N. and Steil, S. and Großmann, N. and Fetzer, R. and Cinchetti, M. and Aeschlimann, M.
    103 (2013)
    We have studied the influence of oxygen exposure at the prototypical interface between cobalt and the organic semiconductor tris(8-hydroxyquinoline) aluminum (III) (Alq3) by photoemission spectroscopy. We find that oxidation of the cobalt leads to a gradual suppression of hybrid interface states, to a progressive change in the work function and to a continuous energetic shift of the molecular orbitals towards higher binding energies. Based on these observations, we propose controlled oxidation of the ferromagnetic electrode as an easy and effective possibility to tune the performance of organic spintronics devices. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4850527
  • Ultrafast electron dynamics in a metallic quantum well nanofilm with spin splitting
    Ruffing, A. and Vollmar, S. and Jakobs, S. and Kaltenborn, S. and Baral, A. and Cinchetti, M. and Mathias, S. and Schneider, H.C. and Aeschlimann, M.
    88 (2013)
    Using time- and angle-resolved two-photon photoemission spectroscopy, we investigate the energy- and momentum-dependent ultrafast electron dynamics in the Rashba spin-split quantum-well nanofilm Bi/Cu(111). We find an expected increase of electron lifetimes towards the band bottom due to a competition of intra- and interband scattering processes. In addition, we find an unexpected peculiar decrease of the lifetimes around the intersection of the split bands. We compare the experimental results with calculated lifetimes due to electron-electron scattering in a model system of a 2D electron gas including a Rashba interaction term and an effective statically screened Coulomb interaction. Although the Rashba model reproduces the increase of lifetimes towards the band bottom well, there is no indication of the experimentally observed decrease around the intersection point in this simple model system. To investigate spin-orbit coupling effects, beyond those contained in a pure Rashba model, we introduce a phenomenological k-dependent spin mixing that leads to a "spin hot spot." It is shown that such a mixing would strongly increase the electron-electron scattering rate around the band intersection and thus improves the agreement with experiment. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.075148
  • All-optical magnetization switching using phase shaped ultrashort laser pulses
    Alebrand, S. and Hassdenteufel, A. and Steil, D. and Bader, M. and Fischer, A. and Cinchetti, M. and Aeschlimann, M.
    209 (2012)
    Switching the magnetization without any external applied magnetic field by using ultrashort circularly polarized laser pulses is a fascinating and challenging topic of modern magnetism. Up to now there is still no consensus about the microscopic mechanism of this so called all-optical switching (AOS) and several different models are suggested. In particular the importance of heat and helicity driven effects is still under discussion. In our experiment we varied the input of heating during the AOS process by utilizing pulse trains, created with a phase pulse shaper, to switch the magnetization. We show that AOS is possible with pulse trains having a temporal delay between the single pulses of more than a picosecond. The minimum switching fluence for pulse trains is found to be higher compared to switching with unshaped pulses. We mainly attribute this behavior to heat loss effects. Further we find that the minimum switching fluence depends on the number of pulses within a pulse train. We discuss our observations in terms of the currently debated theoretical models of AOS. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228500
  • Interplay of heating and helicity in all-optical magnetization switching
    Alebrand, S. and Hassdenteufel, A. and Steil, D. and Cinchetti, M. and Aeschlimann, M.
    85 (2012)
    Contrasting hypotheses have been made about the role played by laser heating and photon helicity in all-optical switching. Here we present an experiment that distinguishes between heating- and helicity-driven effects. We show that even though a minimum amount of circularity is needed to switch, heating contributes to the process. Moreover, we show that the helicity information carried by the exciting laser pulses is more easily transferred into the magnetic material at lower temperatures and that it persists in GdFeCo for at least some picoseconds after optical excitation. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.092401
  • Light-induced magnetization reversal of high-anisotropy TbCo alloy films
    Alebrand, S. and Gottwald, M. and Hehn, M. and Steil, D. and Cinchetti, M. and Lacour, D. and Fullerton, E.E. and Aeschlimann, M. and Mangin, S.
    101 (2012)
    Magnetization reversal using circularly polarized light provides a way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different Tb xCo 1-x ferrimagnetic alloy compositions using fs- and ps-laser pulses and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3 × 10 6 ergs/cm 3. Optical magnetization switching is observed only for alloy compositions where the compensation temperature can be reached through sample heating. © 2012 American Institute of Physics.
    view abstract10.1063/1.4759109
  • Surface spin polarization of the nonstoichiometric Heusler alloy Co 2MnSi
    Wüstenberg, J.-P. and Fetzer, R. and Aeschlimann, M. and Cinchetti, M. and Minár, J. and Braun, J. and Ebert, H. and Ishikawa, T. and Uemura, T. and Yamamoto, M.
    85 (2012)
    Using a combined approach of spin-resolved photoemission spectroscopy, band structure and photoemission calculations we investigate the influence of bulk defects and surface states on the spin polarization of nonstoichiometric Co 2Mn αSi thin films (with α=0.69 and α=1.19) with bulk L2 1 order. We find that for Mn-poor alloys the spin polarization at the Fermi energy (E F) is negative due to the presence of Co Mn antisite and minority surface state contributions. In Mn-rich alloys, the suppression of Co Mn antisites leads to a positive spin polarization at E F, and the influence of minority surface states on the photoelectron spin polarization is reduced. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.064407
  • Temperature dependence of laser-induced demagnetization in Ni: A key for identifying the underlying mechanism
    Roth, T. and Schellekens, A.J. and Alebrand, S. and Schmitt, O. and Steil, D. and Koopmans, B. and Cinchetti, M. and Aeschlimann, M.
    2 (2012)
    The microscopic mechanisms responsible for the ultrafast loss of magnetic order triggered in ferromagnetic metals by optical excitation are still under debate. One of the ongoing controversies is about the thermal origin of ultrafast demagnetization. Although different theoretical investigations support a main driving mechanism of thermal origin, alternative descriptions in terms of coherent interaction between the laser and the spin system or superdiffusive spin transport have been proposed. Another important matter of debate originates from the experimental observation of two time scales in the demagnetization dynamics of the 4f ferromagnet gadolinium. Here, it is still unclear whether it is necessary to invoke two distinct microscopic mechanisms to explain such behavior, or if one single mechanism is indeed sufficient. To uncover the physics behind these two unsolved issues, we explore the dependence of ultrafast-demagnetization dynamics in nickel through a survey of different laser intensities and ambient temperatures. Measurements in a large range of these external parameters are performed by means of the time-resolved magneto-optical Kerr effect and display a pronounced change in the maximum loss of magnetization and in the temporal profile of the demagnetization traces. The most striking observation is that the same material system (nickel) can show a transition from a one-step (one time scale) to a two-step (two time scales) demagnetization, occurring on increasing the ambient temperature. We find that the fluence and the temperature dependence of ultrafast demagnetization-including the transition from one-step to two-step dynamics-are reproduced theoretically assuming only a single scattering mechanism coupling the spin system to the temperature of the electronic system. This finding means that the origin of ultrafast demagnetization is thermal and that only a single microscopic channel is sufficient to describe magnetization dynamics in the 3d ferromagnets on all time scales.
    view abstract10.1103/PhysRevX.2.021006
  • All-optical magnetization recording by tailoring optical excitation parameters
    Steil, D. and Alebrand, S. and Hassdenteufel, A. and Cinchetti, M. and Aeschlimann, M.
    84 (2011)
    We investigate the dependency of all-optical magnetization switching on the properties of the exciting laser pulse by specifically tailoring all accessible laser parameters-pulse duration, wavelength, chirp, and bandwidth-over a wide range. Our results show that all-optical switching can be achieved with picosecond instead of femtosecond laser sources of various wavelengths, which considerably relaxes technological feasibility of this technique. The most striking implication is that, in contrast to all current knowledge, a strong photoinduced nonequilibrium in the electronic system is not necessary to achieve magnetization switching with light. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.224408
  • Driving force of ultrafast magnetization dynamics
    Mueller, B.Y. and Roth, T. and Cinchetti, M. and Aeschlimann, M. and Rethfeld, B.
    13 (2011)
    Irradiating a ferromagnetic material with an ultrashort laser pulse leads to demagnetization on the femtosecond timescale. We implement Elliott-Yafet-type spin-flip scattering, mediated by electron-electron and electron-phonon collisions, in the framework of a spin-resolved Boltzmann equation. Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii) spin-down electrons and (iii) phonons, we trace non-equilibrium electron distributions during and after laser excitation. We identified the driving force for ultrafast magnetization dynamics as the equilibration of temperatures and chemical potentials between electronic subsystems. This principle can be used to easily predict the maximum quenching of magnetization upon ultrashort laser irradiation in any material, as we show for the case of 3d-ferromagnetic nickel. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/13/12/123010
  • Indirect magnetic coupling of manganese porphyrin to a ferromagnetic cobalt substrate
    Chylarecka, D. and Kim, T.K. and Tarafder, K. and Müller, K. and Gödel, K. and Czekaj, I. and Wäckerlin, C. and Cinchetti, M. and Ali, Md.E. and Piamonteze, C. and Schmitt, F. and Wüstenberg, J.-P. and Ziegler, C. and Nolting, F. and Aeschlimann, M. and Oppeneer, P.M. and Ballav, N. and Jung, T.A.
    115 (2011)
    The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface. © 2010 American Chemical Society.
    view abstract10.1021/jp106822s
  • Investigation of the spin-dependent properties of electron doped cobalt-CuPc interfaces
    Steil, S. and Goedel, K. and Ruffing, A. and Sarkar, I. and Cinchetti, M. and Aeschlimann, M.
    161 (2011)
    We have grown metal-organic interfaces by in-situ deposition of ultrathin copper phthalocyanine (CuPc) films on a thin cobalt film on Cu(0 0 1). Evidence for layer-by-layer growth is found. The spin-dependent electronic properties of the Co-CuPc interface and their modification under caesium doping are investigated by spin-resolved photoemission spectroscopy. We observe a doping-induced shift of the highest occupied molecular orbital (HOMO) of CuPc away from the Fermi level (EF), accompanied by the formation of an unpolarised gap-state at 0.7 eV below EF in the high doping regime. Such features are reflected in the behaviour of the detected interfacial spin-polarisation. © 2010 Elsevier B.V.
    view abstract10.1016/j.synthmet.2010.11.031
  • Spin properties of interfaces with organic semiconductors studied by spin- and time-resolved two-photon photoemission
    Cinchetti, M. and Koffler, K. and Steil, S. and Großmann, N. and Aeschlimann, M.
    7937 (2011)
    We review our recent work in the field of organic spintronics, and show that the spin- and time-resolved two-photon photoemission (STR-2PPE) technique can be used to obtain quantitative information about the spin-dependent properties of hybrid organic-inorganic interfaces, as well as about the spin-dependent transport in organic semiconductors. In addition, we present STR-2PPE measurements performed on the Co-copper phthalocyanine (CuPc) system at different temperatures to investigate the microscopic processes repsonsible for the spin-polarization decay in organic semiconductors and at interfaces with such materials. We found no significant temperature dependence of the spin-injection efficiency across the Co-CuPc interface as well as of the spin-polarization decay length in CuPc. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.876459
  • Spin scattering and spin-polarized hybrid interface states at a metal-organic interface
    Methfessel, T. and Steil, S. and Baadji, N. and Großmann, N. and Koffler, K. and Sanvito, S. and Aeschlimann, M. and Cinchetti, M. and Elmers, H.J.
    84 (2011)
    Spin scattering at the interface formed between metallic Fe and Cu-phthalocyanine molecules is investigated by spin-polarized scanning tunneling spectroscopy and spin-resolved photoemission. The results are interpreted using first-principles electronic structure theory. The combination of experimental and theoretical techniques allows us to shed light on the role of hybrid interface states for the spin scattering. We show that Cu-phthalocyanine acts, via hybrid interface states, as a local spin filter up to room temperature both below and above the Fermi energy, E F. At the same time, the molecule behaves as a featureless scattering barrier in a region of about 1 eV around E F. Similar properties are found for both single molecules and self-assembled molecular layers, so that the acquired microscopic knowledge can be transferred to operating devices. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.224403
  • Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co 2Cr 0.6Fe 0.4Al
    Wüstenberg, J.-P. and Steil, D. and Alebrand, S. and Roth, T. and Aeschlimann, M. and Cinchetti, M.
    248 (2011)
    Ultrafast demagnetization upon excitation with intense laser pulses has been observed for a variety of ferromagnetic materials. In half-metals, long demagnetization times are expected due to the band gap for one spin direction. We have investigated the theoretically half-metallic Heusler alloy Co 2Cr 0.6Fe 0.4Al (CCFA), using the time-resolved magnetooptical Kerr effect. A demagnetization time in the range of typical transition metal ferromagnets with lower spin polarization has been found, while magnetization recovery proceeds on a slower time scale. The results are discussed in the context of recent models and experimental results. We propose that for Heusler alloys the initial stage of recovery could be a better measure for the spin polarization than the demagnetization time. A simple estimate leads to a bulk spin polarization value of 0.86 for CCFA. © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201147087
  • Band-structure-dependent demagnetization in the Heusler alloy Co 2Mn1-xFexSi
    Steil, D. and Alebrand, S. and Roth, T. and Krauß, M. and Kubota, T. and Oogane, M. and Ando, Y. and Schneider, H.C. and Aeschlimann, M. and Cinchetti, M.
    105 (2010)
    We investigate the ultrafast demagnetization for two Heusler alloys (Co2Mn1-xFexSi) with a different lineup of the minority band gap and the Fermi level. Even though electronic spin-flip transitions are partially blocked by the band gap in one compound, the respective magnetization dynamics, as measured by the time-resolved Kerr effect, are remarkably similar. Based on a dynamical model that includes momentum and spin-dependent carrier scattering, we show that the magnetization dynamics are dominated by hole spin-flip processes, which are not influenced by the gap.
    view abstract10.1103/PhysRevLett.105.217202
  • Explaining the paradoxical diversity of ultrafast laser-induced demagnetization
    Koopmans, B. and Malinowski, G. and Dalla Longa, F. and Steiauf, D. and Fähnle, M. and Roth, T. and Cinchetti, M. and Aeschlimann, M.
    9 (2010)
    Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.
    view abstract10.1038/nmat2593
  • Tailoring the spin functionality of a hybrid metal-organic interface by means of alkali-metal doping
    Cinchetti, M. and Neuschwander, S. and Fischer, A. and Ruffing, A. and Mathias, S. and Wüstenberg, J.-P. and Aeschlimann, M.
    104 (2010)
    We employ a recently developed purpose-made technique based on spin-resolved two-photon photoemission spectroscopy to study the influence of alkali-metal doping (Cs and Na) on the spin functionality of the interface between a cobalt thin film and the organic semiconductor copper phthalocyanine. We find two alkali-metal-induced effects. First, alkali-metal atoms act as impurities and increase the spin-flip probability for the electrons crossing the interface (detrimental effect). Second, they allow one to modify the interface energy level alignment and, consequently, to enhance the efficiency of spin injection at an arbitrary energy above the Fermi level of the cobalt (intrinsic effect). We show that the intrinsic effect dominates over the detrimental one, opening the possibility to actively tailor the spin functionality of the considered hybrid metal-organic interface by changing the doping concentration. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.104.217602
  • ferromagnetism

  • interfaces

  • magnetism

  • spin dynamics

  • thin films

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