Prof. Dr. Heiko Wende

Experimental Physics
University of Duisburg-Essen


  • Comprehensive investigation of crystallographic, spin-electronic and magnetic structure of (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4: Unraveling the suppression of configuration entropy in high entropy oxides
    Sarkar, A. and Eggert, B. and Witte, R. and Lill, J. and Velasco, L. and Wang, Q. and Sonar, J. and Ollefs, K. and Bhattacharya, S.S. and Brand, R.A. and Wende, H. and de Groot, F.M.F. and Clemens, O. and Hahn, H. and Kruk, R.
    Acta Materialia 226 (2022)
    High entropy oxides (HEOs) are a rapidly emerging class of functional materials consisting of multiple principal cations. The original paradigm of HEOs assumes cationic occupations with the highest possible configurational entropy allowed by the composition and crystallographic structure. However, the fundamental question remains on the actual degree of configurational disorder in HEOs, especially, in systems with a low enthalpy barriers for cation anti-site mixing. Considering the experimental limitations due to the presence of multiple principal cations in HEOs, here we utilize a robust and cross-referenced characterization approach using soft X-ray magnetic circular dichroism, hard X-ray absorption spectroscopy, Mössbauer spectroscopy, neutron powder diffraction and SQUID magnetometry to study the competition between crystal field stabilization energy and configurational entropy governing the cation occupation in a spinel HEO (S-HEO), (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4. In contrast to the previous studies, the derived complete structural and spin-electronic model, (Co0.6Fe0.4)(Cr0.3Fe0.1Mn0.3Ni0.3)2O4, highlights a significant deviation from the hitherto assumed paradigm of entropy-driven non-preferential distribution of cations in HEOs. An immediate correlation of this result can be drawn with bulk as well as the local element specific magnetic properties, which are intrinsically dictated by cationic occupations in spinels. The real local lattice picture presented here provides an alternate viewpoint on ionic arrangement in HEOs, which is of fundamental interest for predicting and designing their structure-dependent functionalities. © 2021 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2021.117581
  • Creating a Ferromagnetic Ground State with Tc Above Room Temperature in a Paramagnetic Alloy through Non-Equilibrium Nanostructuring
    Ye, X. and Fortunato, N. and Sarkar, A. and Geßwein, H. and Wang, D. and Chen, X. and Eggert, B. and Wende, H. and Brand, R.A. and Zhang, H. and Hahn, H. and Kruk, R.
    Advanced Materials (2022)
    Materials with strong magnetostructural coupling have complex energy landscapes featuring multiple local ground states, thus making it possible to switch among distinct magnetic-electronic properties. However, these energy minima are rarely accessible by a mere application of an external stimuli to the system in equilibrium state. A ferromagnetic ground state, with Tc above room temperature, can be created in an initially paramagnetic alloy by nonequilibrium nanostructuring. By a dealloying process, bulk chemically disordered FeRh alloys are transformed into a nanoporous structure with the topology of a few nanometer-sized ligaments and nodes. Magnetometry and Mössbauer spectroscopy reveal the coexistence of two magnetic ground states, a conventional low-temperature spin-glass and a hitherto-unknown robust ferromagnetic phase. The emergence of the ferromagnetic phase is validated by density functional theory calculations showing that local tetragonal distortion induced by surface stress favors ferromagnetic ordering. The study provides a means for reaching conventionally inaccessible magnetic states, resulting in a complete on/off ferromagnetic–paramagnetic switching over a broad temperature range. © 2022 The Authors. Advanced Materials published by Wiley-VCH GmbH.
    view abstract10.1002/adma.202108793
  • Magnetic response of CoFe2O4nanoparticles confined in a PNIPAM microgel network
    Witt, M.U. and Landers, J. and Hinrichs, S. and Salamon, S. and Kopp, J. and Hankiewicz, B. and Wende, H. and Von Klitzing, R.
    Soft Matter 18 (2022)
    The paper addresses coupling of magnetic nanoparticles (MNPs) with the polymer matrix of temperature-sensitive microgels and their response to magnetic fields. Therefore, CoFe2O4@CA (CA = citric acid) NPs are embedded within N-isopropylacrylamid (NIPAM) based microgels. The volume phase transition (VPT) of the magnetic microgels and the respective pure microgels is studied by dynamic light scattering and electrophoretic mobility measurements. The interaction between MNPs and microgel network is studied via magnetometry and AC-susceptometry using a superconducting quantum interference device (SQUID). The data show a significant change of the magnetic properties by crossing the VPT temperature (VPTT). The change is related to the increased confinement of the MNP due to the shrinking of the microgels. Modifying the microgel with hydrophobic allyl mercaptan (AM) affects the swelling ability and the magnetic response, i.e. the coupling of MNPs with the polymer matrix. Modeling the AC-susceptibility data results in an effective size distribution. This distribution represents the varying degree of constraint in MNP rotation and motion by the microgel network. These findings help to understand the interaction between MNPs and the microgel matrix to design multi responsive systems with tunable particle matrix coupling strength for future applications. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d1sm01597d
  • Spin Crossover in a Cobalt Complex on Ag(111)
    Johannsen, S. and Ossinger, S. and Grunwald, J. and Herman, A. and Wende, H. and Tuczek, F. and Gruber, M. and Berndt, R.
    Angewandte Chemie - International Edition 61 (2022)
    The Co-based complex [Co(H2B(pz)(pypz))2] (py=pyridine, pz=pyrazole) deposited on Ag(111) was investigated with scanning tunneling microscopy at ≈5 K. Due to a bis(tridentate) coordination sphere the molecules aggregate mainly into tetramers. Individual complexes in these tetramers undergo reversible transitions between two states with characteristic image contrasts when current is passed through them or one of their neighbors. Two molecules exhibit this bistability while the other two molecules are stable. The transition rates vary linearly with the tunneling current and exhibit an intriguing dependence on the bias voltage and its polarity. We interpret the states as being due to S=1/2 and 3/2 spin states of the Co2+ complex. The image contrast and the orders-of-magnitude variations of the switching yields can be tentatively understood from the calculated orbital structures of the two spin states, thus providing first insights into the mechanism of electron-induced excited spin-state trapping (ELIESST). © 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202115892
  • Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-Pyrazol-1-yl)Pyridine) Complexes
    Suryadevara, N. and Mizuno, A. and Spieker, L. and Salamon, S. and Sleziona, S. and Maas, A. and Pollmann, E. and Heinrich, B. and Schleberger, M. and Wende, H. and Kuppusamy, S.K. and Ruben, M.
    Chemistry - A European Journal 28 (2022)
    Bistable spin-crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2) spin-state switching are desirable for molecule-based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) (bpp) complexes – [Fe(bpp−COOEt)2](X)2⋅CH3NO2 (X=ClO4, 1; X=BF4, 2). Stable spin-state switching – T1/2=288 K; ΔT1/2=62 K – is observed for 1, whereas 2 undergoes above-room-temperature lattice-solvent content-dependent SCO – T1/2=331 K; ΔT1/2=43 K. Variable-temperature single-crystal X-ray diffraction studies of the complexes revealed pronounced molecular reorganizations – from the Jahn-Teller-distorted HS state to the less distorted LS state – and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule-based switching and memory elements. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202103853
  • Uniaxially Aligned 1D Sandwich-Molecular Wires: Electronic Structure and Magnetism
    Kraus, S. and Herman, A. and Huttmann, F. and Bianchi, M. and Stan, R.-M. and Holt, A.J. and Tsukamoto, S. and Rothenbach, N. and Ollefs, K. and Dreiser, J. and Bischof, K. and Wende, H. and Hofmann, P. and Atodiresei, N. and Michely, T.
    Journal of Physical Chemistry C 126 (2022)
    Sandwich-molecular wires consisting of europium and cyclooctatetraene (Cot) were grown in situ on the moiré of graphene with Ir(110). The moiré templates a uniaxial alignment of monolayer EuCot nanowire carpets and multilayer films with the EuCot wire axis along the [001] direction of the Ir substrate. Using angle-resolved photoemission spectroscopy, we investigate the band structure of the wire carpet films. While π-derived bands were not observed experimentally, we find a flat band 1.85 eV below the Fermi energy. Using density-functional theory and X-ray photoelectron spectroscopy and replacing europium through barium in the sandwich-molecular wires, it is concluded that the flat band is derived from Eu 4f states weakly mixed with Eu 5d states and slightly overlapping with Cot π states. X-ray magnetic circular dichroism is employed to characterize the magnetic properties of the EuCot wire carpet films at low temperatures. Clear evidence for an easy-axis magnetization along the wires is found. © 2022 American Chemical Society
    view abstract10.1021/acs.jpcc.1c10625
  • A full gap above the Fermi level: the charge density wave of monolayer VS2
    van Efferen, C. and Berges, J. and Hall, J. and van Loon, E. and Kraus, S. and Schobert, A. and Wekking, T. and Huttmann, F. and Plaar, E. and Rothenbach, N. and Ollefs, K. and Arruda, L.M. and Brookes, N. and Schönhoff, G. and Kummer, K. and Wende, H. and Wehling, T. and Michely, T.
    Nature Communications 12 (2021)
    In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal–insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state. © 2021, The Author(s).
    view abstract10.1038/s41467-021-27094-x
  • Article formation of fe-ni nanoparticle strands in macroscopic polymer composites: Experiment and simulation
    Nadarajah, R. and Tasdemir, L. and Thiel, C. and Salamon, S. and Semisalova, A.S. and Wende, H. and Farle, M. and Barcikowski, S. and Erni, D. and Gökce, B.
    Nanomaterials 11 (2021)
    Magnetic-field-induced strand formation of ferromagnetic Fe-Ni nanoparticles in a PMMA-matrix is correlated with the intrinsic material parameters, such as magnetization, particle size, composition, and extrinsic parameters, including magnetic field strength and viscosity. Since various factors can influence strand formation, understanding the composite fabrication process that maintains the strand lengths of Fe-Ni in the generated structures is a fundamental step in predicting the resulting structures. Hence, the critical dimensions of the strands (length, width, spacing, and aspect ratio) are investigated in the experiments and simulated via different intrinsic and extrinsic parameters. Optimal parameters were found by optical microscopy measurements and finite-element simulations using COMSOL for strand formation of Fe50Ni50 nanoparticles. The anisotropic behavior of the aligned strands was successfully characterized through magnetometry measurements. Compared to the unaligned samples, the magnetically aligned strands exhibit enhanced conductivity, increasing the current by a factor of 1000. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano11082095
  • Drifting inwards in protoplanetary discs: II. The effect of water on sticking properties at increasing temperatures
    Pillich, C. and Bogdan, T. and Landers, J. and Wurm, G. and Wende, H.
    Astronomy and Astrophysics 652 (2021)
    In previous laboratory experiments, we measured the temperature dependence of sticking forces between micrometer grains of chondritic composition. The data showed a decrease in surface energy by a factor ∼5 with increasing temperature. Here, we focus on the effect of surface water on grains. Under ambient conditions in the laboratory, multiple water layers are present. At the low pressure of protoplanetary discs and for moderate temperatures, grains likely only hold a monolayer. As dust drifts inwards, even this monolayer eventually evaporates completely in higher temperature regions. To account for this, we measured the tensile strength for the same chondritic material as was prepared and measured under normal laboratory conditions in our previous work, but now introducing two new preparation methods: drying dust cylinders in air (dry samples), and heating dust pressed into cylinders in vacuum (super-dry samples). For all temperatures up to 1000 K, the data of the dry samples are consistent with a simple increase in the sticking force by a factor of ∼10 over wet samples. Up to 900 K super-dry samples behave like dry samples. However, the sticking forces then exponentially increase up to another factor ∼100 at about 1200 K. The increase in sticking from wet to dry extends a trend that is known for amorphous silicates to multimineral mixtures. The findings for super-dry dust imply that aggregate growth is boosted in a small spatial higherature region around 1200 K, which might be a sweet spot for planetesimal formation. © 2021 ESO.
    view abstract10.1051/0004-6361/202140601
  • Effect of Excess Lead Oxide and Thermal Treatment on Dielectric and Magnetic Properties of Pb(Fe2/3W1/3)O3
    Kroll, E. and Shvartsman, V.V. and Lupascu, D.C. and Schell, J. and Salamon, S. and Wende, H.
    IEEE International Symposium on Applications of Feeroelectric, ISAF 2021, International Symposium on Integrated Functionalities, ISIF 2021 and Piezoresponse Force Microscopy Workshop, PFM 2021 - Proceedings (2021)
    Lead iron tungstate ceramics were synthesized by the columbite method. Two compositions were prepared, one with the stoichiometric ratio and the second one with an additional 2 wt% PbO. To investigate the influence of the synthesis conditions on the physical properties, the compositions were calcined and sintered at different temperatures. Due to the excess PbO, the apparent dielectric permittivity and conductivity decreased. Higher calcination and sintering temperatures led to higher dielectric permittivity as well as higher conductivity values. For all samples, weak ferromagnetism was observed caused by local disorder on the B-site sublattice. © 2021 IEEE.
    view abstract10.1109/ISAF51943.2021.9477333
  • Effect of lattice excitations on transient near-edge x-ray absorption spectroscopy
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Šipr, O. and Ebert, H. and Sokolowski-Tinten, K. and Pentcheva, R. and Bovensiepen, U. and Eschenlohr, A. and Wende, H.
    Physical Review B 104 (2021)
    Time-dependent and constituent-specific spectral changes in soft near-edge x-ray absorption spectroscopy (XAS) of an metal/insulator heterostructure after laser excitation are analyzed at the O K-edge with picosecond time resolution. The oxygen absorption edge of the insulator features a uniform intensity decrease of the fine structure at elevated phononic temperatures, which can be quantified by a simple simulation and fitting procedure presented here. Combining XAS with ultrafast electron diffraction measurements and ab initio calculations demonstrates that the transient intensity changes in XAS can be assigned to a transient lattice temperature. Thus, the sensitivity of transient near-edge XAS to phonons is demonstrated. © 2021 American Physical Society
    view abstract10.1103/PhysRevB.104.144302
  • Epitaxy Induced Highly Ordered Sm2Co17-SmCo5Nanoscale Thin-Film Magnets
    Sharma, S. and Zintler, A. and Günzing, D. and Lill, J. and Meira, D.M. and Eilhardt, R. and Singh, H.K. and Xie, R. and Gkouzia, G. and Major, M. and Radulov, I. and Komissinskiy, P. and Zhang, H. and Skokov, K. and Wende, H. and Takahashi, Y.K. and Ollefs, K. and Molina-Luna, L. and Alff, L.
    ACS Applied Materials and Interfaces (2021)
    Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets. ©
    view abstract10.1021/acsami.1c04780
  • Evolution of the magnetic hyperfine field profiles in an ion-irradiated Fe60Al40film measured by nuclear resonant reflectivity
    Andreeva, M. and Smekhova, A. and Baulin, R. and Repchenko, Y. and Bali, R. and Schmitz-Antoniak, C. and Wende, H. and Sergueev, I. and Schlage, K. and Wille, H.-C.
    Journal of Synchrotron Radiation 28 (2021)
    Nuclear resonant reflectivity (NRR) from an Fe60zAl40film was measured using synchrotron radiation at several grazing angles near the critical angle of total external reflection. Using laterally resolved measurements after irradiation with 20keV Ne+ions of gradually varying fluence of 0-3.0×1014ionscm-2, the progressive creation of the ferromagnetic A2 phase with increasing ion fluence was confirmed. The observed depth selectivity of the method has been explained by application of the standing wave approach. From the time spectra of the nuclear resonant scattering in several reflection directions the depth profiles for different hyperfine fields were extracted. The results show that the highest magnetic hyperfine fields (∼18-23 T) are initially created in the central part of the film and partially at the bottom interface with the SiO2 substrate. The evolution of the ferromagnetic onset, commencing at a fixed depth within the film and propagating towards the interfaces, has been directly observed. At higher fluence (3.0 × 1014ions cm-2) the depth distribution of the ferro­magnetic fractions became more homogeneous across the film depth, in accordance with previous results. © 2021 International Union of Crystallography. All rights reserved.
    view abstract10.1107/S1600577521007694
  • Influence of calcination and sintering temperatures on dielectric and magnetic properties of Pb(Fe0.5Nb0.5)O3 ceramics synthesized by the solid state method
    Bartek, N. and Shvartsman, V.V. and Salamon, S. and Wende, H. and Lupascu, D.C.
    Ceramics International 47 (2021)
    Lead iron niobate, Pb(Fe0.5Nb0.5)O3 (PFN), belongs to the family of multiferroic materials combining ferroelectric and antiferromagnetic ordering. Its properties to a large extent depend on the synthesis conditions. For applications it is important to obtain materials with large polarization and low electric leakage. In this paper we investigate the effect of processing parameters on the structural, electrical and magnetic behaviour of PFN ceramics prepared by the solid state method. The optimal calcination and sintering temperatures are found, which enable us to obtain ceramics with a large polarization Pmax = 28 μC/cm2 and dielectric permittivity εMax ≈ 55 000. We also find that increasing the calcination and sintering temperatures decreases diffuseness of the ferroelectric phase transition and shifts the Néel temperature to lower values, which might be due to a change of the distribution of Fe3+ and Nb5+ across the B-sites of the perovskite towards a more ordered structure. © 2021
    view abstract10.1016/j.ceramint.2021.05.055
  • Influence of the cobalt content in cobalt iron oxides on the electrocatalytic OER activity
    Saddeler, S. and Bendt, G. and Salamon, S. and Haase, F.T. and Landers, J. and Timoshenko, J. and Rettenmaier, C. and Jeon, H.S. and Bergmann, A. and Wende, H. and Roldan Cuenya, B. and Schulz, S.
    Journal of Materials Chemistry A 9 (2021)
    Sub 10 nm cobalt ferrite CoxFe3-xO4 (x ≤ 1.75) nanoparticles and cobalt-rich wüstite (Cox/3Fe(1-x)/3)O nanoparticles (x ≥ 2) were synthesized in a solvothermal approach and characterized by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) as well as energy dispersive X-ray spectroscopy (EDX), IR, Raman, and 57Fe-Mössbauer spectroscopy. Their electrocatalytic activity in the oxygen evolution reaction (OER) was evaluated and the active state formation was tracked by operando X-ray absorption spectroscopy (XAS). Our studies demonstrate that the cobalt-rich wüstite (Cox/3Fe(1-x)/3)O nanoparticles underwent a phase-transformation into the spinels CoxFe3-xO4 (x ≥ 2) under the applied OER conditions. The overpotential η10 at 10 mA cm-2, serving as a benchmark for the OER activity of the cobalt ferrite nanoparticles in alkaline media, was lower than that of magnetite Fe3O4 even with low cobalt concentrations, reaching a minimum of 350 mV for Co2.25Fe0.75O4 with a Tafel slope of 50 mV dec-1. Finally, we identified that the catalytic activity is linked to the nanoparticle size as well as to the degree of Co redox activity and change in coordination during OER. © The Royal Society of Chemistry.
    view abstract10.1039/d1ta06568h
  • Large-scale synthesis of iron oxide/graphene hybrid materials as highly efficient photo-Fenton catalyst for water remediation
    Hammad, M. and Fortugno, P. and Hardt, S. and Kim, C. and Salamon, S. and Schmidt, T.C. and Wende, H. and Schulz, C. and Wiggers, H.
    Environmental Technology and Innovation 21 (2021)
    The Photo-Fenton reaction is an advanced oxidation process to break down organic pollutants in aqueous systems. Moreover, the scalable synthesis and engineering of stable catalysts with a high specific surface area is extremely important for the practical application of the Photo-Fenton process. In the current study, we developed a low-cost method for large-scale production of iron oxide/graphene nanostructures with a controllable graphene loading for the photo-Fenton reaction. Under optimal condition, high efficiencies of degradation (>99%) of methylene blue, rhodamine B, acid orange 7, and phenol at a concentration (60 mg/mL) were reached in 60 min under UV-A irradiation (1.6 mW/cm2) with mineralization of 72, 77, 82, and 48%, respectively. More importantly, the iron oxide/graphene nanocomposites exhibited good stability over a wide range of pH (from 3 to 9) and can be magnetically separated from the solution and repeatedly used with consistent photocatalytic performance. This enhanced removal efficiency of the iron oxide/graphene nanostructure compared to iron oxide nanoparticles is attributed to the accelerated transfer of photo-generated electrons between iron oxide and graphene and its relatively large surface area. The results demonstrate that the iron oxide/graphene system could be potentially utilized for many environmental treatment processes. © 2020 Elsevier B.V.
    view abstract10.1016/j.eti.2020.101239
  • Link between Structural and Optical Properties of CoxFe3- xO4Nanoparticles and Thin Films with Different Co/Fe Ratios
    Kampermann, L. and Klein, J. and Korte, J. and Kowollik, O. and Pfingsten, O. and Smola, T. and Saddeler, S. and Piotrowiak, T.H. and Salamon, S. and Landers, J. and Wende, H. and Ludwig, A. and Schulz, S. and Bacher, G.
    Journal of Physical Chemistry C (2021)
    CoxFe3-xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3-xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface. © 2021 The Authors. Published by American Chemical Society.
    view abstract10.1021/acs.jpcc.0c11277
  • Link between Structural and Optical Properties of CoxFe3-xO4Nanoparticles and Thin Films with Different Co/Fe Ratios
    Kampermann, L. and Klein, J. and Korte, J. and Kowollik, O. and Pfingsten, O. and Smola, T. and Saddeler, S. and Piotrowiak, T.H. and Salamon, S. and Landers, J. and Wende, H. and Ludwig, A. and Schulz, S. and Bacher, G.
    Journal of Physical Chemistry C 125 (2021)
    CoxFe3-xO4nanoparticles (x= 0.4 tox= 2.5) and thin films (x= 0.9 tox= 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+and Fe3+cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3-xO4nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles’ surface. © 2021 The Authors. Published by American Chemical Society
    view abstract10.1021/acs.jpcc.0c11277
  • Mechanism of magnetization reduction in iron oxide nanoparticles
    Köhler, T. and Feoktystov, A. and Petracic, O. and Kentzinger, E. and Bhatnagar-Schöffmann, T. and Feygenson, M. and Nandakumaran, N. and Landers, J. and Wende, H. and Cervellino, A. and Rücker, U. and Kovács, A. and Dunin-Borkowski, R.E. and Brückel, T.
    Nanoscale 13 (2021)
    Iron oxide nanoparticles are presently considered as main work horses for various applications including targeted drug delivery and magnetic hyperthermia. Several questions remain unsolved regarding the effect of size onto their overall magnetic behavior. One aspect is the reduction of magnetization compared to bulk samples. A detailed understanding of the underlying mechanisms of this reduction could improve the particle performance in applications. Here we use a number of complementary experimental techniques including neutron scattering and synchrotron X-ray diffraction to arrive at a consistent conclusion. We confirm the observation from previous studies of a reduced saturation magnetization and argue that this reduction is mainly associated with the presence of antiphase boundaries, which are observed directly using high-resolution transmission electron microscopy and indirectly via an anisotropic peak broadening in X-ray diffraction patterns. Additionally small-angle neutron scattering with polarized neutrons revealed a small non-magnetic surface layer, that is, however, not sufficient to explain the observed loss in magnetization alone. © The Royal Society of Chemistry.
    view abstract10.1039/d0nr08615k
  • Microscopic understanding of particle-matrix interaction in magnetic hybrid materials by element-specific spectroscopy
    Landers, J. and Salamon, S. and Webers, S. and Wende, H.
    Physical Sciences Reviews (2021)
    Mössbauer spectroscopy is a well-known technique to study complex magnetic structures, due to its sensitivity to electronic and magnetic interactions of the probed nucleus with its electronic surrounding. It has also been applied to the emerging fields of magnetic hybrid materials as well as to ferrofluids, as information on the magnetic alignment and the velocity of the probed nucleus, i.e. of the particle it is embedded in, can be inferred from the spectra in addition to the above-mentioned quantities. Considering the wide range of preparation methods and sample properties, including fluids, particle powders, sintered pellets, polymer matrices and viscoelastic hydrogels, a considerable advantage of Mössbauer spectroscopy is the usage of γ-photons. This allows measurements on opaque samples, for which optical experiments are usually not feasible, also making the technique relatively independent of specific sample geometries or preparation. Using iron oxide nanoparticles in glycerol solution as an exemplary material here, the variety of system parameters simultaneously accessible via Mössbauer spectroscopy can be demonstrated: Spectra recorded for particles of different sizes provided information on the particles' Brownian dynamics, including the effect of the shell thickness on their hydrodynamic diameter, the presence (or absence) and ballpark frequency of Neél superspin relaxation as well as the particles' average magnetic orientation in external magnetic fields. For single-core particles, this resulted in the observation of standard Langevin-type alignment behavior. Mössbauer spectra additionally provide information on the absolute degree of spin alignment, also allowing the determination of the degree of surface spin canting, which limits the maximum magnetization of ferrofluid samples. Analyzing the alignment behavior of agglomerated particles for comparison, we found a completely different trend, in which spin alignment was further hindered by the competition of easy magnetic directions. More complex particle dynamics are observed when going from ferrofluids to hybrid materials, where the particle mobility and alignability depends not only on the particles' shape and material, but also on the matrices' inner structure and the acting force-transfer mechanism between particles and the surrounding medium. In ferrohydrogels for example, particle mobility in terms of Mössbauer spectroscopy was probed for different crosslinker concentrations, resulting in widely different mesh-sizes of the polymer network and different degrees of freedom. While a decrease in particle dynamics is clearly visible in Mössbauer spectroscopy upon rising crosslinker density, complementary AC-susceptometry experiments indicated no Brownian motion on the expected timescales. This apparent contradiction could, however, be explained by the different timescales of the experiments, probing either the relatively free Brownian motion on ultrashort timescales or the more bound state preventing extensive particle motion by interaction with the trapping mesh walls in the millisecond regime. However, it should also be considered that the effect of the surroundings on particle rotation in AC-susceptometry may also differ from the variation in translational motion, probed by Mössbauer spectroscopy. Being sensitive mainly to translational motion also results in a wide range of particles to be accessible for studies via Mössbauer spectroscopy, including larger agglomerates embedded in polymers, intended for remote-controlled heating. Despite the agglomerates' wide distribution in effective diameters, information on particle motion was found to be in good agreement with AC-susceptometry experiments at ultralow frequencies in and above the polymer melting region, while additionally giving insight into Neél relaxation of the individual nanoparticles and their magnetic structure. © 2020 Joachim Landers et al., published by De Gruyter, Berlin/Boston 2020.
    view abstract10.1515/psr-2019-0116
  • On the structure-property relationships of (Al, Ga, In)-doped spinel cobalt ferrite compounds: A combined experimental and DFT study
    Naveed-Ul-Haq, M. and Hussain, S. and Webers, S. and Salamon, S. and Ahmad, I. and Bibi, T. and Hameed, A. and Wende, H.
    Physical Chemistry Chemical Physics 23 (2021)
    We report a combined experimental and theoretical study of pure and doped cobalt ferrite where 25% of Fe3+ ions were replaced by Al3+, Ga3+, and In3+ ions, respectively, i.e., CoFe1.5X0.5O4 (X = Al, Ga, and In). The ferrite compositions were successfully synthesized using the solid-state reaction method. The X-ray powder diffraction method established that all ferrite samples had a spinel unit cell structure with the Fd3m (No. 227) space group. The lattice constants of ferrites increased from 8.382 Å (for undoped CoFe2O4) to 8.520 Å (for In-doped cobalt ferrite) in direct relation to the dopant ion size. The magnetic properties were obtained at 4.3 K and 300 K. At 4.3 K, the In-doped CoFe2O4 showed the highest saturation magnetic moment of 4.68 μB f.u.-1, while Al-doped CoFe2O4 showed the smallest value of 2.72 μB f.u.-1. The Fe3+ distribution among the spinel tetrahedral and octahedral sites was determined from the Mössbauer spectra. From ultraviolet-visible diffuse reflectance spectroscopy the direct optical bandgaps were determined, which have values between 1.20 eV and 1.28 eV for these ferrites. The ferrite compositions were also studied theoretically using plane-wave density functional theory using the CASTEP code where it was revealed that arrangements of the non-magnetic cations at the tetrahedral and octahedral sites strongly influence the electronic structure, the bandgap value, and the net magnetic moment per formula unit. Light Al3+ ions at the octahedral site give a low value of the net magnetic moment while the heavier Ga3+ and In3+ ions at the tetrahedral sites of the spinel give an enhanced magnetic moment. The magnetic moment values obtained from theoretical calculations match very well with the experimental values. Moreover, the theoretical calculations reveal that there exists a strong p-d hybridization among the oxygen and magnetic ions, which is affected by the non-magnetic dopant ions. The change in hybridization with the non-magnetic ion doping is responsible for the altered magnetic moments of the doped ferrites. Thus, our study provides a comprehensive investigation covering the synthesis and characterization of ferrites along with a good understanding of the phenomenon of how non-magnetic ion doping into spinel ferrites provides a method to tune the electronic and magnetic properties of the spinel ferrite. This journal is © the Owner Societies.
    view abstract10.1039/d1cp02625a
  • Spatially resolved GHz magnetization dynamics of a magnetite nano-particle chain inside a magnetotactic bacterium
    Feggeler, T. and Meckenstock, R. and Spoddig, D. and Zingsem, B.W. and Ohldag, H. and Wende, H. and Farle, M. and Winklhofer, M. and Ollefs, K.J.
    Physical Review Research 3 (2021)
    Understanding magnonic properties of nonperiodic magnetic nanostructures requires real-space imaging of ferromagnetic resonance modes with spatial resolution well below the optical diffraction limit and sampling rates in the 5-100 GHz range. Here, we demonstrate element-specific scanning transmission x-ray microscopy-detected ferromagnetic resonance (STXM-FMR) applied to a chain of dipolarly coupled nano-particles (40-50 nm particle size) inside a single cell of a magnetotactic bacterium Magnetospirillum magnetotacticum. The ferromagnetic resonance mode of the nano-particle chain driven at 6.748 GHz and probed with 50 nm x-ray focus size was found to have a uniform phase response but non-uniform amplitude response along the chain segments due to the superposition of dipolar coupled modes of chain segments and individual particles, in agreement with micromagnetic simulations. © 2021 Published by the American Physical Society
    view abstract10.1103/PhysRevResearch.3.033036
  • Spin polarization and magnetotransport properties of systematically disordered Fe60Al40 thin films
    Borisov, K. and Ehrler, J. and Fowley, C. and Eggert, B. and Wende, H. and Cornelius, S. and Potzger, K. and Lindner, J. and Fassbender, J. and Bali, R. and Stamenov, P.
    Physical Review B 104 (2021)
    We investigate the evolution of spin polarization, spontaneous Hall angle (SHA), saturation magnetization, and Curie temperature of B2-ordered Fe60Al40 thin films under varying antisite disorder, induced by Ne+-ion irradiation. The spin polarization increases monotonically as a function of ion fluence. A relatively high polarization of 46% and a SHA of 3.1% are achieved on 40 nm films irradiated with 2×1016 ions/cm2 at 30 keV. An interesting divergence in the trends of the magnetization and SHA is observed for low disorder concentrations. The high spin polarization and its broad tunability range make ion-irradiated Fe60Al40 a promising material for application in spin electronic devices. © 2021 American Physical Society.
    view abstract10.1103/PhysRevB.104.134417
  • Strain- And field-induced anisotropy in hybrid elastomers with elongated filler nanoparticles
    Seifert, J. and Günzing, D. and Webers, S. and Dulle, M. and Kruteva, M. and Landers, J. and Wende, H. and Schmidt, A.M.
    Soft Matter 17 (2021)
    The implementation of anisotropy to functional materials is a key step towards future smart materials. In this work, we evaluate the influence of preorientation and sample architecture on the strain-induced anisotropy in hybrid elastomers containing covalently attached elongated magnetic filler particles. Accordingly, silica coated spindle-type hematite nanoparticles are incorporated into poly(dimethylsiloxane)-based elastomers, and two types of composite architectures are compared: on the one hand a conventional architecture of filled, covalently crosslinked elastomers, and on the other hybrid elastomers that are crosslinked exclusively by covalent attachment of the polymer chains to the particle surface. By the application of external strain and with magnetic fields, the orientational order of the elongated nanoparticles can be manipulated, and we investigate the interplay between strain, magnetic order, and orientational order of the particles by combining 2D small angle X-ray scattering experiments under strain and fields with Mössbauer spectroscopy under similar conditions, and supplementary angular-dependent magnetization experiments. The converging information is used to quantify the order in these interesting materials, while establishing a direct link between the magnetic properties and the spatial orientation of the embedded magnetic nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0sm02104k
  • Superparamagnetic nanoparticles with LC polymer brush shell as efficient dopants for ferronematic phases
    Koch, K. and Kundt, M. and Barkane, A. and Nadasi, H. and Webers, S. and Landers, J. and Wende, H. and Eremin, A. and Schmidt, A.M.
    Physical Chemistry Chemical Physics 23 (2021)
    Liquid crystal (LC) based magnetic materials consisting of LC hosts doped with functional magnetic nanoparticles enable optical switching of the mesogens at moderate magnetic field strengths and thereby open the pathway for the design of novel smart devices. A promising route for the fabrication of stable ferronematic phases is the attachment of a covalently bound LC polymer shell onto the surface of nanoparticles. With this approach, ferronematic phases based on magnetically blocked particles and the commercial LC 4-cyano-4′-pentylbiphenyl (5CB) liquid crystal were shown to have a sufficient magnetic sensitivity, but the mechanism of the magneto-nematic coupling is unidentified. To get deeper insight into the coupling modes present in these systems, we prepared ferronematic materials based on superparamagnetic particles, which respond to external fields with internal magnetic realignment instead of mechanical rotation. This aims at clarifying whether the hard coupling of the magnetization to the particle's orientation (magnetic blocking) is a necessary component of the magnetization-nematic director coupling mechanism. We herein report the fabrication of a ferronematic phase consisting of surface-functionalized superparamagnetic Fe3O4 particles and 5CB. We characterize the phase behavior and investigate the magneto-optical properties of the new ferronematic phase and compare it to the ferronematic system containing magnetically blocked CoFe2O4 particles to get information about the origin of the magneto-nematic coupling. © the Owner Societies.
    view abstract10.1039/d1cp03005a
  • The Effect of Water on the 2-Propanol Oxidation Activity of Co-Substituted LaFe1−CoxO3 Perovskites
    Dreyer, M. and Cruz, D. and Hagemann, U. and Zeller, P. and Heidelmann, M. and Salamon, S. and Landers, J. and Rabe, A. and Ortega, K.F. and Najafishirtari, S. and Wende, H. and Hartmann, N. and Knop-Gericke, A. and Schlögl, R. and Behrens, M.
    Chemistry - A European Journal (2021)
    Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1−xCoxO3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x>0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202102791
  • The Roles of Composition and Mesostructure of Cobalt-Based Spinel Catalysts in Oxygen Evolution Reactions
    Rabe, A. and Büker, J. and Salamon, S. and Koul, A. and Hagemann, U. and Landers, J. and Friedel Ortega, K. and Peng, B. and Muhler, M. and Wende, H. and Schuhmann, W. and Behrens, M.
    Chemistry - A European Journal (2021)
    By using the crystalline precursor decomposition approach and direct co-precipitation the composition and mesostructure of cobalt-based spinels can be controlled. A systematic substitution of cobalt with redox-active iron and redox-inactive magnesium and aluminum in a cobalt spinel with anisotropic particle morphology with a preferred 111 surface termination is presented, resulting in a substitution series including Co3O4, MgCo2O4, Co2FeO4, Co2AlO4 and CoFe2O4. The role of redox pairs in the spinels is investigated in chemical water oxidation by using ceric ammonium nitrate (CAN test), electrochemical oxygen evolution reaction (OER) and H2O2 decomposition. Studying the effect of dominant surface termination, isotropic Co3O4 and CoFe2O4 catalysts with more or less spherical particles are compared to their anisotropic analogues. For CAN-test and OER, Co3+ plays the major role for high activity. In H2O2 decomposition, Co2+ reveals itself to be of major importance. Redox active cations in the structure enhance the catalytic activity in all reactions. A benefit of a predominant 111 surface termination depends on the cobalt oxidation state in the as-prepared catalysts and the investigated reaction. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202102400
  • Towards field-effect controlled graphene-enhanced Raman spectroscopy of cobalt octaethylporphyrin molecules
    Sleziona, S. and Rauls, S. and Heckhoff, T. and Christen, L. and Pollmann, E. and Madauß, L. and Franzka, S. and Lorke, A. and Wende, H. and Schleberger, M.
    Nanotechnology 32 (2021)
    During the last decade graphene-enhanced Raman spectroscopy has proven to be a powerful tool to detect and analyze minute amounts of molecules adsorbed on graphene. By using a graphene-based field-effect device the unique opportunity arises to gain a deeper insight into the coupling of molecules and graphene as graphene's Fermi level can be controlled by the transistor`s gate voltage. However, the fabrication of such a device comes with great challenges because of contaminations stemming from processing the device inevitably prevent direct adsorption of the molecules onto graphene rendering it unsuitable for field-effect controlled graphene-enhanced Raman spectroscopy measurements/experiments. In this work, we solve this problem by establishing two different fabrication procedures for such devices, both of which are in addition compatible with large area and scalable production requirements. As a first solution, selective argon cluster irradiation is shown to be an efficient way to remove resist residues after processing. We provide evidence that after the irradiation the enhancement of the molecular Raman signal can indeed be measured, demonstrating that this procedure cleans graphene's surface sufficiently enough for direct molecular adsorption. As a second solution, we have developed a novel stacking method to encapsulate the molecules in between two graphene layers to protect the underlying graphene and molecular layer from the harsh conditions during the photolithography process. This method combines the advantages of dry stacking, which leads to a perfectly clean interface, and wet stacking processes, which can easily be scaled up for large area processing. Both approaches yield working graphene transistors with strong molecular Raman signals stemming from cobalt octaehtylporphyrin, a promising and prototypical candidate for spintronic applications, and are therefore suitable for graphene based molecular sensing applications. © 2021 The Author(s). Published by IOP Publishing Ltd Printed in the UK
    view abstract10.1088/1361-6528/abde60
  • Towards laser printing of magnetocaloric structures by inducing a magnetic phase transition in iron-rhodium nanoparticles
    Nadarajah, R. and Landers, J. and Salamon, S. and Koch, D. and Tahir, S. and Doñate-Buendía, C. and Zingsem, B. and Dunin-Borkowski, R.E. and Donner, W. and Farle, M. and Wende, H. and Gökce, B.
    Scientific Reports 11 (2021)
    The development of magnetocaloric materials represents an approach to enable efficient and environmentally friendly refrigeration. It is envisioned as a key technology to reduce CO2 emissions of air conditioning and cooling systems. Fe-Rh has been shown to be one of the best-suited materials in terms of heat exchange per material volume. However, the Fe-Rh magnetocaloric response depends on its composition. Hence, the adaptation of material processing routes that preserve the Fe-Rh magnetocaloric response in the generated structures is a fundamental step towards the industrial development of this cooling technology. To address this challenge, the temperature-dependent properties of laser synthesized Fe-Rh nanoparticles and the laser printing of Fe-Rh nanoparticle inks are studied to generate 2D magnetocaloric structures that are potentially interesting for applications such as waste heat management of compact electrical appliances or thermal diodes, switches, and printable magnetocaloric media. The magnetization and temperature dependence of the ink’s γ-FeRh to B2-FeRh magnetic transition is analyzed throughout the complete process, finding a linear increase of the magnetization M (0.8 T, 300 K) up to 96 Am2/kg with ca. 90% of the γ-FeRh being transformed permanently into the B2-phase. In 2D structures, magnetization values of M (0.8 T, 300 K) ≈ 11 Am2/kg could be reached by laser sintering, yielding partial conversion to the B2-phase equivalent to long-time heating temperature of app. 600 K, via this treatment. Thus, the proposed procedure constitutes a robust route to achieve the generation of magnetocaloric structures. © 2021, The Author(s).
    view abstract10.1038/s41598-021-92760-5
  • 3D printing of magnetic parts by laser powder bed fusion of iron oxide nanoparticle functionalized polyamide powders
    Hupfeld, T. and Salamon, S. and Landers, J. and Sommereyns, A. and Doñate-Buendía, C. and Schmidt, J. and Wende, H. and Schmidt, M. and Barcikowski, S. and Gökce, B.
    Journal of Materials Chemistry C 8 (2020)
    The development of new feedstock materials is a central prerequisite for advances in Additive Manufacturing (AM). To increase the breadth of potential applications for 3D and 4D printing of polymers, micro- and nano-additives incorporated into the feedstock material play an important role. In this context, magnetic materials are of great interest. Our study describes a way to fabricate polymer powders for laser powder bed fusion (PBF-LB) with a homogeneous, well-dispersed coating of iron oxide nanoparticles. Without the addition of chemical precursors, spherical superparamagnetic FeOxnanoparticles with monomodal size distribution below 10 nm are generated from FeOxmicropowder by laser fragmentation in liquid. The adsorption of the nanoparticles on polyamide (PA12) powder is conducted directly in an aqueous dispersion after laser fragmentation, followed by drying, powder analysis and PBF-LB processing.ViaMössbauer spectroscopy and magnetometry, we determined that the saturation magnetization and structure of the iron oxide nanoparticles were not influenced by PBF-LB processing, and the magnetic properties were successfully transferred to the final 3D-printed magnetic part. © The Royal Society of Chemistry 2020.
    view abstract10.1039/d0tc02740e
  • Applications
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    In this chapter, a short introduction to possible applications utilizing organic molecules interacting with ferromagnetic substrates will be given. By explaining the spin filtering concept, studies with Cu-phthalocyanine as well as zinc methyl phenalenyl molecules will be reviewed. The hybridization of the electronic states of the molecules at the interface to those of substrates is a crucial point to realize functional hybrid metalorganic interfaces. A large magnetic anisotropy is found for the hybridized phenalenyl layer, which is important for the interface magnetoresistance effect used in this spin filter concept study. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_8
  • Biocompatible magnetic fluids of co-doped iron oxide nanoparticles with tunable magnetic properties
    Dutz, S. and Buske, N. and Landers, J. and Gräfe, C. and Wende, H. and Clement, J.H.
    Nanomaterials 10 (2020)
    Magnetite (Fe3O4) particles with a diameter around 10 nm have a very low coercivity (Hc) and relative remnant magnetization (Mr/Ms), which is unfavorable for magnetic fluid hyperthermia. In contrast, cobalt ferrite (CoFe2O4) particles of the same size have a very high Hc and Mr/Ms, which is magnetically too hard to obtain suitable specific heating power (SHP) in hyperthermia. For the optimization of the magnetic properties, the Fe2+ ions of magnetite were substituted by Co2+ step by step, which results in a Co doped iron oxide inverse spinel with an adjustable Fe2+ substitution degree in the full range of pure iron oxide up to pure cobalt ferrite. The obtained magnetic nanoparticles were characterized regarding their structural and magnetic properties as well as their cell toxicity. The pure iron oxide particles showed an average size of 8 nm, which increased up to 12 nm for the cobalt ferrite. For ferrofluids containing the prepared particles, only a limited dependence of Hc and Mr/Ms on the Co content in the particles was found, which confirms a stable dispersion of the particles within the ferrofluid. For dry particles, a strong correlation between the Co content and the resulting Hc and Mr/Ms was detected. For small substitution degrees, only a slight increase in Hc was found for the increasing Co content, whereas for a substitution of more than 10% of the Fe atoms by Co, a strong linear increase in Hc and Mr/Ms was obtained. Mössbauer spectroscopy revealed predominantly Fe3+ in all samples, while also verifying an ordered magnetic structure with a low to moderate surface spin canting. Relative spectral areas of Mössbauer subspectra indicated a mainly random distribution of Co2+ ions rather than the more pronounced octahedral site-preference of bulk CoFe2O4. Cell vitality studies confirmed no increased toxicity of the Co-doped iron oxide nanoparticles compared to the pure iron oxide ones. Magnetic heating performance was confirmed to be a function of coercivity as well. The here presented non-toxic magnetic nanoparticle system enables the tuning of the magnetic properties of the particles without a remarkable change in particles size. The found heating performance is suitable for magnetic hyperthermia application. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10061019
  • Controlling the oxidation of magnetic and electrically conductive solid-solution iron-rhodium nanoparticles synthesized by laser ablation in liquids
    Nadarajah, R. and Tahir, S. and Landers, J. and Koch, D. and Semisalova, A.S. and Wiemeler, J. and El-Zoka, A. and Kim, S.-H. and Utzat, D. and Möller, R. and Gault, B. and Wende, H. and Farle, M. and Gökce, B.
    Nanomaterials 10 (2020)
    This study focuses on the synthesis of FeRh nanoparticles via pulsed laser ablation in liquid and on controlling the oxidation of the synthesized nanoparticles. Formation of monomodal γ-FeRh nanoparticles was confirmed by transmission electron microscopy (TEM) and their composition confirmed by atom probe tomography (APT). For these particles, three major contributors to oxidation were analysed: (1) dissolved oxygen in the organic solvents, (2) the bound oxygen in the solvent and (3) oxygen in the atmosphere above the solvent. The decrease of oxidation for optimized ablation conditions was confirmed through energy-dispersive X-ray (EDX) and Mössbauer spectroscopy. Furthermore, the time dependence of oxidation was monitored for dried FeRh nanoparticles powders using ferromagnetic resonance spectroscopy (FMR). By magnetophoretic separation, B2-FeRh nanoparticles could be extracted from the solution and characteristic differences of nanostrand formation between γ-FeRh and B2-FeRh nanoparticles were observed. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano10122362
  • Corrigendum to ‘Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3-NiFe2O4 multiferroics: A relationship between phase-connectivity and interface coupling’ (Acta Materialia (2018) 144 (305–313), (S1359645417309072), (10.1016/j.actamat.2017.10.048))
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Trivedi, H. and Salamon, S. and Webers, S. and Wende, H. and Hagemann, U. and Schröder, J. and Lupascu, D.C.
    Acta Materialia 187 (2020)
    The authors regret to inform the readers of Acta Materialia that Figure 10 of the article contains mistakes, none of which change the overall interpretation of the data nor the main point of the paper, but which should be rectified, nonetheless. 1. There was a calculation error during the conversion from applied voltage to the electric field and finally to the converse ME coupling coefficient. This was amended, with the correct data being shown in Table 1, to more clearly show the composition-dependent change to the reader. The overall interpretation remains unaffected, with the sample BCZTNFO30 displaying the highest ME coefficient.2. The value of the ME coefficient for the BCZTNFO50 sample was recorded at a different frequency (1.7 Hz) than the other compositions (8 Hz), which lead to a strongly deviating result for this sample. In order to obtain a dataset with consistent parameters, a single data point from the magnetic field dependent measurement regime was used, the results of which are shown in Figure 11 of the original manuscript. The slope and thus the converse ME coupling coefficient were calculated from a linear function between this point and the point of origin, which is justified by the fact that our measurement values show high stability and precision, making it highly unlikely for significant deviations to result from this approximation. The combination of parameters (magnetic field, temperature, electric field) represents the optimum for this sample, as also used for the original measurement, in addition to the now matching frequency of 8 Hz. This updated data point has also been included in Table 1 and Figure 1.3. Furthermore, the dataset of the BCZTNFO20 sample currently shown in Figure 10 of the manuscript is not the correct one, evident by the lower point density, higher amount of noise and the deviating frequency of 11 Hz. This dataset was replaced with the correct one for this sample, as shown in Figure 2, recorded at the same point density and frequency (8 Hz) as all the other samples of this set.From this updated set of data, new values of ME coefficients were calculated: 16.9(6) ps/m for BCZTNFO20, 90.1(6) ps/m for BCZTNFO30, 34.9(2) ps/m for BCZTNFO40 and 25.4 ps/m for BCZTNFO50. This does not change the conclusions that were drawn in the manuscript, as sample BCTNFO30 still has the highest value of the converse ME coupling coefficient. However, all of the values are increased at least twofold due to the corrected voltage conversion, clearly showing that this NFO-based sample set exhibits greatly improved ME response compared to previously used CFO-based materials. Furthermore, thanks to the now correct coefficient for BCZTNFO50, the composition dependent peak is much clearer, with the coupling coefficient consistently dropping when we move away from the optimum BCZTNFO30 composition to either side (see Figure 1), giving more merit to our original interpretation. The significant change of the measurement signal for BCZTNFO50 we observed when going from 1.7 Hz to 8 Hz is most probably caused by the fact that this sample has a lower resistivity of only 1 GΩ compared to the 30 GΩ of BCZTNFO30 (see Figure 3). This presumably induces a strong increase of frequency-dependence of the ME coefficient through Maxwell-Wagner relaxation due to the different dielectric permittivities of the two constituents that make up the sample, further exacerbated by the higher temperature at which this measurement was performed. These findings were discussed in greater detail in the dissertation of one of the co-authors [1]. Another minor correction needs to be made regarding Figure 1 of the original manuscript: There are typo mistakes on all four panels of Figure 1, where the phase of BCZT was incorrectly labeled as P4/mmm, which should be labeled as P4/mm. The same should be understood for Table 1 and wherever P4/mmm is mentioned in the text. The authors would like to apologise for any inconvenience caused. © 2020 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2020.01.054
  • Drifting inwards in protoplanetary discs i Sticking of chondritic dust at increasing temperatures
    Bogdan, T. and Pillich, C. and Landers, J. and Wende, H. and Wurm, G.
    Astronomy and Astrophysics 638 (2020)
    Sticking properties rule the early phases of pebble growth in protoplanetary discs in which grains regularly travel from cold, water-rich regions to the warm inner part. This drift affects composition, grain size, morphology, and water content as grains experience ever higher temperatures. In this study we tempered chondritic dust under vacuum up to 1400 K. Afterwards, we measured the splitting tensile strength of millimetre-sized dust aggregates. The deduced effective surface energy starts out as γe = 0.07 J m-2. This value is dominated by abundant iron-oxides as measured by Mössbauer spectroscopy. Up to 1250 K, γe continuously decreases by up to a factor five. Olivines dominate at higher temperature. Beyond 1300 K dust grains significantly grow in size. The γe no longer decreases but the large grain size restricts the capability of growing aggregates. Beyond 1400 K aggregation is no longer possible. Overall, under the conditions probed, the stability of dust pebbles would decrease towards the star. In view of a minimum aggregate size required to trigger drag instabilities it becomes increasingly harder to seed planetesimal formation closer to a star. © ESO 2020.
    view abstract10.1051/0004-6361/202038120
  • Effect of Mn and Ba Codoping on a Magnetic Spin Cycloid of Multiferroic Bismuth Ferrite Nanoparticles
    Dubey, A. and Escobar Castillo, M. and Landers, J. and Salamon, S. and Wende, H. and Hagemann, U. and Gemeiner, P. and Dkhil, B. and Shvartsman, V.V. and Lupascu, D.C.
    Journal of Physical Chemistry C 124 (2020)
    Bismuth ferrite (BFO) is the drosophila of research in multiferroic materials due to its simultaneous magnetic and electric ordering at room temperature. The unfortunate detail is its antiferromagnetic ordering, which practically cancels magnetization and magnetoelectric coupling of the crystals. To induce finite coupling, dopants have been introduced with a certain success so far. Nanoparticles (NPs) can additionally constrain the formation of the magnetic cycloid in BFO due to size confinement. Doping nanoparticles can thus potentially provide a sizeable magnetization of BFO, making applications in computer memories and hyperthermia cancer treatment feasible. We show that the codoping of BFO NPs by Ba and Mn balances the electrochemical equilibrium, reduces the particle size, and shifts the magnetic phase transition to lower temperatures. The ferroelectric properties are retained and the remanent magnetization is increased by 1 order of magnitude: Bi0.95Ba0.05Fe0.95Mn0.05O3 possesses a remanent magnetization of 0.277 Am2/kg. Our Mössbauer studies reveal that two effects drive this increase: partial destruction of the spin cycloid due to Mn and increased spin canting due to Ba doping inducing local stress fields. This dopant combination and particular concentration improve the effective magnetization value exceptionally well. © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.0c05778
  • Electron correlation and spin transition
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    Theoretical treatment of functional metalorganics is non-trivial for the metal centers with narrow bands (3d, 4d of transition metals or 4f bands of rare-earth metals), featuring a sizeable Coulomb interaction. An interplay between crystal field, spin-orbit coupling and Coulomb interaction expresses the properties of the molecule. Correlated metal centers, immersed in the electron bath of organic ring makes it ideal to treat with Anderson’s impurity model. In this chapter, we will focus on the description of electron correlation in functional metalorganics with the aid of density functional theory, combined with a many body approach. For most of the illustrative purposes, we will consider iron porphyrin (FeP) molecule. The chapter will reveal the importance of the treatment of explicit electron correlation in order to accurately identify the spin transition, magnetic anisotropy, Kondo effect etc., which are key ingredients for molecular spintronics and electronics. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_5
  • Electronic structure of isolated molecules
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    Phthalocyanine molecules with a 3d transition metal in the center, like MnPc, FePc, CoPc, NiPc and CuPc, have attracted a huge interest in the last decades due to the large number of possible applications. Experimental and theoretical gas phase studies are an important reference to understand the properties of the molecules, as well as how they can be modified and manipulated upon deposition on substrates or in supramolecular conformations. However, in several 3d metal phthalocyanines the electronic structure of the single molecule is still under debate even after several spectroscopical studies and computational works have been performed. This is mostly due to the highly correlated 3d electrons of the metal atoms, which pose a challenge for the theory. In addition, the experiments to determine the electronic structure are often carried out in different conditions (on thick films or in gas phase for example), and this can lead to different results. The following chapter provides an overview of the theoretical and experimental results and debates related to the electronic structure of gas phase MnPc, FePc, CoPc, NiPc and CuPc. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_4
  • Exchange bias effect in bulk multiferroic BiFe0.5Sc0.5O3
    Fertman, E.L. and Fedorchenko, A.V. and Desnenko, V.A. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H. and Vaisburd, A.I. and Stanulis, A. and Ramanauskas, R. and Olekhnovich, N.M. and Pushkarev, A.V. and Radyush, Y.V. and Khalyavin, D.D. and Salak, A.N.
    AIP Advances 10 (2020)
    Below the Néel temperature, TN ∼220 K, at least two nano-scale antiferromagnetic (AFM) phases coexist in the polar polymorph of the BiFe0.5Sc0.5O3 perovskite; one of these phases is a weak ferromagnetic. Non-uniform structure distortions induced by high-pressure synthesis lead to competing AFM orders and a nano-scale spontaneous magnetic phase separated state of the compound. Interface exchange coupling between the AFM domains and the weak ferromagnetic domains causes unidirectional anisotropy of magnetization, resulting in the exchange bias (EB) effect. The EB field, HEB, and the coercive field strongly depend on temperature and the strength of the cooling magnetic field. HEB increases with an increase in the cooling magnetic field and reaches a maximum value of about 1 kOe at 5 K. The exchange field vanishes above TN with the disappearance of long-range magnetic ordering. The effect is promising for applications in electronics as it is large enough and as it is tunable by temperature and the magnetic field applied during cooling. © 2020 Author(s).
    view abstract10.1063/1.5135586
  • Experimental techniques
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    Different spectroscopic methods can be used to characterize the electronic structure of a system of interest, i.e. molecular, solid or adsorbate samples. The different techniques give complementary information about the geometric and electronic structure of the system. By Photoelectron Spectroscopy (PES), Auger and resonant photoemission (RPES) the occupied electronic levels can be studied, whereas X-ray Absorption Spectroscopy (XAS) gives information about the unoccupied valence states of the system in presence, however, of a core hole. Magnetic information can be obtained from X-ray Magnetic Circular Dichroism (XMCD). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_2
  • Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance
    Hammad, M. and Hardt, S. and Mues, B. and Salamon, S. and Landers, J. and Slabu, I. and Wende, H. and Schulz, C. and Wiggers, H.
    Journal of Alloys and Compounds 824 (2020)
    Magnetic nanoparticle-mediated hyperthermia has shown great potential in cancer therapy. However, upscaling of the synthesis of iron oxide nanoparticle with the required narrow size distribution remains challenging. This paper describes the reproducible and scalable synthesis of citric acid-functionalized iron oxide nanoparticles optimized for hyperthermia treatment. Iron oxide nanoparticles were synthesized by a spray flame method, which is eco-friendly and cost-effective. To the best of our knowledge, this is the first study reporting spray-flame synthesis of small iron oxide nanoparticles (approx. 7 nm) with narrow size distribution (polydispersity index ≪ 0.1). The citric acid-coated iron oxide nanoparticles revealed a hydrodynamic size of approx. 37 nm and a high magnetic saturation of 69 Am2/kg at room temperature. The magnetic hyperthermia study showed a significantly enhanced value of the intrinsic loss power (4.8 nHm2/kg), which is 1.5-fold higher than the best commercially available equivalents. The improved heating efficiency and small hydrodynamic size of citric acid-coated iron oxide nanoparticles demonstrate that the system could potentially be used as a nanoplatform for hyperthermia treatment. © 2020 Elsevier B.V.
    view abstract10.1016/j.jallcom.2020.153814
  • Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6 H1.6
    Terwey, A. and Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Eggert, B. and Ollefs, K. and Brabänder, V. and Radulov, I. and Skokov, K. and Faske, T. and Hu, M.Y. and Zhao, J. and Alp, E.E. and Giacobbe, C. and Gutfleisch, O. and Wende, H.
    Physical Review B 101 (2020)
    We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 in terms of the vibrational (phonon) density of states (VDOS), which we determined with Fe57 nuclear resonant inelastic X-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy ΔSlat across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change ΔSlat of (3.2±1.9)JkgK across the FM-to-PM transition contributes with ∼35% significantly and cooperatively to the total isothermal entropy change ΔSiso. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.064415
  • Influence of ligands
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    The adsorption of small molecules on the metal center of 3d metal phthalocyanines alters the bonding scheme of the molecule and can induce different spin states and magnetic moments, as well as trigger more exotic effects like Kondo resonance. Soft X-ray spectroscopy studies and computational studies have highlighted this kind of effects in magnetic molecules like FePc, CoPc and MnPc by adsorption of for example CO, NO, O$$:2$$ molecules. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_7
  • Interaction with substrates
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    Organometallic molecules have attracted interest because their properties can be varied by changing ligands, metal center, end groups etc. which makes them candidates for various applications. Special attention has been paid to hybrid systems of molecules and substrates as possible building blocks for future electronic and magnetic devices. In view of such devices phthalocyanine molecules are advantageous because they can adsorb flat on metallic or semiconducting substrates. Aiming to understand the magnetic properties of the molecules and their interplay with substrates and ligands the focus will be on the paramagnetic Pc molecules i.e. Mn, Fe, Co and CuPc and their interaction with (metallic) substrates and nonmagnetic TMPCs such as NiPc and ZnPc are only briefly mentioned. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_6
  • Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations
    Eggert, B. and Gruner, M.E. and Ollefs, K. and Schuster, E. and Rothenbach, N. and Hu, M.Y. and Zhao, J. and Toellner, T.S. and Sturhahn, W. and Pentcheva, R. and Cuenya, B.R. and Alp, E.E. and Wende, H. and Keune, W.
    Physical Review Materials 4 (2020)
    We combine Fe57 Mössbauer spectroscopy and Fe57 nuclear resonant inelastic x-ray scattering (NRIXS) on nanoscale polycrystalline [bcc-Fe57/MgO] multilayers with various Fe-layer thicknesses and layer-resolved density-functional-theory (DFT)-based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure (where ML denotes monolayer) to unravel the interface-related atomic vibrational properties of a multilayer system. Being consistent in theory and experiment, we observe enhanced hyperfine magnetic fields Bhf in the multilayers as compared to Bhf in bulk bcc Fe; this effect is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.044402
  • Introduction
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    Phthalocyanine molecules with a 3d transition metal in the centre, like MnPc, FePc, CoPc, NiPc and CuPc, have attracted a huge interest in the last decades due to the large number of possible applications. Experimental and theoretical gas phase studies are an important reference to understand the properties of the molecules, as well as how they can be modified and manipulated upon deposition on substrates or in supramolecular conformations. However, in several 3d metal phthalocyanines, the electronic structure of the single molecule is still under debate even after several spectroscopic studies and computational works have been performed. This is mostly due to the highly correlated 3d electrons of the metal atoms, which pose a challenge for the theory. In addition, the experiments to determine the electronic structure are often carried out in different conditions (on thick films or in gas phase for example), and this can lead to different results. The following chapter provides an overview of the theoretical and experimental results and debates related to the electronic structure of gas phase MnPc, FePc, CoPc, NiPc and CuPc. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_1
  • Magnetic Properties and Mössbauer Spectroscopy of Fe3O4/CoFe2O4 Nanorods
    Hähsler, M. and Landers, J. and Nowack, T. and Salamon, S. and Zimmermann, M. and Heißler, S. and Wende, H. and Behrens, S.
    Inorganic Chemistry 59 (2020)
    Fe3O4/CoFe2O4 nanorods were obtained via a simple seed-mediated synthesis. Nanorods were used as seeds to grow CoFe2O4 by thermal codecomposition of the cobalt(II) and iron(III) acetylacetonate precursors. The growth process was monitored by electron microscopy (SEM, TEM), and the resulting nanorods were characterized by powder X-ray diffraction analysis and IR and Raman spectroscopy. Magnetometry and AC susceptometry studies revealed a distribution of Néel relaxation times with an average blocking temperature of 140 K and a high-field magnetization of 42 Am2/kg. Complementarily recorded 57Fe-Mössbauer spectra were consistent with the Fe3O4/CoFe2O4 spinel structure and exhibited considerable signs of spin frustration, which was correlated to the internal and surface structure of the nanorods. © 2020 American Chemical Society.
    view abstract10.1021/acs.inorgchem.9b03267
  • Magnetic properties of rare-earth and transition metal based perovskite type high entropy oxides
    Witte, R. and Sarkar, A. and Velasco, L. and Kruk, R. and Brand, R.A. and Eggert, B. and Ollefs, K. and Weschke, E. and Wende, H. and Hahn, H.
    Journal of Applied Physics 127 (2020)
    High entropy oxides (HEOs) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e., five or more) sharing one lattice site, which crystallize in a single phase structure. One complex example of the rather young HEO family is the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview of the magnetic properties of three perovskite type high entropy oxides. The compounds have a rare-earth site that is occupied by five different rare-earth elements, while the transition metal site is occupied by a single transition metal. In this way, a comparison to the parent binary oxides, namely, the orthocobaltites, -chromites, and -ferrites, is possible. X-ray absorption near edge spectroscopy, magnetometry, and Mössbauer spectroscopy are employed to characterize these complex materials. In general, we find surprising similarities to the magnetic properties of the binary oxides despite the chemical disorder on the rare-earth site. However, distinct differences and interesting magnetic properties are also observed such as noncollinearity, spin reorientation transitions, and large coercive fields of up to 2 T at ambient temperature. Both the chemical disorder on the rare-earth A-site and the nature of the transitional metal on the B-site play an important role in the physical properties of these high entropy oxides. © 2020 Author(s).
    view abstract10.1063/5.0004125
  • Magnetic response of FeRh to static and dynamic disorder
    Eggert, B. and Schmeink, A. and Lill, J. and Liedke, M.O. and Kentsch, U. and Butterling, M. and Wagner, A. and Pascarelli, S. and Potzger, K. and Lindner, J. and Thomson, T. and Fassbender, J. and Ollefs, K. and Keune, W. and Bali, R. and Wende, H.
    RSC Advances 10 (2020)
    Atomic scale defects generated using focused ion as well as laser beams can activate ferromagnetism in initially non-ferromagnetic B2 ordered alloy thin film templates. Such defects can be induced locally, confining the ferromagnetic objects within well-defined nanoscale regions. The characterization of these atomic scale defects is challenging, and the mechanism for the emergence of ferromagnetism due to sensitive lattice disordering is unclear. Here we directly probe a variety of microscopic defects in systematically disordered B2 FeRh thin films that are initially antiferromagnetic and undergo a thermally-driven isostructural phase transition to a volatile ferromagnetic state. We show that the presence of static disorder i.e., the slight deviations of atoms from their equilibrium sites is sufficient to induce a non-volatile ferromagnetic state at room temperature. A static mean square relative displacement of 9 × 10-4 Å-2 is associated with the occurrence of non-volatile ferromagnetism and replicates a snapshot of the dynamic disorder observed in the thermally-driven ferromagnetic state. The equivalence of static and dynamic disorder with respect to the ferromagnetic behavior can provide insights into the emergence of ferromagnetic coupling as well as achieving tunable magnetic properties through defect manipulations in alloys. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0ra01410a
  • Magneto-structural correlations in a systematically disordered B2 lattice
    Ehrler, J. and Sanyal, B. and Grenzer, J. and Zhou, S. and Böttger, R. and Eggert, B. and Wende, H. and Lindner, J. and Fassbender, J. and Leyens, C. and Potzger, K. and Bali, R.
    New Journal of Physics 22 (2020)
    Ferromagnetism in certain B2 ordered alloys such as Fe60Al40 can be switched on, and tuned, via antisite disordering of the atomic arrangement. The disordering is accompanied by a ∼1 % increase in the lattice parameter. Here we performed a systematic disordering of B2 Fe60Al40 thin films, and obtained correlations between the order parameter (S), lattice parameter (a 0), and the induced saturation magnetization (M s). As the lattice is gradually disordered, a critical point occurs at 1-S = 0.6 and a 0 = 2.91 Å, where a sharp increase of the M s is observed. DFT calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, whereas above, it is largely the increase of a 0 in the disordered state that determines the M s. The insights obtained here can be useful for achieving tailored magnetic properties in alloys through disordering. © 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/ab944a
  • Non-standing spin-waves in confined micrometer-sized ferromagnetic structures under uniform excitation
    Pile, S. and Feggeler, T. and Schaffers, T. and Meckenstock, R. and Buchner, M. and Spoddig, D. and Zingsem, B. and Ney, V. and Farle, M. and Wende, H. and Ohldag, H. and Ney, A. and Ollefs, K.
    Applied Physics Letters 116 (2020)
    A non-standing characteristic of directly imaged spin-waves in confined micrometer-sized ultrathin Permalloy (Ni 80 Fe 20) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations, the presence of the spin-wave modes excited in the Permalloy stripes along with the quasi-uniform modes was observed. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing characteristic of the spin-waves. Also, it was shown by micromagnetic simulations and confirmed using STXM-FMR results that the observed characteristic of the spin-waves can be influenced by the local magnetic fields in different sample geometries. © 2020 Author(s).
    view abstract10.1063/1.5139881
  • Role of intermediate 4 f states in tuning the band structure of high entropy oxides
    Sarkar, A. and Eggert, B. and Velasco, L. and Mu, X. and Lill, J. and Ollefs, K. and Bhattacharya, S.S. and Wende, H. and Kruk, R. and Brand, R.A. and Hahn, H.
    APL Materials 8 (2020)
    High entropy oxides (HEOs) are single-phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show the reversible control of optical properties in a rare-earth (RE) based HEO-(Ce0.2La0.2Pr0.2Sm0.2Y0.2)O2-δ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic band structure underpinning the observed optical phenomena. Heat treatment of the HEO under a vacuum atmosphere followed by reheat treatment in air results in a reversible change in the bandgap energy, from 1.9 eV to 2.5 eV. The finding is consistent with the reversible changes in the oxidation state and related f-orbital occupancy of Pr. However, no pertinent changes in the phase composition or crystal structure are observed upon the vacuum heat treatment. Furthermore, annealing of this HEO under a H2 atmosphere, followed by reheat treatment in air, results in even larger but still a reversible change in the bandgap energy from 1.9 eV to 3.2 eV. This is accompanied by a disorder-order type crystal structure transition and changes in the O 2p-RE 5d hybridization evidenced from X-ray absorption near-edge spectra (XANES). The O K and RE M4,5/L3 XANES indicate that the presence of Ce and Pr (in 3+/4+ states) leads to the formation of intermediate 4f energy levels between the O 2p and the RE 5d gap in HEO. It is concluded that heat treatment under reducing/oxidizing atmospheres affects these intermediate levels, thus offering the possibility to tune the bandgap energy in HEOs. © 2020 Author(s).
    view abstract10.1063/5.0007944
  • Scale-dependent particle diffusivity and apparent viscosity in polymer solutions as probed by dynamic magnetic nanorheology
    Hess, M. and Gratz, M. and Remmer, H. and Webers, S. and Landers, J. and Borin, D. and Ludwig, F. and Wende, H. and Odenbach, S. and Tschöpe, A. and Schmidt, A.M.
    Soft Matter 16 (2020)
    In several upcoming rheological approaches, including methods of micro- and nanorheology, the measurement geometry is of critical impact on the interpretation of the results. The relative size of the probe objects employed (as compared to the intrinsic length scales of the sample to be investigated) becomes of crucial importance, and there is increasing interest to investigate the dynamic processes and mobility in nanostructured materials. A combination of different rheological approaches based on the rotation of magnetically blocked nanoprobes is used to systematically investigate the size-dependent diffusion behavior in aqueous poly(ethylene glycol) (PEG) solutions with special attention paid to the relation of probe size to characteristic length scales within the polymer solutions. We employ two types of probe particles: nickel rods of hydrodynamic length Lh between 200 nm and 650 nm, and cobalt ferrite spheres with diameter dh between 13 nm and 23 nm, and examine the influence of particle size and shape on the nanorheological information obtained in model polymer solutions based on two related, dynamic-magnetic approaches. The results confirm that as long as the investigated solutions are not entangled, and the particles are much larger than the macromolecular correlation length, a good accordance between macroscopic and nanoscopic results, whereas a strong size-dependent response is observed in cases where the particles are of similar size or smaller than the radius of gyration Rg or the correlation length ξ of the polymer solution. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c9sm00747d
  • Soft X-Ray Magnetic Circular Dichroism of Vanadium in the Metal–Insulator Two-Phase Region of Paramagnetic V2O3 Doped with 1.1% Chromium
    Schmitz, D. and Schmitz-Antoniak, C. and Radu, F. and Ryll, H. and Luo, C. and Bhandary, S. and Biermann, S. and Siemensmeyer, K. and Wende, H. and Ivanov, S. and Eriksson, O.
    Physica Status Solidi (B) Basic Research 257 (2020)
    V2O3 doped with 1.1% Cr is investigated at its isostructural correlation-driven metal–insulator transition near room temperature in its paramagnetic state with X-ray magnetic circular dichroism (XMCD) spectroscopy in external magnetic fields. A relative XMCD amplitude of about 2 permille is observed at the L2,3 absorption edges of vanadium as expected for magnetic moment per mass values of the order of 1 J T−1 kg−1 from magnetometry and the literature. Across the metal–insulator transition, the vanadium XMCD spectral shape significantly changes. According to atomic multiplet simulations, these changes are due to a changing orbital occupation indicating a changing phase composition. According to estimates used in this study, the dipole moment of the spin density distribution (Formula presented.) in the bulk increases such that the effective vanadium spin moment increases by a few percent with temperature in the two-phase region. Thereby, it partially compensates for the decrease in the relative XMCD amplitude due to a decreasing alignment of the paramagnetic moments. After a few minor temperature cycles, the sample is in a two-phase state in which the XMCD and X-ray linear dichroism spectra hardly depend on the temperature, and the specific electrical resistance is intermediate, showing only a weak sign of the metal–insulator transition. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201900456
  • Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902051
  • Theoretical methods
    Herper, H.C. and Brena, B. and Puglia, C. and Bhandary, S. and Wende, H. and Eriksson, O. and Sanyal, B.
    SpringerBriefs in Applied Sciences and Technology (2020)
    In this chapter, the theoretical methods required for the description of structure, electronic structure and magnetism of magnetic molecules in the gas phase and in the adsorbed configurations will be discussed. The main workhorse of the theoretical methods is the density functional theory that provides a materials-specific description of electronic structure, which is quite sufficient for many of the materials. However, in the present context of magnetic molecules, one needs to go beyond standard approximations in density functional theory. In this regard, some of the crucial characteristics in the electronic structure and magnetism will be discussed such as electron correlation, van der Waals interaction, band gaps, magnetic anisotropy and spin-dipole moments. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2020.
    view abstract10.1007/978-981-15-3719-6_3
  • CoTPP molecules deposited on graphene/Ni (111): Quenching of the antiferromagnetic interaction induced by gold intercalation
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Wende, H. and Otero, E. and Affronte, M.
    Journal of Applied Physics 125 (2019)
    In this work, we investigated the effect of Au-intercalation on the magnetic coupling between a sub-monolayer of Co-Tetraphenylporphyrin molecules and a graphene-covered Ni(111) single crystal. Using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and its field dependence were probed. Cobalt strongly couples antiferromagnetically to the nickel substrate, also through the graphene layer. The intercalation of graphene with gold leads to a complete removal of this coupling. © 2019 Author(s).
    view abstract10.1063/1.5063562
  • Effects of ta substitution on the microstructure and transport properties of hf-doped nbfesb half-heusler thermoelectric materials
    Farahi, N. and Stiewe, C. and Truong, D.Y.N. and Shi, Y. and Salamon, S. and Landers, J. and Eggert, B. and Wende, H. and De Boor, J. and Kleinke, H. and Müller, E.
    ACS Applied Energy Materials 2 (2019)
    This investigation demonstrates the effect of partial substitutions of Nb by refractory Ta on the microstructure and thermoelectric properties of Hf-doped NbFeSb materials. All the synthesized samples show a heavily doped semiconducting character with the electrical conductivity higher than 3500 ω-1 cm-1 at 326 K. Furthermore, the samples containing Ta display consistently lower (∼10-13%) thermal conductivity of ∼7 W m-1 K-1 at 350 K compared to a value of ∼8 W m-1 K-1 at the same temperature for the nonsubstituted sample. The vivid impact of Ta substitutions on reducing the lattice thermal conductivity of NbFeSb based materials is chiefly due to the lattice disorder originating from the mass difference between Ta and Nb atoms, resulting in ∼28% reduction in lattice thermal conductivity of the Nb0.73Hf0.12Ta0.15FeSb sample at 350 K compared to the nonsubstituted sample. The results of our Mößbauer spectroscopy measurements exclude the possibility of mixed Fe occupancies. Although magnetic properties were not strongly modified by the Ta substitution, Nb0.83Hf0.12Ta0.05FeSb shows a magnetic phase transition at ∼150 K, which is not observed in Nb0.88Hf0.12FeSb. This could be caused by extrinsic defects and microstructure induced by Ta addition. All samples exhibit a similar maximum dimensionless figure of merit value, zTmax, of ∼0.75 at 800 K, which is comparable to the high efficiency materials previously reported in this system and makes them potential candidates to be utilized as p-type legs in half-Heusler based thermoelectric generators (TEG). Copyright © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b01711
  • Epitaxial strain adaptation in chemically disordered FeRh thin films
    Witte, R. and Kruk, R. and Wang, D. and Schlabach, S. and Brand, R.A. and Gruner, M.E. and Wende, H. and Hahn, H.
    Physical Review B 99 (2019)
    Strain and strain adaptation mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaptation mechanism in a thin film model system as a function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g., epitaxial boundary condition. It is shown by means of high-resolution x-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response is explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a≈0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaptation behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.134109
  • Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator
    Huttmann, F. and Rothenbach, N. and Kraus, S. and Ollefs, K. and Arruda, L.M. and Bernien, M. and Thonig, D. and Delin, A. and Fransson, J. and Kummer, K. and Brookes, N.B. and Eriksson, O. and Kuch, W. and Michely, T. and Wende, H.
    Journal of Physical Chemistry Letters 10 (2019)
    We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M 5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)μ B and (+7.0 ± 0.6)μ B , respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M 5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower. © 2019 American Chemical Society.
    view abstract10.1021/acs.jpclett.8b03711
  • Extracting the dynamic magnetic contrast in time-resolved X-ray transmission microscopy
    Schaffers, T. and Feggeler, T. and Pile, S. and Meckenstock, R. and Buchner, M. and Spoddig, D. and Ney, V. and Farle, M. and Wende, H. and Wintz, S. and Weigand, M. and Ohldag, H. and Ollefs, K. and Ney, A.
    Nanomaterials 9 (2019)
    Using a time-resolved detection scheme in scanning transmission X-ray microscopy (STXM), we measured element resolved ferromagnetic resonance (FMR) at microwave frequencies up to 10 GHz and a spatial resolution down to 20 nm at two different synchrotrons. We present different methods to separate the contribution of the background from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the GHz microwave excitation and the X-ray pulses generated by the synchrotron, as well as the opening angle of the precession at FMR can be quantified. A detailed analysis for homogeneous and inhomogeneous magnetic excitations demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect. The dynamic magnetic contrast in time-resolved STXM has the potential be a powerful tool to study the linear and nonlinear, magnetic excitations in magnetic micro-and nano-structures with unique spatial-temporal resolution in combination with element selectivity. © 2019 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/nano9070940
  • FEM analysis of a multiferroic nanocomposite: comparison of experimental data and numerical simulation
    Labusch, M. and Lemke, V. and Schmitz-Antoniak, C. and Schröder, J. and Webers, S. and Wende, H.
    Archive of Applied Mechanics 89 (2019)
    In this contribution, we analyze the properties of two-phase magneto-electric (ME) composites. Such ME composite materials have raised scientific attention in the last decades due to many possible applications in a wide range of technical devices. Since the effective magneto-electric properties significantly depend on the microscopic morphology, we investigate in more detail the changes in the in-plane polarization due to an applied magnetic field. It was shown in previous works that it is possible to grow vertically aligned nanopillars of magnetostrictive cobalt ferrite in a piezoelectric barium titanate matrix by pulsed laser deposition. Based on x-ray linear dichroism, the displacements of titanate ions in the matrix material can be measured due to an applied magnetic field near the boundary of the interface between the matrix and the nanopillars. Here, we focus on (1–3) fiber-induced composites, based on previous experiments, where cobalt ferrite nanopillars are embedded in a barium titanate matrix. In the numerical simulations, we adjusted the boundary value problem to match the experimental setup and compare the results with previously made assumptions of the in-plane polarizations. A further focus is taken on the deformation behavior of the nanopillar over its whole height. Such considerations validate the assumption of the distortion of the nanopillars under an external magnetic field. Furthermore, we analyze the resulting magneto-electric coupling coefficient. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s00419-019-01534-z
  • High-entropy oxides: An emerging prospect for magnetic rare-earth transition metal perovskites
    Witte, R. and Sarkar, A. and Kruk, R. and Eggert, B. and Brand, R.A. and Wende, H. and Hahn, H.
    Physical Review Materials 3 (2019)
    It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single-phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high-entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the multielement phase diagram. Among the already realized structures there is the highly complex class of rare-earth transition element perovskites. This fascinating class of materials generated by applying the innovative concept of high-entropy stabilization provides a new and manyfold research space with promise of discoveries of unprecedented properties and phenomena. The present study provides a first investigation of the magnetic properties of selected compounds of this novel class of materials. Comprehensive studies by DC and AC magnetometry are combined with element specific spectroscopy in order to understand the interplay between magnetic exchange and the high degree of chemical disorder in the systems. We observe a predominant antiferromagnetic behavior in the single-phase materials, combined with a small ferromagnetic contribution. The latter can be attributed to either small ferromagnetic clusters or configurations in the antiferromagnetic matrix or a possible spin canting. In the long term perspective it is proposed to screen the properties of this family of compounds with high throughput methods, including combined experimental and theoretical approaches. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.034406
  • In-Field Orientation and Dynamics of Ferrofluids Studied by Mössbauer Spectroscopy
    Landers, J. and Salamon, S. and Remmer, H. and Ludwig, F. and Wende, H.
    ACS Applied Materials and Interfaces 11 (2019)
    By studying the response behavior of ferrofluids of 6-22 nm maghemite nanoparticles in glycerol solution exposed to external magnetic fields, we demonstrate the ability of Mössbauer spectroscopy to access a variety of particle dynamics and static magnetic particle characteristics at the same time, offering an extensive characterization of ferrofluids for in-field applications; field-dependent particle alignment and particle mobility in terms of Brownian motion have been extracted simultaneously from a series of Mössbauer spectra for single-core particles as well as for particle agglomerates. Additionally, information on Néel superspin relaxation and surface spin frustration could be directly inferred from this analysis. Parameters regarding Brownian particle dynamics, as well as Néel-type relaxation behavior, obtained via Mössbauer spectroscopy, have been verified by complementary AC-susceptometry experiments, modulating the AC-field amplitude, and using an extended frequency range of 10-1 to 106 Hz, while field-dependent particle alignment has been cross-checked via magnetometry. © 2018 American Chemical Society.
    view abstract10.1021/acsami.8b16356
  • Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. and Bovensiepen, U. and Sokolowski-Tinten, K. and Eschenlohr, A.
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.174301
  • Nanoscale Physical and Chemical Structure of Iron Oxide Nanoparticles for Magnetic Particle Imaging
    Hufschmid, R. and Landers, J. and Shasha, C. and Salamon, S. and Wende, H. and Krishnan, K.M.
    Physica Status Solidi (A) Applications and Materials Science 216 (2019)
    In this work, the role of the nanoscale chemical and magnetic structure on relaxation dynamics of iron oxide nanoparticles in the context of magnetic particle imaging (MPI) is investigated with Mössbauer spectroscopy (MS) and electron energy loss spectroscopy (EELS). Two samples of 27 nm monodisperse iron oxide nanoparticles are compared, with and without an additional oxidation optimization step, with corresponding differences in structure and properties. Iron oxide nanoparticles synthesized in the presence of sufficient oxygen form single crystalline, inverse-spinel magnetite (Fe 3 O 4 ) and display magnetic properties suitable for MPI. A secondary wüstite (FeO) phase is observed in the diffraction pattern of unoptimized nanoparticles, which is antiferromagnetic and therefore unsuitable for MPI. Mössbauer spectra confirm the composition of the optimized nanoparticles to be ≈70% magnetite, with the remaining 30% oxidized to maghemite; in contrast, the as-synthesized particles (without the oxidation step) contained about 40% wüstite and 60% magnetite. The authors use scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to probe iron 2p-3d electronic transitions and correlate their intensities with the oxidation state with sub-nanometer spatial resolution. The optimally oxidized nanoparticles are uniform in crystallography and phase, while the mixed phase nanoparticles are core-shell wüstite/magnetite. Further confirming the core-shell structure of the mixed phase nanoparticles, considerable spin canting in the in-field Mössbauer spectrum, likely caused by interface coupling, is observed. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssa.201800544
  • Porous purple glass-a cobalt imidazolate glass with accessible porosity from a meltable cobalt imidazolate framework
    Frentzel-Beyme, L. and Kloß, M. and Pallach, R. and Salamon, S. and Moldenhauer, H. and Landers, J. and Wende, H. and Debus, J. and Henke, S.
    Journal of Materials Chemistry A 7 (2019)
    We report the first microporous cobalt imidazolate glass obtained from a meltable cobalt-based zeolitic imidazolate framework, ZIF-62(Co). Crystalline ZIF-62(Co) is constructed from Co2+ cations and two different imidazolate-type linkers, namely conventional imidazolate and benzimidazolate. The microporous framework melts at ∼430 °C and converts into a glass upon cooling to room temperature. X-Ray total scattering and Raman spectroscopy reveal that the local structure of the glass and the crystalline parent material are very similar. Magnetic measurements and X-ray diffraction uncover that ZIF-62(Co) partially decomposes upon melting and glass formation resulting in the reduction of ∼3% of the Co2+ ions to metallic cobalt. Most importantly, the ZIF glass retains almost 50% of the porosity of crystalline ZIF-62(Co). Our results pave the way for the realisation of metal-organic framework glasses containing open shell metal ions, as well as the application of these porous glasses in gas separation, energy storage and catalysis. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ta08016j
  • Size effects on rotational particle diffusion in complex fluids as probed by Magnetic Particle Nanorheology
    Hess, M. and Roeben, E. and Rochels, P. and Zylla, M. and Webers, S. and Wende, H. and Schmidt, A.M.
    Physical Chemistry Chemical Physics 21 (2019)
    Rheological approaches based on micro- or nanoscopic probe objects are of interest due to the low volume requirement, the option of spatially resolved probing, and the minimal-invasive nature often connected to such probes. For the study of microstructured systems or biological environments, such methods show potential for investigating the local, size-dependent diffusivity and particle-matrix interactions. For the latter, the relative length scale of the used probes compared to the size of the structural units of the matrix becomes relevant. In this study, a rotational-dynamic approach based on Magnetic Particle Nanorheology (MPN) is used to extract size- and frequency-dependent nanorheological properties by using an otherwise well-established polymer model system. We use magnetically blocked CoFe2O4 nanoparticles as tracers and systematically vary their hydrodynamic size by coating them with a silica shell. On the polymer side, we employ aqueous solutions of poly(ethylene glycol) (PEG) by varying molar mass M and volume fraction ϕ. The complex Brownian relaxation behavior of the tracer particles in solutions of systematically varied composition is investigated by means of AC susceptometry (ACS), and the results provide access to frequency dependent rheological properties. The size-dependent particle diffusivity is evaluated based on theoretical descriptions and macroscopic measurements. The results allow the classification of the investigated compositions into three regimes, taking into account the probe particle size and the length scales of the polymer solution. While a fuzzy cross-over is indicated between the well-known macroscopic behavior and structurally dominated spectra, where the hydrodynamic radius is equal to the radius of gyration of the polymer (rh ∼ Rg), the frequency-related scaling behavior is dominated by the correlation length ξ respectively by the tube diameter a in entangled solutions for rh < Rg. This journal is © the Owner Societies.
    view abstract10.1039/c9cp04083h
  • Tuning the optical, structural and multiferroic properties of Bismuth Ferrite (BiFeO3) Nanoparticles by Doping with Ba
    Dubey, A. and Castillo, M.E. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H.
    2019 IEEE International Symposium on Applications of Ferroelectrics, ISAF 2019 - Proceedings (2019)
    Bismuth Ferrite Nanoparticles (BiFeO3 NPs) are interesting single-phase multiferroic materials due to their notable magnetoelectric properties at room temperature. We enhance the magnetization of BFO NPs via doping 5 % Ba at the Bi site. These NPs were synthesized by a modified and reproducible sol-gel technique and further characterized by XRD (x-ray diffraction), VSM (Vibrating Sample Magnetometer), PFM (Piezoresponse Force Microscopy), DSC (Differential Scanning Calorimetry) and UV-Vis (Absorption spectroscopy) techniques. Rietveld refinement unveils a lattice distortion and shows that the average crystallite size is reduced from 43.7 to 38.4 nm after Ba incorporation. 5 % Ba enhances the ferromagnetism from 0.71 to 0.86 Am2/kg at 300 K. DSC curves confirm no significant variation in magnetic transition temperature (TN) after Ba doping. Moreover, PFM data show the ferroelectric behavior of 5 % Ba doped NPs with reduced piezoresponse. UV-Vis spectra show the presence of crystal field transition and a doubly degenerate d-d transition, whereas the band gap of 5 % Ba doped BFO NPs increases from 2.18 eV for BFO NPs to 2.23 eV. © 2019 IEEE.
    view abstract10.1109/ISAF43169.2019.9034963
  • Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6 Si1.4 by nuclear resonant inelastic x-ray scattering
    Landers, J. and Salamon, S. and Keune, W. and Gruner, M.E. and Krautz, M. and Zhao, J. and Hu, M.Y. and Toellner, T.S. and Alp, E.E. and Gutfleisch, O. and Wende, H.
    Physical Review B 98 (2018)
    Magnetocaloric LaFe13-xSix-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent Fe57 nuclear resonant inelastic x-ray scattering measurements (in a field μ0H of ∼0.7 T) of the vibrational (phonon) density of states, VDOS, in LaFe11.6Si1.4 across the metamagnetic isostructural first-order phase transition at TC∼192 K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at TC. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across TC, occurring within a small temperature interval of ΔT≤12 K around TC. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to TC, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states D(EF) at the Fermi energy EF. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy ΔSvib of +6.9±2.6 J/(kg K) and in the vibrational specific heat ΔCvib of +2.7±1.6 J/(kg K) upon heating. The increase in magnitude of the vibrational entropy |ΔSvib|=6.9 J/(kg K) of the Fe sublattice at TC upon heating is substantial, if compared with the magnitude of the isothermal entropy change |ΔSiso| of 14.2 J/(kg K) in a field change ΔB from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. We demonstrate that ΔSvib obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change ΔSiso at the phase transition of LaFe13-xSix. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.024417
  • Effect of substrate orientation on local magnetoelectric coupling in bi-layered multiferroic thin films
    Naveed-Ul-Haq, M. and Webers, S. and Trivedi, H. and Salamon, S. and Wende, H. and Usman, M. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Nanoscale 10 (2018)
    In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr06041j
  • Fluoride doped γ-Fe2O3 nanoparticles with increased MRI relaxivity
    Jones, N.E. and Burnett, C.A. and Salamon, S. and Landers, J. and Wende, H. and Lazzarini, L. and Gibbs, P. and Pickles, M. and Johnson, B.R.G. and Evans, D.J. and Archibald, S.J. and Francesconi, M.G.
    Journal of Materials Chemistry B 6 (2018)
    Iron oxide nanoparticles (IONs) are being actively researched and experimented with as contrast agents for Magnetic Resonance Imaging (MRI), as well as image-directed delivery of therapeutics. The efficiency of an MRI contrast agent can be described by its longitudinal and transverse relaxivities, r1 and r2. γ-Fe2O3 nanoparticles-doped with fluoride in a controlled manner and functionalised with citric acid-showed a 3-fold increase in r1 and a 17-fold increase in r2 in a magnetic field of 3 T and almost 6-fold increase in r1 and a 14-fold increase in r2 at 11 T. Following fluorination, PXRD shows that the crystal structure of γ-Fe2O3 is maintained, Mössbauer spectroscopy shows that the oxidation state of the Fe cation is unchanged and HREM shows that the particle size does not vary. However, magnetisation curves show a large increase in the coercive field, pointing towards a large increase in the magnetic anisotropy for the fluorinated nanoparticles compared to the un-doped γ-Fe2O3 nanoparticles. Therefore, a chemically induced increase in magnetic anisotropy appears to be the most relevant parameter responsible for the large increase in relaxivity for γ-Fe2O3 nanoparticles. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8tb00360b
  • Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications
    Scheibel, F. and Gottschall, T. and Taubel, A. and Fries, M. and Skokov, K.P. and Terwey, A. and Keune, W. and Ollefs, K. and Wende, H. and Farle, M. and Acet, M. and Gutfleisch, O. and Gruner, M.E.
    Energy Technology 6 (2018)
    Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first-order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First-order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first-order magnetocaloric materials such as Ni–Mn-based Heusler alloys, FeRh, La(FeSi)13-based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size- and time-dependence of the MCE at magnetostructural transitions. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ente.201800264
  • Local probe of irradiation-induced structural changes and orbital magnetism in Fe60Al40 thin films via an order-disorder phase transition
    La Torre, E. and Smekhova, A. and Schmitz-Antoniak, C. and Ollefs, K. and Eggert, B. and Cöster, B. and Walecki, D. and Wilhelm, F. and Rogalev, A. and Lindner, J. and Bali, R. and Banerjee, R. and Sanyal, B. and Wende, H.
    Physical Review B 98 (2018)
    Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick Fe60Al40 thin films along the order-disorder (B2→A2) phase transition initiated by 20-keV Ne+ ion irradiation with fluences of (0.75-6)×1014ionscm-2. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and ∼1% of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at ∼60 eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed Fe60Al40 model structures for the ordered (B2) and the disordered (A2) phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.024101
  • Moment-Volume Coupling in La(Fe1−xSix)13
    Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Krautz, M. and Zhao, J. and Hu, M.Y. and Toellner, T. and Alp, E.E. and Gutfleisch, O. and Wende, H.
    Physica Status Solidi (B) Basic Research 255 (2018)
    We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe1−xSix)13 by means of first-principles calculations combined with the fixed-spin moment approach. We find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. In the same spirit, the dependence of the average local Fe moment on the Si content can account for the occurence of first- and second-order transitions in the system. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700465
  • Probing magnetic coupling between LnPc2 (Ln = Tb, Er) molecules and the graphene/Ni (111) substrate with and without Au-intercalation: Role of the dipolar field
    Corradini, V. and Candini, A. and Klar, D. and Biagi, R. and De Renzi, V. and Lodi Rizzini, A. and Cavani, N. and Del Pennino, U. and Klyatskaya, S. and Ruben, M. and Velez-Fort, E. and Kummer, K. and Brookes, N.B. and Gargiani, P. and Wende, H. and Affronte, M.
    Nanoscale 10 (2018)
    Lanthanides (Ln) bis-phthalocyanine (Pc), the so-called LnPc2double decker, are a promising class of molecules with a well-defined magnetic anisotropy. In this work, we investigate the magnetic properties of LnPc2 molecules UHV-deposited on a graphene/Ni(111) substrate and how they modify when an Au layer is intercalated between Ni and graphene. X-ray absorption spectroscopy (XAS), and linear and magnetic circular dichroism (XLD and XMCD) were used to characterize the systems and probe the magnetic coupling between LnPc2 molecules and the Ni substrate through graphene, both gold-intercalated and not. Two types of LnPc2 molecules (Ln = Tb, Er) with a different magnetic anisotropy (easy-axis for Tb, easy-plane for Er) were considered. XMCD shows an antiferromagnetic coupling between Ln and Ni(111) even in the presence of the graphene interlayer. Au intercalation causes the vanishing of the interaction between Tb and Ni(111). In contrast, in the case of ErPc2, we found that the gold intercalation does not perturb the magnetic coupling. These results, combined with the magnetic anisotropy of the systems, suggest the possible importance of the magnetic dipolar field contribution for determining the magnetic behaviour. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7nr06610d
  • Strong converse magnetoelectric effect in (Ba,Ca)(Zr,Ti)O3 - NiFe2O4 multiferroics: A relationship between phase-connectivity and interface coupling
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Trivedi, H. and Salamon, S. and Webers, S. and Wende, H. and Hagemann, U. and Schröder, J. and Lupascu, D.C.
    Acta Materialia 144 (2018)
    Studying multiferroic magnetoelectrics has been a focus field for the last decade and a half, and the exploration of new materials is one of the several aspects of this quest. Here we report on the synthesis and characterization of NiFe2O4-based multiferroic composites which employ (Ba,Ca)(Zr,Ti)O3 as the ferroelectric/piezoelectric component and NiFe2O4 as the magnetostrictive phase. We find that these composites show excellent magnetoelectric properties. Especially the composite with 30 vol% of NiFe2O4 has a converse ME coefficient approximately two times larger than the previously reported one for BaTiO3-CoFe2O4 composites. A relationship between the phase connectivity within these composites and the ME properties was explored by the time of flight secondary ion mass microscopy. We believe that our investigation will be helpful for the design of magnetoelectric materials as components of sensors and memory devices. © 2017 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2017.10.048
  • Suppression of the Verwey Transition by Charge Trapping
    Schmitz-Antoniak, C. and Schmitz, D. and Warland, A. and Darbandi, M. and Haldar, S. and Bhandary, S. and Sanyal, B. and Eriksson, O. and Wende, H.
    Annalen der Physik (2018)
    The Verwey transition in Fe3O4 nanoparticles with a mean diameter of 6.3 nm is suppressed after capping the particles with a 3.5 nm thick shell of SiO2. By X-ray absorption spectroscopy and its associated X-ray magnetic circular dichroism this suppression can be correlated to localized Fe2+ states and a reduced double exchange visible in different site-specific magnetization behavior in high magnetic fields. The results are discussed in terms of charge trapping at defects in the Fe3O4/ SiO2 interface and the consequent difficulties in the formation of the common phases of Fe3O4. By comparison to X-ray absorption spectra of bare Fe3O4 nanoparticles in course of the Verwey transition, particular changes in the spectral shape could be correlated to changes in the number of unoccupied d states for Fe ions at different lattice sites. These findings are supported by density functional theory calculations. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/andp.201700363
  • The Role of Composition of Uniform and Highly Dispersed Cobalt Vanadium Iron Spinel Nanocrystals for Oxygen Electrocatalysis
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Lunkenbein, T. and Greiner, M.T. and Masliuk, L. and Salamon, S. and Landers, J. and Schlögl, R. and Wende, H. and Pentcheva, R. and Schulz, S. and Behrens, M.
    ACS Catalysis 8 (2018)
    Cation substitution in transition-metal oxides is an important approach to improve electrocatalysts by the optimization of their composition. Herein, we report on phase-pure spinel-type CoV2-xFexO4 nanoparticles with 0 ≤ x ≤ 2 as a new class of bifunctional catalysts for the oxygen evolution (OER) and oxygen reduction reactions (ORR). The mixed-metal oxide catalysts exhibit high catalytic activity for both OER and ORR that strongly depends on the V and Fe content. CoV2O4 is known to exhibit a high conductivity, while in CoFe2O4 the cobalt cation distribution is expected to change due to the inversion of the spinel structure. The optimized catalyst, CoV1.5Fe0.5O4, shows an overpotential for the OER of â300 mV for 10 mA cm-2 with a Tafel slope of 38 mV dec-1 in alkaline electrolyte. DFT+U+SOC calculations on cation ordering confirm the tendency toward the inverse spinel structure with increasing Fe concentration in CoV2-xFexO4 that starts to dominate already at low Fe contents. The theoretical results also show that the variations of oxidation states are related to the surface region, where the redox activity was found experimentally to be manifested in the transformation of V3+ ↠V2+. The high catalytic activity, facile synthesis, and low cost of the CoV2-xFexO4 nanoparticles render them very promising for application in bifunctional electrocatalysis. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b03529
  • Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose
    Waag, F. and Gökce, B. and Kalapu, C. and Bendt, G. and Salamon, S. and Landers, J. and Hagemann, U. and Heidelmann, M. and Schulz, S. and Wende, H. and Hartmann, N. and Behrens, M. and Barcikowski, S.
    Scientific Reports 7 (2017)
    Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm-2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec-1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials. © 2017 The Author(s).
    view abstract10.1038/s41598-017-13333-zf
  • Effect of Al3+ modification on cobalt ferrite and its impact on the magnetoelectric effect in BCZT–CFO multiferroic composites
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Constantinescu, G. and Trivedi, H. and Salamon, S. and Landers, J. and Wende, H. and Lupascu, D.C.
    Journal of Materials Science (2017)
    One of the methods to enhance the functional properties of two-phase multiferroic magnetoelectrics is to increase magnetostriction of the ferrite phase. Al3+-modified cobalt ferrite Co(Al0.5Fe1.5)O4 shows better magnetostriction than unmodified cobalt ferrite. It is used in combination with (Ba,Ca)(Zr,Ti)O3 which has very good piezoelectric properties, to form a multiferroic composite. The composite shows good magnetoelectric characteristics, both macroscopically (converse magnetoelectric coefficient of 11 ps/m) and microscopically. Al3+ proves to be the best non-magnetic dopant to enhance magnetostriction in CoFe2O4 and thus the magnetoelectric coefficient. © 2017 Springer Science+Business Media, LLC
    view abstract10.1007/s10853-017-1444-4
  • Effect of etching on exchange bias in CoFe/IrMn bilayers studied by soft X-ray XMCD and resonant magnetic reflectometry
    O'Donnell, D. and Smekhova, A. and Wende, H. and Fan, R. and Steadman, P. and Du, Y. and Hassan, S. and Dobrynin, A.
    2017 IEEE International Magnetics Conference, INTERMAG 2017 (2017)
    Exchange biased (EB) antiferromagnetic (AFM) - ferromagnetic (FM) bilayers are essential parts of modern spintronic devices, such as spin valve sensors or magnetic random access memory. © 2017 IEEE.
    view abstract10.1109/INTMAG.2017.8007643
  • Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa8e7c
  • Ferromagnetic resonance of MBE-grown FeRh thin films through the metamagnetic phase transition
    Heidarian, A. and Stienen, S. and Semisalova, A. and Yuan, Y. and Josten, E. and Hübner, R. and Salamon, S. and Wende, H. and Gallardo, R.A. and Grenzer, J. and Potzger, K. and Bali, R. and Facsko, S. and Lindner, J.
    Physica Status Solidi (B) Basic Research 254 (2017)
    An FeRh thin film of 33 nm thickness grown by molecular beam epitaxy (MBE) has been investigated with respect to its temperature dependent magnetic properties by means of ferromagnetic resonance (FMR). Within the ferromagnetic regime, that is at temperatures above the antiferromagnetic-to-ferromagnetic phase transition, the resonance field decreases with decreasing temperature reflecting an increasing magnetization. Within the temperature regime of the phase transition, the resonance field behaves non-monotonically, that is it suddenly increases with decreasing temperature. The observed asymmetric shape of the FMR absorption line is discussed with respect to a possible small magnetic inhomogeneity of the film coupled to the main ferromagnetic volume of B2 ordered, equiatomic FeRh. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700145
  • Is There a Temperature Limit in Planet Formation at 1000 K?
    Demirci, T. and Teiser, J. and Steinpilz, T. and Landers, J. and Salamon, S. and Wende, H. and Wurm, G.
    Astrophysical Journal 846 (2017)
    Dust drifting inward in protoplanetary disks is subject to increasing temperatures. In laboratory experiments, we tempered basaltic dust between 873 K and 1273 K and find that the dust grains change in size and composition. These modifications influence the outcome of self-consistent low speed aggregation experiments showing a transition temperature of 1000 K. Dust tempered at lower temperatures grows to a maximum aggregate size of 2.02 ±0.06 mm, which is 1.49 ±0.08 times the value for dust tempered at higher temperatures. A similar size ratio of 1.75 ±0.16 results for a different set of collision velocities. This transition temperature is in agreement with orbit temperatures deduced for observed extrasolar planets. Most terrestrial planets are observed at positions equivalent to less than 1000 K. Dust aggregation on the millimeter-scale at elevated temperatures might therefore be a key factor for terrestrial planet formation. © 2017. The American Astronomical Society. All rights reserved.
    view abstract10.3847/1538-4357/aa816c
  • Magnetic properties of the Bi0.65La0.35Fe0.5Sc0.5O3 perovskite
    Fedorchenko, A.V. and Fertman, E.L. and Desnenko, V.A. and Kotlyar, O.V. and Čižmár, E. and Shvartsman, V.V. and Lupascu, D.C. and Salamon, S. and Wende, H. and Salak, A.N. and Khalyavin, D.D. and Olekhnovich, N.M. and Pushkarev, A.V. and Radyush, Yu.V. and Feher, A.
    Acta Physica Polonica A 131 (2017)
    Magnetic properties of polycrystalline multiferroic Bi0.65La0.35Fe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions were studied using a SQUID magnetometer technique. The temperature dependent static magnetic moment M was measured in both zero-field-cooled and field-cooled modes over the temperature range of 5-300 K in low magnetic field H = 0.02 kOe. The field dependent magnetization M(H) was measured in magnetic fields up to 50 kOe at different temperatures up to 230 K after zero-field cooling procedure. A long-range magnetic ordering of the antiferromagnetic type with a weak ferromagnetic contribution takes place below TN ≈ 220 K. Magnetic hysteresis loops taken below TN show a huge coercive field up to Hc ≈ 10 kOe, while the magnetic moment does not saturate up to 50 kOe. A strong effect of magnetic field on the magnetic properties of the compound has been found. Below TN ≈ 220 K the derivatives of the initial magnetization curves demonstrate the existence of a temperature-dependent anomaly in fields of H = 15÷25 kOe. The nature of the anomaly is unknown and requires additional study.
    view abstract10.12693/APhysPolA.131.1069
  • Magnetism in a graphene- 4f-3d hybrid system
    Huttmann, F. and Klar, D. and Atodiresei, N. and Schmitz-Antoniak, C. and Smekhova, A. and Martínez-Galera, A.J. and Caciuc, V. and Bihlmayer, G. and Blügel, S. and Michely, T. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    We create an interface of graphene with a metallic and magnetic support that leaves its electronic structure largely intact. This is achieved by exposing epitaxial graphene on ferromagnetic thin films of Co and Ni to vapor of the rare earth metal Eu at elevated temperatures, resulting in the intercalation of an Eu monolayer in between graphene and its substrate. The system is atomically well defined, with the Eu monolayer forming a (3×3)R30 superstructure with respect to the graphene lattice. Thereby, we avoid the strong hybridization with the (Ni,Co) substrate 3d states that otherwise drastically modify the electronic structure of graphene. This picture is suggested by our x-ray absorption spectroscopy measurements which show that after Eu intercalation the empty 2p states of C atoms resemble more the ones measured for graphite in contrast to graphene directly bound to 3d ferromagnetic substrates. We use x-ray magnetic circular dichroism at the Co and Ni L2,3 and Eu M4,5 as an element-specific probe to investigate magnetism in these systems. An antiferromagnetic coupling between Eu and Co/Ni moments is found, which is so strong that a magnetic moment of the Eu layer can be detected at room temperature. Density functional theory calculations confirm the antiferromagnetic coupling and provide an atomic insight into the magnetic coupling mechanism. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.075427
  • Mobility investigations of magnetic nanoparticles in biocomposites
    Müller, R. and Zhou, M. and Liebert, T. and Landers, J. and Salamon, S. and Webers, S. and Dellith, A. and Borin, D. and Heinze, T. and Wende, H.
    Materials Chemistry and Physics 193 (2017)
    Biocompatible composites are presented, consisting of magnetite nanoparticles embedded into a matrix of meltable dextran ester, which can be softened under an induced alternating magnetic field and may thereby allow magnetically controlled release applications. Temperature dependent mobility investigations of magnetic nanoparticles in the molten composites were carried out by optical microscopy, magnetometry, AC susceptibility and Mössbauer spectroscopy measurements. Optical microscopy shows a movement of agglomerates and texturing in the micrometer scale, whereas AC-susceptometry and Mössbauer spectroscopy investigations reveal that the particles perform diffusive Brownian motion in the liquid polymer melt as separated particles rather than as large agglomerates. © 2017 Elsevier B.V.
    view abstract10.1016/j.matchemphys.2017.02.046
  • Planetesimal Formation in the Warm, Inner Disk: Experiments with Tempered Dust
    De Beule, C. and Landers, J. and Salamon, S. and Wende, H. and Wurm, G.
    Astrophysical Journal 837 (2017)
    It is an open question how elevated temperatures in the inner parts of protoplanetary disks influence the formation of planetesimals. We approach this problem here by studying the tensile strength of granular beds with dust samples tempered at different temperatures. We find via laboratory experiments that tempering at increasing temperatures is correlated with an increase in cohesive forces. We studied dust samples of palagonite (JSC Mars-1a) which were tempered for up to 200 hr at temperatures between 600 and 1200 K, and measured the relative tensile strengths of highly porous dust layers once the samples cooled to room temperature. Tempering increases the tensile strength from 800 K upwards. This change is accompanied by mineral transformations, the formation of iron oxide crystallites as analyzed by Mössbauer spectroscopy, changes in the number size distribution, and the morphology of the surface visible as cracks in larger grains. These results suggest a difference in the collisional evolution toward larger bodies with increasing temperature as collisional growth is fundamentally based on cohesion. While high temperatures might also increase sticking (not studied here), compositional evolution will already enhance the cohesion and the possibility of growing larger aggregates on the way toward planetesimals. This might lead to a preferred in situ formation of inner planets and explain the observed presence of dense inner planetary systems. © 2017. The American Astronomical Society. All rights reserved.
    view abstract10.3847/1538-4357/837/1/59
  • Role of Composition and Size of Cobalt Ferrite Nanocrystals in the Oxygen Evolution Reaction
    Chakrapani, K. and Bendt, G. and Hajiyani, H. and Schwarzrock, I. and Lunkenbein, T. and Salamon, S. and Landers, J. and Wende, H. and Schlögl, R. and Pentcheva, R. and Behrens, M. and Schulz, S.
    ChemCatChem 9 (2017)
    Sub-10 nm CoFe2O4 nanoparticles with different sizes and various compositions obtained by (partial) substitution of Co with Ni cations have been synthesized by using a one-pot method from organic solutions by the decomposition of metal acetylacetonates in the presence of oleylamine. The electrocatalytic activity of CoFe2O4 towards the oxygen evolution reaction (OER) is clearly enhanced with a smaller size (3.1 nm) of the CoFe2O4 nanoparticles (compared with 4.5 and 5.9 nm). In addition, the catalytic activity is improved by partial substitution of Co with Ni, which also leads to a higher degree of inversion of the spinel structure. Theoretical calculations attribute the positive catalytic effect of Ni owing to the lower binding energy differences between adsorbed O and OH compared with pure cobalt or nickel ferrites, resulting in higher OER activity. Co0.5Ni0.5Fe2O4 exhibited a low overpotential of approximately 340 mV at 10 mA cm−2, a smaller Tafel slope of 51 mV dec−1, and stability over 30 h. The unique tunability of these CoFe2O4 nanocrystals provides great potential for their application as an efficient and competitive anode material in the field of electrochemical water splitting as well as for systematic fundamental studies aiming at understanding the correlation of composition and structure with performance in electrocatalysis. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201700376
  • Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201702248
  • A new (Ba, Ca) (Ti, Zr)O 3 based multiferroic composite with large magnetoelectric effect
    Naveed-Ul-Haq, M. and Shvartsman, V.V. and Salamon, S. and Wende, H. and Trivedi, H. and Mumtaz, A. and Lupascu, D.C.
    Scientific Reports 6 (2016)
    The lead-free ferroelectric 0.5Ba(Zr 0.2 Ti 0.8)O 3 - 0.5(Ba 0.7 Ca 0.3)TiO 3 (BCZT) is a promising component for multifunctional multiferroics due to its excellent room temperature piezoelectric properties. Having a composition close to the polymorphic phase boundary between the orthorhombic and tetragonal phases, it deserves a case study for analysis of its potential for modern electronics applications. To obtain magnetoelectric coupling, the piezoelectric phase needs to be combined with a suitable magnetostrictive phase. In the current article, we report on the synthesis, dielectric, magnetic, and magnetoelectric characterization of a new magnetoelectric multiferroic composite consisting of BCZT as a piezoelectric phase and CoFe 2 O 4 (CFO) as the magnetostrictive phase. We found that this material is multiferroic at room temperature and manifests a magnetoelectric effect larger than that of BaTiO 3 -CoFe 2 O 4 bulk composites with similar content of the ferrite phase.
    view abstract10.1038/srep32164
  • Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films
    Wolloch, M. and Gruner, M.E. and Keune, W. and Mohn, P. and Redinger, J. and Hofer, F. and Suess, D. and Podloucky, R. and Landers, J. and Salamon, S. and Scheibel, F. and Spoddig, D. and Witte, R. and Roldan Cuenya, B. and Gutfleisch, O. and Hu, M.Y. and Zhao, J. and Toellner, T. and Alp, E.E. and Siewert, M. and Entel, P. and Pentcheva, R. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the X point depend critically on the Fe magnetic moment and atomic volume. Analyzing these nonvibrational modes leads to the discovery of a stable monoclinic ground-state structure, which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy, and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling. © 2016 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License.
    view abstract10.1103/PhysRevB.94.174435
  • Mastering hysteresis in magnetocaloric materials
    Gutfleisch, O. and Gottschall, T. and Fries, M. and Benke, D. and Radulov, I. and Skokov, K.P. and Wende, H. and Gruner, M. and Acet, M. and Entel, P. and Farle, M.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374 (2016)
    Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratoryscale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2 P-type compounds. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rsta.2015.0308
  • Molecular Design for Tailoring a Single-Source Precursor for Bismuth Ferrite
    Bendt, G. and Schiwon, R. and Salamon, S. and Landers, J. and Hagemann, U. and Limberg, C. and Wende, H. and Schulz, S.
    Inorganic Chemistry 55 (2016)
    Nearly phase-pure bismuth ferrite particles were formed by thermolysis of the single-source precursor [Cp(CO)2FeBi(OAc)2] (1) in octadecene at 245 °C, followed by subsequent calcination at 600 °C for 3 h. In contrast, the slightly modified compound [Cp(CO)2FeBi(O2CtBu)2] (2) yielded only mixtures of different bismuth oxide phases, revealing the distinctive influence of molecular design in material synthesis. The chemical composition, morphology, and crystallinity of the resulting materials were investigated by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. In addition, the optical properties were investigated by Fourier transform infrared and UV-vis spectroscopies, showing a strong band gap absorption in the visible range at 590 nm (2.2 eV). The magnetic behavior was probed by vibrating-sample and superconducting quantum interference device magnetometry, as well as 57Fe Mössbauer spectroscopy. © 2016 American Chemical Society.
    view abstract10.1021/acs.inorgchem.6b00951
  • Multiferroic Clusters: A New Perspective for Relaxor-Type Room-Temperature Multiferroics
    Henrichs, L. F. and Cespedes, O. and Bennett, J. and Landers, J. and Salamon, S. and Heuser, C. and Hansen, T. and Helbig, T. and Gutfleisch, O. and Lupascu, D. C. and Wende, H. and Kleemann, W. and Bell, A. J.
    Advanced Functional Materials 26 (2016)
    Multiferroics are promising for sensor and memory applications, but despite all efforts invested in their research no single-phase material displaying both ferroelectricity and large magnetization at room-temperature has hitherto been reported. This situation has substantially been improved in the novel relaxor ferroelectric single-phase (BiFe0.9Co0.1O3)(0.4)-(Bi1/2K1/2TiO3)(0.6), where polar nanoregions (PNR) transform into static-PNR as evidenced by piezoresponse force microscopy (PFM) and simultaneously enable congruent multiferroic clusters (MFC) to emerge from inherent strongly magnetic Bi(Fe,Co)O-3 rich regions as verified by magnetic force microscopy (MFM) and secondary ion mass spectrometry. The material's exceptionally large Neel temperature T-N = 670 +/- 10 K, as found by neutron diffraction, is proposed to be a consequence of ferrimagnetic order in MFC. On these MFC, exceptionally large direct and converse magnetoelectric (ME) coupling coefficients, approximate to 1.0 x 10(-5) s m(-1) at room-temperature, are measured by PFM and MFM, respectively. It is expected that the non-ergodic relaxor properties which are governed by the Bi1/2K1/2TiO3 component to play a vital role in the strong ME coupling, by providing an electrically and mechanically flexible environment to MFC. This new class of non-ergodic relaxor multiferroics bears great potential for applications. Especially the prospect of a ME nanodot storage device seems appealing.
    view abstract10.1002/adfm.201503335
  • Recipe for High Moment Materials with Rare-earth and 3d Transition Metal Composites
    Autieri, C. and Kumar, P.A. and Walecki, D. and Webers, S. and Gubbins, M.A. and Wende, H. and Sanyal, B.
    Scientific Reports 6 (2016)
    Materials with high volume magnetization are perpetually needed for the generation of sufficiently large magnetic fields by writer pole of magnetic hard disks, especially for achieving increased areal density in storage media. In search of suitable materials combinations for this purpose, we have employed density functional theory to predict the magnetic coupling between iron and gadolinium layers separated by one to several monolayers of 3d transition metals (Sc-Zn). We demonstrate that it is possible to find ferromagnetic coupling for many of them and in particular for the early transition metals giving rise to high moment. Cr and Mn are the only elements able to produce a significant ferromagnetic coupling for thicker spacer layers. We also present experimental results on two trilayer systems Fe/Sc/Gd and Fe/Mn/Gd. From the experiments, we confirm a ferromagnetic coupling between Fe and Gd across a 3 monolayers Sc spacer or a Mn spacer thicker than 1 monolayer. In addition, we observe a peculiar dependence of Fe/Gd magnetic coupling on the Mn spacer thickness.
    view abstract10.1038/srep29307
  • Relay-Like Exchange Mechanism through a Spin Radical between TbPc2 Molecules and Graphene/Ni(111) Substrates
    Marocchi, S. and Candini, A. and Klar, D. and Van Den Heuvel, W. and Huang, H. and Troiani, F. and Corradini, V. and Biagi, R. and De Renzi, V. and Klyatskaya, S. and Kummer, K. and Brookes, N.B. and Ruben, M. and Wende, H. and Del Pennino, U. and Soncini, A. and Affronte, M. and Bellini, V.
    ACS Nano 10 (2016)
    We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices. © 2016 American Chemical Society.
    view abstract10.1021/acsnano.6b04107
  • Reversed ageing of Fe3O4 nanoparticles by hydrogen plasma
    Schmitz-Antoniak, C. and Schmitz, D. and Warland, A. and Svechkina, N. and Salamon, S. and Piamonteze, C. and Wende, H.
    Scientific Reports 6 (2016)
    Magnetite (Fe3O4) nanoparticles suffer from severe ageing effects when exposed to air even when they are dispersed in a solvent limiting their applications. In this work, we show that this ageing can be fully reversed by a hydrogen plasma treatment. By x-ray absorption spectroscopy and its associated magnetic circular dichroism, the electronic structure and magnetic properties were studied before and after the plasma treatment and compared to results of freshly prepared magnetite nanoparticles. While aged magnetite nanoparticles exhibit a more γ-Fe2O3 like behaviour, the hydrogen plasma yields pure Fe3O4 nanoparticles. Monitoring the temperature dependence of the intra-atomic spin dipole contribution to the dichroic spectra gives evidence that the structural, electronic and magnetic properties of plasma treated magnetite nanoparticles can outperform the ones of the freshly prepared batch. © 2016, Nature Publishing Group. All rights reserved.
    view abstract10.1038/srep20897
  • Simultaneous Study of Brownian and Néel Relaxation Phenomena in Ferrofluids by Mössbauer Spectroscopy
    Landers, J. and Salamon, S. and Remmer, H. and Ludwig, F. and Wende, H.
    Nano Letters 16 (2016)
    We demonstrate the ability of Mössbauer spectroscopy to simultaneously investigate Brownian motion and Néel relaxation in ferrofluidic samples. For this purpose, Mössbauer spectra of coated iron oxide nanoparticles with core diameters of 6.0-26.4 nm dissolved in 70 vol % glycerol solution were recorded in the temperature range of 234-287 K and compared to low-temperature spectra without Brownian motion. By comparison to theory, we were able to determine the particle coating thickness and the dynamic viscosity of the fluid from the broadening of the absorption lines (Brownian motion), as well as the state of Néel relaxation. Results from Mössbauer spectroscopy were crosschecked by AC-susceptometry at several temperatures for Brownian motion and in the high-frequency regime (100 Hz-1 MHz) for Néel relaxation. © 2016 American Chemical Society.
    view abstract10.1021/acs.nanolett.5b04409
  • Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate
    Candini, A. and Klar, D. and Marocchi, S. and Corradini, V. and Biagi, R. and De Renzi, V. and Del Pennino, U. and Troiani, F. and Bellini, V. and Klyatskaya, S. and Ruben, M. and Kummer, K. and Brookes, N.B. and Huang, H. and Soncini, A. and Wende, H. and Affronte, M.
    Scientific Reports 6 (2016)
    Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc 2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.
    view abstract10.1038/srep21740
  • Tailoring magnetic frustration in strained epitaxial FeRh films
    Witte, R. and Kruk, R. and Gruner, M.E. and Brand, R.A. and Wang, D. and Schlabach, S. and Beck, A. and Provenzano, V. and Pentcheva, R. and Wende, H. and Hahn, H.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy, and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.104416
  • The Direct and the Converse Magnetoelectric Effect in Multiferroic Cobalt Ferrite–Barium Titanate Ceramic Composites
    Etier, M. and Shvartsman, V.V. and Salamon, S. and Gao, Y. and Wende, H. and Lupascu, D.C. and Raveau, B.
    Journal of the American Ceramic Society 99 (2016)
    We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe2O4)—barium titanate (BaTiO3) ceramic composites with (0-3) connectivity. Both the converse magnetoelectric coefficient, αC, and the direct voltage magnetoelectric coefficient, αE, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1−x) CoFe2O4–xBaTiO3 with x = 0.5 yielding values αC = 25 psm−1 and αE = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO3 content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO3 content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites. © 2016 The American Ceramic Society
    view abstract10.1111/jace.14362
  • Doping of inorganic materials in microreactors-preparation of Zn doped Fe3O4 nanoparticles
    Simmons, M.D. and Jones, N. and Evans, D.J. and Wiles, C. and Watts, P. and Salamon, S. and Escobar Castillo, M. and Wende, H. and Lupascu, D.C. and Francesconi, M.G.
    Lab on a Chip - Miniaturisation for Chemistry and Biology 15 (2015)
    Microreactor systems are now used more and more for the continuous production of metal nanoparticles and metal oxide nanoparticles owing to the controllability of the particle size, an important property in many applications. Here, for the first time, we used microreactors to prepare metal oxide nanoparticles with controlled and varying metal stoichiometry. We prepared and characterised Zn-substituted Fe3O4 nanoparticles with linear increase of Zn content (ZnxFe3-xO4 with 0 ≤ x ≤ 0.48), which causes linear increases in properties such as the saturation magnetization, relative to pure Fe3O4. The methodology is simple and low cost and has great potential to be adapted to the targeted doping of a vast array of other inorganic materials, allowing greater control on the chemical stoichiometry for nanoparticles prepared in microreactors. © 2015 The Royal Society of Chemistry.
    view abstract10.1039/c5lc00287g
  • Element-resolved thermodynamics of magnetocaloric lafe13-xsix
    Gruner, M.E. and Keune, W. and Roldan Cuenya, B. and Weis, C. and Landers, J. and Makarov, S.I. and Klar, D. and Hu, M.Y. and Alp, E.E. and Zhao, J. and Krautz, M. and Gutfleisch, O. and Wende, H.
    Physical Review Letters 114 (2015)
    By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13-xSix. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.114.057202
  • Element-specific electronic structure and magnetic properties of an epitaxial Ni51.6Mn32.9Sn15.5 thin film at the austenite-martensite transition
    Krumme, B. and Auge, A. and Herper, H.C. and Opahle, I. and Klar, D. and Teichert, N. and Joly, L. and Ohresser, P. and Landers, J. and Kappler, J.P. and Entel, P. and Hütten, A. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    An austenite-martensite transition was observed in a 100-nm-thick Ni51.6Mn32.9Sn15.5 film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of MS≈260 K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment ml/mS can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of B=3 T reverses the change of the electronic structure of Mn and reduces the ratio of ml/mS from 13.5 to ≈1 % indicating a field-induced reverse martensitic transition. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.214417
  • Influence of film thickness and composition on the martensitic transformation in epitaxial Ni-Mn-Sn thin films
    Teichert, N. and Auge, A. and Yüzüak, E. and Dincer, I. and Elerman, Y. and Krumme, B. and Wende, H. and Yildirim, O. and Potzger, K. and Hütten, A.
    Acta Materialia 86 (2015)
    Two series of epitaxial Ni-Mn-Sn thin films of different thickness are investigated for the thickness and composition dependence of the martensitic transformation. Thin films ranging in thickness from 20 to 200 nm (series A) and 10 to 100 nm (series B) were prepared by magnetron co-sputtering and deposited on heated MgO(0 0 1) substrates. The structural characterization was done by temperature-dependent X-ray diffraction measurements. Magnetization and resistivity measurements were performed to investigate the transformation characteristics. We find a strong influence of the film thickness on the relative amount of material undergoing the martensitic transformation, the temperature range of the transformation, and the transformation temperatures. The main contribution originates from the rigid substrate which delays the transformation of the Ni-Mn-Sn near the interface and even leads to a layer of residual austenite at low temperatures. Another issue are size effects which presumably broaden the martensitic transformation and decrease the transformation temperatures. By variation of the thin film composition we find changes of the substrate influence due to a different mismatch between the lattice of MgO and austenite. A better phase compatibility between martensite and austenite, denoted by λ2, not only results in a smaller hysteresis but is also beneficial for the transformation of material close to the substrate. © 2014 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2014.12.019
  • Local electronic and magnetic properties of pure and Mn-containing magnetocaloric LaFe13-xSix compounds inferred from Mössbauer spectroscopy and magnetometry
    Makarov, S.I. and Krautz, M. and Salamon, S. and Skokov, K. and Teixeira, C.S. and Gutfleisch, O. and Wende, H. and Keune, W.
    Journal of Physics D: Applied Physics 48 (2015)
    Manganese containing La-Fe-Si alloys are important magnetocaloric compounds, since Mn atoms prevent segregation of hydrogen in partially hydrogenated La-Fe-Mn-Si alloys when their Curie temperature is tuned to room temperature by hydrogen. The effect of Mn alloying on the Fe atomic magnetic moment μ<inf>Fe</inf> is still rather unexplored. Therefore, we investigated the (local) magnetic and electric hyperfine interactions in the strongly magnetocaloric compound LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf> and, for comparison, LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 57Fe Mössbauer spectroscopy, and the global magnetic properties by vibrating sample magnetometry. The NaZn<inf>13</inf> structure was confirmed by x-ray diffraction. Two non-equivalent Fe lattice sites are known to exist in this material: the (96i) sites (Fe<inf>II</inf>) of low local symmetry, and the highly symmetrical (8b) sites (Fe<inf>I</inf>). At room temperature in the paramagnetic state, the electric hyperfine parameters of Fe atoms on both sites were obtained. At low temperatures (4.8 K), the observed magnetically split nuclear Zeeman sextets with broad apparent lines were analyzed in terms of a distribution P(B<inf>hf</inf>) of hyperfine magnetic fields B<inf>hf</inf>. The average hyperfine field 〈B<inf>hf</inf>〉, originating predominantly from Fe<inf>II</inf> sites, was found to be rather high (30.7(1) T at 4.8 K) for LaFe<inf>11.6</inf>Si<inf>1.4</inf>, and the approximate relation 〈B<inf>hf</inf>〉 = Aμ<inf>Fe</inf> is confirmed for Fe<inf>II</inf> sites, with A = 14.2 T/μ<inf>B</inf>. 〈B<inf>hf</inf>〉 is significantly reduced (to 27.7(1) T at 4.8 K) for the Mn-containing sample LaFe<inf>11.3</inf>Mn<inf>0.3</inf>Si<inf>1.4</inf>, providing evidence for a reduction by 9.7% of the average Fe atomic moment μ<inf>Fe</inf> from ∼2.16 μ<inf>B</inf> to a value of ∼1.95 μ<inf>B</inf> by Mn substitution of Fe. Our Mössbauer results are in good agreement with magnetometry, which reveals a reduction of the saturation magnetization of M<inf>s</inf> = 163.1(1) Am2 kg-1 of LaFe<inf>11.6</inf>Si<inf>1.4</inf> by 10.5% due to Mn substitution. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/30/305006
  • Magnetic interplay between two different lanthanides in a tris-phthalocyaninato complex: a viable synthetic route and detailed investigation in the bulk and on the surface
    Lan, Y. and Klyatskaya, S. and Ruben, M. and Fuhr, O. and Wernsdorfer, W. and Candini, A. and Corradini, V. and Lodi Rizzini, A. and Del Pennino, U. and Troiani, F. and Joly, L. and Klar, D. and Wende, H. and Affronte, M.
    Journal of Materials Chemistry C 3 (2015)
    Future applications of molecular units in quantum information technologies require a fine control at the single molecule level. This includes the choice of each functional element, the intramolecular interaction and the robustness of molecules when dispersed on a substrate. Keeping these goals in mind, we designed and synthesized a heterometallic phthalocyaninato-complex including two different lanthanides in each moiety, namely [PcDyPcTbPc∗] (Pc being phthalocyanines; and Pc∗ being 2,3,9,10,16,17,23,24-octahexyl-substituted phthalocyanines). Full magnetic characterization was performed down to the mK temperature range on bulk microcrystals by means of AC susceptibility, DC magnetization (including microSQUID) and specific heat measurements. A weak, yet sizeable, interaction between the two lanthanides is clearly detected by different techniques, altering the magnetic behavior of the single lanthanide as observed in the parent [LnPc2] complexes. Isolated [PcDyPcTbPc∗] molecules dispersed on HOPG and the Au surface by liquid phase deposition are proven to maintain their main chemical and magnetic features by combined XPS, XAS and XMCD analysis and to lie with one Pc ligand flat to the surface. Opening of a small but sizable hysteresis loop at 1.8 K is directly observed on both Tb and Dy sites proving the retention of magnetization at the single molecule level. © The Royal Society of Chemistry.
    view abstract10.1039/c5tc02011e
  • Magnetic ordering in magnetic shape memory alloy Ni-Mn-In-Co
    Ollefs, K. and Schöppner, C. and Titov, I. and Meckenstock, R. and Wilhelm, F. and Rogalev, A. and Liu, J. and Gutfleisch, O. and Farle, M. and Wende, H. and Acet, M.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    Structural and magnetic properties across the martensite-austenite phase transitions in the shape memory alloy Ni-Mn-In-Co are studied using complementary experimental techniques: ferromagnetic resonance, macroscopic magnetization measurements, and x-ray magnetic circular dichroism in the temperature range from 5 to 450 K. Ferromagnetic resonance experiments show coexisting antiferromagnetic and ferromagnetic correlations for the martensite phase and ferromagnetic and paramagnetic correlations in the austenite phase. Magnetization measurements reveal spin-glass-like behavior for T<30 K and Ni and Co K-edge x-ray magnetic circular dichroism measurements confirm an assignment of a ferromagnetic resonance line purely to Ni (and Co) for a wide temperature range from 125 to 225 K. Hence a combined analysis of ferromagnetic resonance and x-ray magnetic circular dichroism allows us to attribute particular magnetic resonance signals to individual elemental species in the alloy. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.224429
  • Magnetoelectric coupling on multiferroic cobalt ferrite-barium titanate ceramic composites with different connectivity schemes
    Etier, M. and Schmitz-Antoniak, C. and Salamon, S. and Trivedi, H. and Gao, Y. and Nazrabi, A. and Landers, J. and Gautam, D. and Winterer, M. and Schmitz, D. and Wende, H. and Shvartsman, V.V. and Lupascu, D.C.
    Acta Materialia 90 (2015)
    In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)-barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3-0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0-3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3-0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3-0) sample compared to the traditional (0-3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms. © 2015 Acta Materialia Inc.
    view abstract10.1016/j.actamat.2015.02.032
  • Measuring the magnetoelectric effect across scales
    Lupascu, D.C. and Wende, H. and Etier, M. and Nazrabi, A. and Anusca, I. and Trivedi, H. and Shvartsman, V.V. and Landers, J. and Salamon, S. and Schmitz-Antoniak, C.
    GAMM Mitteilungen 38 (2015)
    Magnetoelectric coupling is the material based coupling between electric and magnetic fields without recurrence to electrodynamics. It can arise in intrinsic multiferroics as well as in composites. Intrinsic multiferroics rely on atomistic coupling mechanisms, or coupled crystallographic order parameters, and even more complex mechanisms. They typically require operating temperatures much below T = 0C in order to exhibit their coupling effects. Room temperature applications are thus excluded. Consequently, composites have been designed to circumvent this limitation. They rely on field coupling between magnetostrictive and piezoelectric materials or in more advanced scenarios on quantum coupling in between both phases. This overview will describe experimental techniques and their particular limitations in accessing these coupling phenomena at different scales. Strain coupling is the dominant coupling mechanism at the macroscale as well as down to the micrometer. At the nanoscale more subtle effects can arise and some care has to be taken when investigating local coupling at interfaces using scanning probe techniques, e. g. due to semiconductor effects, field screening, or gradient and surface effects. At the smallest length scale atomic or molecular coupling can be tested using X-ray dichroism or probe atoms like 57Fe in Mössbauer spectroscopy. We display a selection of measuring techniques at the different scales and outline possible pitfalls for experimentalists as well as theoreticians when using material parameters extracted from such experimental work. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/gamm.201510003
  • Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide
    Liedke, M.O. and Anwand, W. and Bali, R. and Cornelius, S. and Butterling, M. and Trinh, T.T. and Wagner, A. and Salamon, S. and Walecki, D. and Smekhova, A. and Wende, H. and Potzger, K.
    Journal of Applied Physics 117 (2015)
    Magnetic phase transition in the Fe<inf>60</inf>Al<inf>40</inf> transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 °C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and Mössbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600 °C) has been found, whereas higher temperatures showed an increase in the vacancy concentration. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4919014
  • Particle-Matrix Interaction in Cross-Linked PAAm-Hydrogels Analyzed by Mössbauer Spectroscopy
    Landers, J. and Roeder, L. and Salamon, S. and Schmidt, A.M. and Wende, H.
    Journal of Physical Chemistry C 119 (2015)
    The constrained motion of spindle-shaped hematite nanoparticles of about 400 nm in PAAm-hydrogels with different degrees of cross-link density is measured utilizing the line broadening observable in Mössbauer spectra at 265-293 K. A slight decrease of nanoparticle mobility is observed upon increasing cross-link density. Mössbauer spectra of the same nanoparticles in 60 wt % sucrose solution, used as reference material, display line broadening of the same magnitude as the hydrogel samples, indicating a similar degree of motion at the atomic scale and the time scale of the Mössbauer experiment. AC (alternating current) susceptibility data indicate that the magnetic relaxation of the nanoparticles in sucrose solution mainly occurs by Brownian motion, while the absence of magnetic loss within the investigated frequency range observed in measurements on hydrogel samples reveals very limited particle mobility. This apparent contradiction between results on particle dynamics in hydrogels by Mössbauer spectroscopy and AC susceptibility measurements is explained in terms of constrained particle mobility at atomic scales. © 2015 American Chemical Society.
    view abstract10.1021/acs.jpcc.5b03697
  • The role of weak interlayer coupling in the spin-reorientation of perpendicular ultrathin Co-Fe-B/MgO-based heterostructures
    Gareev, R.R. and Zbarsky, V. and Landers, J. and Soldatov, I. and Schäfer, R. and Münzenberg, M. and Wende, H. and Grünberg, P.
    Applied Physics Letters 106 (2015)
    Ultrathin magnetic tunneling structures implicate fundamental interlayer exchange coupling between magnetic layers. Here, we describe its important role in the spin-reorientation transition of weakly coupled perpendicular ultrathin Ta/Co-Fe-B/MgO/Co-Fe-B/Ta heterostructures. Near the spin-reorientation, the domain structure is quite sensitive to weak interlayer exchange coupling. Antiferromagnetic coupling stabilizes homogeneous perpendicular magnetization at the remanence, whereas ferromagnetic coupling favors in-/out-of-plane stripe domains. Close to the spin-reorientation transition, even the subtle changes of interlayer exchange coupling can lead to reversible switching between stable in- and out-of-plane states. Our results suggest that this multi-stability caused by the interplay of perpendicular anisotropy and weak interlayer coupling can be utilized in perpendicular spin torque devices operating under reduced spin currents. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4915323
  • Correlation of superparamagnetic relaxation with magnetic dipole interaction in capped iron-oxide nanoparticles
    Landers, J. and Stromberg, F. and Darbandi, M. and Schöppner, C. and Keune, W. and Wende, H.
    Journal of Physics Condensed Matter 27 (2014)
    Six nanometer sized iron-oxide nanoparticles capped with an organic surfactant and/or silica shell of various thicknesses have been synthesized by a microemulsion method to enable controllable contributions of interparticle magnetic dipole interaction via tunable interparticle distances. Bare particles with direct surface contact were used as a reference to distinguish between interparticle interaction and surface effects by use of Mössbauer spectroscopy. Superparamagnetic relaxation behaviour was analyzed by SQUID-magnetometry techniques, showing a decrease of the blocking temperature with decreasing interparticle interaction energies kBT 0 obtained by AC susceptibility. A many-state relaxation model enabled us to describe experimental Mössbauer spectra, leading to an effective anisotropy constant Keff ≈ 45 kJm-3 in case of weakly interacting particles, consistent with results from ferromagnetic resonance. Our unique multi-technique approach, spanning a huge regime of characteristic time windows from about 10 s to 5 ns, provides a concise picture of the correlation of superparamagnetic relaxation with interparticle magnetic dipole interaction. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0953-8984/27/2/026002
  • Ferromagnetic exchange coupling between Fe phthalocyanine and Ni(111) surface mediated by the extended states of graphene
    Candini, A. and Bellini, V. and Klar, D. and Corradini, V. and Biagi, R. and De Renzi, V. and Kummer, K. and Brookes, N.B. and Del Pennino, U. and Wende, H. and Affronte, M.
    Journal of Physical Chemistry C 118 (2014)
    The interface spin coupling mechanism is studied in a hybrid structure made of Fe phthalocyanine molecules sublimed in ultrahigh vacuum on graphene grown on the magnetic substrate Ni(111). By using synchrotron X-ray magnetic circular dichroism, the field-dependent magnetization of the isolated FePc molecules and of the Ni substrate has been measured at low temperature (8 K). Along with density functional theory calculations, the role of the graphene interlayer in transmitting the magnetic coupling is addressed. Both experiments and theory show a ferromagnetic coupling between the molecules and the substrate which is weakened by the insertion of graphene. DFT calculations indicate that the key role is played by the π orbitals of graphene, which hybridize with the underlying magnetic Ni, giving rise to a sizable spin polarized continuum at the molecular interface. The resulting overlap with the Fe orbitals favors a direct coupling of ferromagnetic nature, as evidenced by our spin density distribution plots. © 2014 American Chemical Society.
    view abstract10.1021/jp5041663
  • Field-regulated switching of the magnetization of Co-porphyrin on graphene
    Klar, D. and Bhandary, S. and Candini, A. and Joly, L. and Ohresser, P. and Klyatskaya, S. and Schleberger, M. and Ruben, M. and Affronte, M. and Eriksson, O. and Sanyal, B. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    Different magnetic coupling mechanisms have been identified for a few monolayers of Co-porphyrin molecules deposited on a graphene-covered Ni(111) single crystal. A relatively strong antiferromagnetic coupling of the first molecular layer via graphene to the Ni crystal in comparison to a weaker intermolecular coupling gives rise to a complex field-dependent response of this hybrid system. By continuously increasing the magnetic field strength, the net magnetization of the molecular system switches from antiparallel to parallel to the field direction at 2.5 T. Utilizing x-ray absorption spectroscopy and x-ray magnetic circular dichroism, the element-specific magnetization and field dependence was probed. The nature of the magnetic couplings is identified by means of density functional theory and orbital-dependent susceptibilities. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.144411
  • Hysteretic behaviour in a vacuum deposited submonolayer of single ion magnets
    Klar, D. and Candini, A. and Joly, L. and Klyatskaya, S. and Krumme, B. and Ohresser, P. and Kappler, J.-P. and Ruben, M. and Wende, H.
    Dalton Transactions 43 (2014)
    With element-specific X-ray absorption spectroscopy and X-ray magnetic circular dichroism we have investigated submonolayer coverages of TbPc 2 and DyPc2 molecules sublimated on highly ordered pyrolytic graphite. We have studied the field dependence of the magnetization of the central lanthanide ion at very low temperatures. Even in zero applied magnetic field we still observe a remanence in the magnetization. Since there are neither intermolecular coupling nor magnetic interactions with the substrate, this remanent behaviour results just from single-ion anisotropy. On the very inert surface of graphite at temperatures between 0.5 K and 2 K the spin relaxation is slow enough to observe a memory effect in the timescale of the experimental measurements. © 2014 the Partner Organisations.
    view abstract10.1039/c4dt01005a
  • Mössbauer study of temperature-dependent cycloidal ordering in BiFeO3 nanoparticles
    Landers, J. and Salamon, S. and Escobar Castillo, M. and Lupascu, D.C. and Wende, H.
    Nano Letters 14 (2014)
    To study the effects of different temperatures and particle sizes on the anharmonic cycloidal spin structure in BiFeO3 nanoparticles, Mössbauer spectroscopy was applied to three sets of particles with different mean diameters in the range of 54 nm to 1.6 μm at temperatures between 4.2 and 800 K. The paramagnetic transition showed a distinct broadening upon decreasing particle size with Néel temperatures decreasing from 652 to 631 K. The anharmonicity of the long-range cycloidal structure, calculated from experimental Mössbauer spectra, is revealed to decrease upon rising temperature, starting at 150-200 K and reaching the harmonic state at about 400 K. © 2014 American Chemical Society.
    view abstract10.1021/nl5031375
  • The dipole moment of the spin density as a local indicator for phase transitions
    Schmitz, D. and Schmitz-Antoniak, C. and Warland, A. and Darbandi, M. and Haldar, S. and Bhandary, S. and Eriksson, O. and Sanyal, B. and Wende, H.
    Scientific Reports 4 (2014)
    The intra-atomic magnetic dipole moment-frequently called T zterm-plays an important role in the determination of spin magnetic moments by X-ray absorption spectroscopy for systems with nonspherical spin density distributions. In this work, we present the dipole moment as a sensitive monitor to changes in the electronic structure in the vicinity of a phase transiton. In particular, we studied the dipole moment at the Fe2+ and Fe3+ sites of magnetite as an indicator for the Verwey transition by a combination of X-ray magnetic circular dichroism and density functional theory. Our experimental results prove that there exists a local change in the electronic structure at temperatures above the Verwey transition correlated to the known spin reorientation. Furthermore, it is shown that measurement of the dipole moment is a powerful tool to observe this transition in small magnetite nanoparticles for which it is usually screened by blocking effects in classical magnetometry.
    view abstract10.1038/srep05760
  • Antiferromagnetic coupling of TbPc2 molecules to ultrathin Ni and Co films
    Klar, D. and Klyatskaya, S. and Candini, A. and Krumme, B. and Kummer, K. and Ohresser, P. and Corradini, V. and de Renzi, V. and Biagi, R. and Joly, L. and Kappler, J.-P. and Pennino, U.D. and Affronte, M. and Wende, H. and Ruben, M.
    Beilstein Journal of Nanotechnology 4 (2013)
    The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface. © 2013 Klar et al.
    view abstract10.3762/bjnano.4.36
  • Blocked-micropores, surface functionalized, bio-compatible and silica-coated iron oxide nanocomposites as advanced MRI contrast agent
    Darbandi, M. and Laurent, S. and Busch, M. and Li, Z.-A. and Yuan, Y. and Krüger, M. and Farle, M. and Winterer, M. and Vander Elst, L. and Muller, R.N. and Wende, H.
    Journal of Nanoparticle Research 15 (2013)
    Biocompatible magnetic nanoparticles have been found promising in several biomedical applications for tagging, imaging, sensing and separation in recent years. In this article, a systematic study of the design and development of surface-modification schemes for silica-coated iron oxide nanoparticles (IONP) via a one-pot, in situ method at room temperature is presented. Silica-coated IONP were prepared in a water-in-oil microemulsion, and subsequently the surface was modified via addition of organosilane reagents to the microemulsion system. The structure and the morphology of the as synthesized nanoparticles have been investigated by means of transmission electron microscopy (TEM) and measurement of N2 adsorption-desorption. Electron diffraction and high-resolution transmission electron microscopic (TEM) images of the nanoparticles showed the highly crystalline nature of the IONP structures. Nitrogen adsorption indicates microporous and blocked-microporous structures for the silica-coated and amine functionalized silica-coated IONP, respectively which could prove less cytotoxicity of the functionalized final product. Besides, the colloidal stability of the final product and the presence of the modified functional groups on top of surface layer have been proven by zeta-potential measurements. Owing to the benefit from the inner IONP core and the hydrophilic silica shell, the as-synthesized nanocomposites were exploited as an MRI contrast enhancement agent. Relaxometric results prove that the surface functionalized IONP have also signal enhancement properties. These surface functionalized nanocomposites are not only potential candidates for highly efficient contrast agents for MRI, but could also be used as ultrasensitive biological-magnetic labels, because they are in nanoscale size, having magnetic properties, blocked-microporous and are well dispersible in biological environment. © 2013 Springer Science+Business Media Dordrecht.
    view abstract10.1007/s11051-013-1664-8
  • Effect of particle size on ferroelectric and magnetic properties of BiFeO3 nanopowders
    Castillo, M.E. and Shvartsman, V.V. and Gobeljic, D. and Gao, Y. and Landers, J. and Wende, H. and Lupascu, D.C.
    Nanotechnology 24 (2013)
    The ferroelectric and magnetic behaviour of multiferroic BiFeO3 nanoparticles has been studied using piezoresponse force microscopy (PFM), Mössbauer spectroscopy and SQUID magnetometry. The results of the PFM studies indicate a decay of the spontaneous polarization with decreasing particle size. Nevertheless, particles with diameter ∼50 nm still manifest ferroelectric behaviour. At the same time these particles are weakly ferromagnetic. The Mössbauer spectroscopy studies prove that the weak ferromagnetic state is due to non-compensated surface spins rather than distortions of the cycloidal spin structure characteristic for bulk BiFeO 3. © 2013 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/24/35/355701
  • Electric in-plane polarization in multiferroic CoFe2O 4/BaTiO3 nanocomposite tuned by magnetic fields
    Schmitz-Antoniak, C. and Schmitz, D. and Borisov, P. and De Groot, F.M.F. and Stienen, S. and Warland, A. and Krumme, B. and Feyerherm, R. and Dudzik, E. and Kleemann, W. and Wende, H.
    Nature Communications 4 (2013)
    Ferrimagnetic CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix are an example for a two-phase magnetoelectrically coupled system. They operate at room temperature and are free of any resource-critical rare-earth element, which makes them interesting for potential applications. Prior studies succeeded in showing strain-mediated coupling between the two subsystems. In particular, the electric properties can be tuned by magnetic fields and the magnetic properties by electric fields. Here we take the analysis of the coupling to a new level utilizing soft X-ray absorption spectroscopy and its associated linear dichroism. We demonstrate that an in-plane magnetic field breaks the tetragonal symmetry of the (1,3)-type CoFe2O4/BaTiO3 structures and discuss it in terms of off-diagonal magnetostrictive-piezoelectric coupling. This coupling creates staggered in-plane components of the electric polarization, which are stable even at magnetic remanence due to hysteretic behaviour of structural changes in the BaTiO3 matrix. The competing mechanisms of clamping and relaxation effects are discussed in detail. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms3051
  • Iron porphyrin molecules on Cu(001): Influence of adlayers and ligands on the magnetic properties
    Herper, H.C. and Bernien, M. and Bhandary, S. and Hermanns, C.F. and Krüger, A. and Miguel, J. and Weis, C. and Schmitz-Antoniak, C. and Krumme, B. and Bovenschen, D. and Tieg, C. and Sanyal, B. and Weschke, E. and Czekelius, C. and Kuch, W. and Wende, H. and Eriksson, O.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The structural and magnetic properties of Fe octaethylporphyrin molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and x-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a √2×2√2R45° reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT, we calculate the spin dipolar term 7T(θ), which is also important in view of the magnetic anisotropy of the molecule. The measured x-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of 7T(θ), e.g., on Cu(001), 7T(θ) amounts to -2.07 μB for normal incidence leading to a reduction of the effective spin moment (m s+7T(θ)). Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the x-ray absorption are clearly related to residual Cl ligands. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.174425
  • Magnetoelectric effect in (0-3) CoFe 2O 4-BaTiO 3 (20/80) composite ceramics prepared by the organosol route
    Etier, M. and Shvartsman, V.V. and Gao, Y. and Landers, J. and Wende, H. and Lupascu, D.C.
    Ferroelectrics 448 (2013)
    In this paper we report on a new approach to synthesize core/shell cobalt iron oxide/barium titanate composite nanoparticles combining the co-precipitation and organosol crystallization techniques. The weight fraction of CoFe2O4 and BaTiO3 was 20% and 80% respectively. The obtained core/shell powder was used to sinter (0-3) composite multiferroic ceramics. Ferroelectric, magnetic, and magnetoelectric properties of the ceramics were studied. It was found that the value of the converse magnetoelectric coefficient, c, reaches 4.4·10-12 s·m-1 at the magnetic field μ 0 H dc = 0.15 T and T = 285 K. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/00150193.2013.822292
  • Manipulation of spin state of iron porphyrin by chemisorption on magnetic substrates
    Bhandary, S. and Brena, B. and Panchmatia, P.M. and Brumboiu, I. and Bernien, M. and Weis, C. and Krumme, B. and Etz, C. and Kuch, W. and Wende, H. and Eriksson, O. and Sanyal, B.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L2,3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S=2) from an intermediate spin state (S=1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.024401
  • Oxygen-tuned magnetic coupling of Fe-phthalocyanine molecules to ferromagnetic Co films
    Klar, D. and Brena, B. and Herper, H.C. and Bhandary, S. and Weis, C. and Krumme, B. and Schmitz-Antoniak, C. and Sanyal, B. and Eriksson, O. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The coupling of submonolayer coverages of Fe-phthalocyanine molecules on bare and oxygen-covered ferromagnetic Co(001) films was studied by x-ray-absorption spectroscopy, especially the x-ray magnetic circular dichroism, in combination with density functional theory. We observe that the magnetic moments of the paramagnetic molecules are aligned even at room temperature, resulting from a magnetic coupling to the substrate. While the magnetization of the Fe ions directly adsorbed on the Co surface is parallel to the magnetization of the Co film, the introduction of an oxygen interlayer leads to an antiparallel alignment. As confirmed by theory, the coupling strength is larger for the system FePc/Co than for FePc/O/Co, causing a stronger temperature dependence of the Fe magnetization for the latter system. Furthermore, the calculations reveal that the coupling mechanism changes due to the O layer from mostly direct exchange to Co of the bare surface to a 180° antiferromagnetic superexchange via the O atoms. Finally, by comparing the experimental x-ray-absorption spectra at the N K edge with the corresponding calculations, the contribution of the individual orbitals has been determined and the two inequivalent N atoms of the molecules could be distinguished. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.224424
  • Revelation of the crucial interactions in spin-hybrid systems by means of X-ray absorption spectroscopy
    Wende, H.
    Journal of Electron Spectroscopy and Related Phenomena 189 (2013)
    Spin-hybrid systems consisting of magnetic molecules on surfaces are studied by means of X-ray absorption spectroscopy. The relevant magnetic interactions of paramagnetic molecules on ferromagnetic surfaces are analyzed utilizing the element specificity of the X-ray magnetic circular dichroism revealing the magnetic coupling for these systems. By the help of X-ray natural dichroism the orientation of the molecules on the surfaces and the electronic structure is probed. The structural properties are correlated with the magnetic characteristics to achieve a more complete understanding of the spin-hybrid systems. The possibility to tailor the magnetic coupling for these systems is demonstrated by modifying the interface of the molecules and the substrate by using an intermediate layer of atomic oxygen. As an outlook it will be discussed how a spin crossover behavior of the molecules might be achieved in specific spin-hybrid systems by employing graphene. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.elspec.2013.04.010
  • Room temperature switching of a neutral molecular iron(ii) complex
    Schäfer, B. and Rajnák, C. and Šalitroš, I. and Fuhr, O. and Klar, D. and Schmitz-Antoniak, C. and Weschke, E. and Wende, H. and Ruben, M.
    Chemical Communications 49 (2013)
    Abrupt room temperature switching (T<inf>c</inf> = 295 K with a 5 K hysteresis) was achieved in a neutral FeII complex based on a 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine ligand. Structural characterization and spin crossover study (via SQUID magnetometry, photoexcitation and X-ray absorption spectroscopy) in the solid state are described. © The Royal Society of Chemistry 2013.
    view abstract10.1039/c3cc46624h
  • Spin injection, transport, and relaxation in spin light-emitting diodes: Magnetic field effects
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8813 (2013)
    Efficient electrical spin injection into semiconductor based devices at room temperature is one of the most important requirements for the development of applicable spintronic devices in the near future and is thus an important and very active research field. Here we report experimental results for the electrical spin injection in spin light-emitting diodes (spin-LEDs) without external magnetic fields at room temperature. Our devices consist of a Fe/Tb multilayer spin injector with remanent out-of-plane magnetization, an MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. Using a series of samples with different injection path lengths allows us to experimentally determine the spin relaxation during vertical transport from the spin injector to the active region at room temperature. In combination with our concept for remanent spin injection, we are additionally able to investigate the influence of an external magnetic field on the spin relaxation process during transport. While the spin relaxation length at room temperature without external magnetic field is determined to be 27 nm, this value almost doubles if an external magnetic field of 2 Tesla is applied in Faraday geometry. This demonstrates that the results for spin injection and spin relaxation obtained with or without magnetic field can hardly be compared. The efficiency of spin-induced effects is overestimated as long as magnetic fields are involved. Since strong magnetic fields are not acceptable in application settings, this may lead to wrong conclusions and potentially impairs proper device development. © 2013 Copyright SPIE.
    view abstract10.1117/12.2023324
  • Spin relaxation in spin light-emitting diodes: Effects of magnetic field and temperature
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8623 (2013)
    We report experimental results on the electron spin relaxation length during vertical transport in spin lightemitting diodes (LEDs). Our devices are GaAs based LEDs with InAs quantum dots in the active region, an MgO tunnel barrier and an Fe/Tb multilayer spin injector with perpendicular magnetic anisotropy, i.e. remanent out-of-plane magnetization, enabling efficient electrical spin injection in magnetic remanence. Additionally, our devices can be operated at room temperature. A series of samples with different injection path lengths allows us to experimentally determine the spin relaxation length in our devices. In combination with operation in magnetic remanence, we are able to determine the spin relaxation length without the influence of external magnetic fields and at room temperature and find it to be 27 nm. Applying an additional external magnetic field, we find that at a field strength of 2 T, this relaxation length almost doubles, which is in good agreement with spin relaxation times in GaAs. Temperature control of our samples allows us to measure the temperature dependence of the spin relaxation length. At 200 K, the spin relaxation length doubles to 50 nm and reaches 80 nm at 30 K, in good agreement with theoretic calculations. Our results show that polarization values obtained with spin-LEDs inside strong magnetic fields and at low temperatures are not comparable to those in remanence and at room temperature. However, the transfer of efficient spintronic devices to such applicationenabling settings is absolutely necessary and will be a major challenge considering the enormous differences in spin relaxation. © 2013 SPIE.
    view abstract10.1117/12.2001511
  • Spin relaxation length in quantum dot spin LEDs
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 10 (2013)
    We analyse the spin relaxation length during vertical electron transport in spin light-emitting diode devices at room temperature. We obtain a spin relaxation length of 27 nm in remanence. When a magnetic field is applied, spin relaxation is significantly reduced during transport to the active region of the device. This results in a nearly doubled spin relaxation length at 2T magnetic field strength. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201200689
  • Surface-induced dechlorination of FeOEP-Cl** on Cu(111)
    Van Vörden, D. and Lange, M. and Schaffert, J. and Cottin, M.C. and Schmuck, M. and Robles, R. and Wende, H. and Bobisch, C.A. and Möller, R.
    ChemPhysChem 14 (2013)
    To be or not to be chlorinated: When octaethylporphyrin iron(III) chloride (FeOEP-Cl) molecules are sublimated onto Cu(111) surfaces, two different molecular species are observed through scanning tunneling microscopy, showing either a protrusion or a depression at the center. In combination with van der Waals-corrected density functional calculations, our experiments reveal that one species corresponds to FeOEP-Cl molecules with the chlorine atom pointing away from the surface, whereas the other species has been dechlorinated. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201300497
  • Antiferromagnetic coupling across silicon regulated by tunneling currents
    Gareev, R.R. and Schmid, M. and Vancea, J. and Back, C.H. and Schreiber, R. and Bürgler, D. and Schneider, C.M. and Stromberg, F. and Wende, H.
    Applied Physics Letters 100 (2012)
    We report on the enhancement of antiferromagnetic coupling in epitaxial Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using the ballistic electron magnetic microscopy, we established that the hot-electron collector current reflects magnetization alignment and the magnetocurrent exceeds 200% at room temperature. The saturation magnetic field for the collector current corresponding to the parallel alignment of magnetizations rises up with the tunneling current, thus demonstrating stabilization of the antiparallel alignment and increasing antiferromagnetic coupling. We connect the enhancement of antiferromagnetic coupling with local dynamic spin torques mediated by spin-polarized tunneling electrons. © 2012 American Institute of Physics.
    view abstract10.1063/1.3675872
  • Antiferromagnetic coupling in combined Fe/Si/MgO/Fe structures with controlled interface diffusion
    Gareev, R. and Stromberg, F. and Buchmeier, M. and Keune, W. and Back, C. and Wende, H.
    Applied Physics Express 5 (2012)
    We study antiferromagnetic coupling and interface diffusion in Fe/Si/MgO/Fe structures grown by molecular beam epitaxy. The Fe/Si/Fe samples with a 1.2-nm-thick Si spacer demonstrate antiferromagnetic coupling J 1 ∼- 1:5 mJ/m 2 and prevailing interdiffusion at the top Si/Fe interface, as revealed by conversion electron Mössbauer spectroscopy. For combined Si/MgO spacers with 0.9-nm-thick Si, interdiffusion continuously reduces upon changing the MgO thickness from 0.3 to 0.5nm accompanied by a decrease of antiferromagnetic coupling from |J 1| ∼ 1 mJ/m2 to |J 1| ∼ 0:002mJ/m 2. We emphasize that monolayer-scaled engineering of insulating spacers is a promising tool for the precise control of antiferromagnetic coupling and interface diffusion. © 2012 The Japan Society of Applied Physics.
    view abstract10.1143/APEX.5.033003
  • Cobalt ferrite/barium titanate core/shell nanoparticles
    Etier, M. and Gao, Y. and Shvartsman, V.V. and Elsukova, A. and Landers, J. and Wende, H. and Lupascu, D.C.
    Ferroelectrics 438 (2012)
    Cobalt ferrite/barium titanate nanoparticles with a core/shell structure were synthesized by combining co-precipitation and organosol methods. The average particle size was about 110 nm with an average shell thickness of about 40 nm. Dielectric and magnetic properties of the particles were studied using impedance and Mössbauer spectroscopy, respectively. The particles are promising for fabrication of multiferroic ceramics with the core-shell structure. Copyright © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2012.743773
  • Depth-selective electronic and magnetic properties of a Co2MnSi tunnel magneto-resistance electrode at a MgO tunnel barrier
    Krumme, B. and Ebke, D. and Weis, C. and Makarov, S.I. and Warland, A. and Hütten, A. and Wende, H.
    Applied Physics Letters 101 (2012)
    We investigated the electronic structure as well as the magnetic properties of a Co2MnSi film on MgO(100) element-specifically at the interface to a MgO tunnel barrier by means of X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The electronic structure of the Co atoms as a function of the capping layer thickness remained unchanged, whereas the XA spectra of Mn indicate an increase of the unoccupied d states. The experimental findings are consistent with the interfacial structure proposed in the work by B. Hülsen [Phys. Rev. Lett. 103, 046802 (2009)], where a MnSi layer is present at the interface to the MgO with oxygen atoms at top positions in the first MgO layer. © 2012 American Institute of Physics.
    view abstract10.1063/1.4769180
  • Effect of silica capping on the oxidation of Fe 3O 4 nanoparticles in dispersion revealed by x-ray absorption spectroscopy
    Warland, A. and Antoniak, C. and Darbandi, M. and Weis, C. and Landers, J. and Keune, W. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Fe 3O 4 nanoparticles have been investigated as they are biocompatible and their surface can be functionalized. We synthesized iron oxide nanoparticles using a water-in-oil microemulsion method. Bare and silica-coated iron oxide nanoparticles of a core size of 6 nm dispersed in ethanol have been investigated by means of x-ray absorption spectroscopy (XAS). Due to a dedicated experimental setup the particles can be measured directly in dispersion. XAS allows us to disentangle the contributions of the Fe2 + and Fe3 + ions and therefore to estimate the amount of Fe 3O 4 in the particles. In case of the silica coated particles a high amount of magnetite was obtained. In contrast, the bare nanoparticles showed indications of a further oxidation into γ-Fe 2O 3 even in dispersion. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.235113
  • Induced magnetism on silicon in Fe 3Si quasi-Heusler compound
    Antoniak, C. and Herper, H.C. and Zhang, Y.N. and Warland, A. and Kachel, T. and Stromberg, F. and Krumme, B. and Weis, C. and Fauth, K. and Keune, W. and Entel, P. and Wu, R.Q. and Lindner, J. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Element-specific magnetic moments of epitaxially grown Fe 3Si in D0 3 crystal symmetry were analyzed by means of x-ray absorption spectroscopy and its associated magnetic circular dichroism. To detect the weak magnetization induced at the Si sites, measurements were performed at both the Si K edge and the Si L 3,2 edges. By band structure calculations based on either the SPR-KKR method or FLAPW with GGA, the spectroscopic features could be reproduced and provide an insight to the underlying physics. In addition, comparison of the experimental data to calculated spectra made it possible for us to estimate the induced effective spin and orbital magnetic moment of Si in our sample, i.e., μseff=(-0.011±0.005) μ B and μ l=(-0.003±0.003) μ B, respectively. The sign and the order of magnitude of the tiny orbital magnetic moment has been confirmed by application of the magneto-optical sum rule. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.214432
  • Intrinsic magnetism and collective magnetic properties of size-selected nanoparticles
    Antoniak, C. and Friedenberger, N. and Trunova, A. and Meckenstock, R. and Kronast, F. and Fauth, K. and Farle, M. and Wende, H.
    NanoScience and Technology 79 (2012)
    Using size-selected spherical FePt nanoparticles and cubic Fe/Fe-oxide nanoparticles as examples, we discuss the recent progress in the determination of static and dynamic properties of nanomagnets. Synchroton radiation-based characterization techniques in combination with detailed structural, chemical and morphological investigations by transmission and scanning electron microscopy allow the quantitative correlation between element-specific magnetic response and spin structure on the one hand and shape, crystal and electronic structure of the particles on the other hand. Examples of measurements of element-specific hysteresis loops of single 18 nm sized nanocubes are discussed. Magnetic anisotropy of superparamagnetic ensembles and their dynamic magnetic response are investigated by ferromagnetic resonance as a function of temperature at different microwave frequencies. Such investigations allow the determination of the magnetic relaxation and the extraction of the average magnetic anisotropy energy density of the individual particles. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_11
  • Magnetic field dependence of the spin relaxation length in spin light-emitting diodes
    Höpfner, H. and Fritsche, C. and Ludwig, As. and Ludwig, Ar. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Physics Letters 101 (2012)
    We investigate the spin relaxation length during vertical electron transport in spin-light emitting diode devices as a function of magnetic field strength at room temperature. In most publications on spin relaxation in optoelectronic devices, strong magnetic fields are used to achieve perpendicular-to-plane magnetization of the spin injection contacts. We show experimentally that high magnetic field strengths significantly reduce spin relaxation during transport to the active region of the device. We obtain a spin relaxation length of 27(3) nm in magnetic remanence and at room temperature, which nearly doubles at 2 T magnetic field strength. © 2012 American Institute of Physics.
    view abstract10.1063/1.4752162
  • Magnetoelectric properties of 0.2CoFe 2O 4-0. 8BaTiO 3 composite prepared by organic method
    Etier, M. and Gao, Y. and Shvartsman, V.V. and Lupascu, D.C. and Landers, J. and Wende, H.
    Proceedings of 2012 21st IEEE Int. Symp. on Applications of Ferroelectrics held jointly with 11th IEEE European Conference on the Applications of Polar Dielectrics and IEEE PFM, ISAF/ECAPD/PFM 2012 (2012)
    We have successfully synthesized CoFe 2O 4/BaTiO 3 composite nanoparticles with core-shell structure using a new organosol crystallization method. The weight fraction of the ferroelectric and the ferrimagnetic phase was 80% and 20%, respectively. The nanopowders were sintered to form a ceramic composite. The ceramics exhibits the magnetoelectric effect. The value of the converse magnetoelectric coefficient α c measured by a modified SQUID susceptometer reaches 4.4·10 -12 s·m -1 at the magnetic field μ 0H dc = 0.15 T and T = 285 K. © 2012 IEEE.
    view abstract10.1109/ISAF.2012.6297820
  • Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method
    Darbandi, M. and Stromberg, F. and Landers, J. and Reckers, N. and Sanyal, B. and Keune, W. and Wende, H.
    Journal of Physics D: Applied Physics 45 (2012)
    Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mössbauer spectroscopy at 4.2K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mössbauer spectral parameters provided evidence for the existence of a high fraction of Fe 3O 4 (magnetite) in the IONP. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/45/19/195001
  • Room temperature spin relaxation in quantum dot based spin-optoelectronic devices
    Höpfner, H. and Li, M. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8260 (2012)
    Spin-optoelectronic devices have become a field of intensive research in the past few years. Here we present electrical spin injection into spin light-emitting diodes both at room temperature and in magnetic remanence. Our devices consist of a Fe/Tb multilayer spin injection structure with remanent out-of-plane magnetization, a MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. The ground state emission and first excited state emission both show circularly polarized emission in remanence, i.e. without external magnetic fields which is due to spin injection from our ferromagnetic contact. Using a series of samples with varying transport path lengths between the spin injector and the active region, we investigate the spin relaxation length during vertical carrier transport through our devices. Due to our spin injector with remanent out-of-plane magnetization this spin relaxation can be investigated without the need for external magnetic fields which would possibly influence the spin relaxation process. The decrease in circular polarization with increasing injection path length is found to be exponential, indicating drift-based transport which is in accordance with theoretic calculations. From the exponential decay the spin relaxation length of 26 nm as well as a lower bound for the spin injection efficiency of 25% are calculated. Additionally, influences of magnetic field, temperature and current density in the devices on the spin relaxation process are discussed. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.907821
  • Structural properties of amorphous metal carbides: Theory and experiment
    Kádas, K. and Andersson, M. and Holmström, E. and Wende, H. and Karis, O. and Urbonaite, S. and Butorin, S.M. and Nikitenko, S. and Kvashnina, K.O. and Jansson, U. and Eriksson, O.
    Acta Materialia 60 (2012)
    By means of theoretical modeling and experimental synthesis and characterization, we investigate the structural properties of amorphous Zr-Si-C. Two chemical compositions are selected: Zr0.31Si0.29 C0.40 and Zr0.60Si0.33C0.07. Amorphous structures are generated in the theoretical part of our work by the stochastic quenching (SQ) method, and detailed comparison is made regarding the structure and density of the experimentally synthesized films. These films are analyzed experimentally using X-ray absorption spectroscopy, transmission electron microscopy and X-ray diffraction. Our results demonstrate a remarkable agreement between theory and experiment concerning bond distances and atomic coordination of this complex amorphous metal carbide. The demonstrated power of the SQ method opens up avenues for theoretical predictions of amorphous materials in general. © 2012 Acta Materialia Inc. Published by Elsevier Ltd.
    view abstract10.1016/j.actamat.2012.04.044
  • Tuning magnetic relaxation by oblique deposition
    Barsukov, I. and Landeros, P. and Meckenstock, R. and Lindner, J. and Spoddig, D. and Li, Z.-A. and Krumme, B. and Wende, H. and Mills, D.L. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 85 (2012)
    Oblique deposition conditions of Si were used to create a periodic compositional defect matrix in Fe 3Si/MgO(001) thin films. The modified growth conditions provoke shadow effects, which lead to a two-magnon scattering channel with twofold symmetry in the film plane. Its axis is controlled by the sample orientation with respect to the Si evaporator. Angular-dependent ferromagnetic resonance data reveal an enhanced magnetic-relaxation rate induced by the dipolar interactions originating from these artificially created defect structures, while magnetic anisotropy is shown to be influenced negligibly. Experimental results agree well with the developed theoretical approach allowing one to distinguish different relaxation channels. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.85.014420
  • A guideline for atomistic design and understanding of ultrahard nanomagnets
    Antoniak, C. and Gruner, M.E. and Spasova, M. and Trunova, A.V. and Römer, F.M. and Warland, A. and Krumme, B. and Fauth, K. and Sun, S. and Entel, P. and Farle, M. and Wende, H.
    Nature Communications 2 (2011)
    Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms1538
  • Co2+xFe1-xSi/MgO(001) Heusler alloys: Influence of off-stoichiometry and lattice distortion on the magnetic properties in bulk and on MgO(001)
    Herper, H.C. and Krumme, B. and Ebke, D. and Antoniak, C. and Weis, C. and Warland, A. and Htten, A. and Wende, H. and Entel, P.
    Journal of Applied Physics 109 (2011)
    We investigate the influence of lattice distortion and off-stoichiometry on the electronic and magnetic properties of Co2+xFe1-x Si Heusler alloys in their L21 bulk phase and on MgO(001) using the density functional theory. Our investigations show that Co excess does not significantly change the magnetic properties, whereas an increase of the Fe content can reduce the spin polarization. In addition, the influence of off-stoichiometry on x-ray absorption spectra is studied. For comparison, x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements have been carried out for Co2FeSi/MgO(001) at the Co and Fe L2,3 edges. © 2011 American Institute of Physics.
    view abstract10.1063/1.3559487
  • Electrical spin injection in InAs quantum dots at room temperature and adjustment of the emission wavelength for spintronic applications
    Ludwig, Ar. and Roescu, R. and Rai, A.K. and Trunov, K. and Stromberg, F. and Li, M. and Soldat, H. and Ebbing, A. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D.
    Journal of Crystal Growth 323 (2011)
    We have observed room temperature (RT) electrical spin injection in an InAs quantum dot (QD) light emitting diode (LED) grown on a p-type GaAs substrate from a ferromagnetic Fe/Tb electrode with strong out-of-plane anisotropy in remanence, i.e. without applied magnetic field. The QDs in the LED emit at 1275 nm (ground state luminescence), which is beyond the range for highly sensitive detectors, and therefore not optimum for various applications, e.g. quantum information studies. We will present two different ways to blue-shift the emission wavelength and discuss the advantages and drawbacks of the experiments. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jcrysgro.2010.09.087
  • Graphene as a reversible spin manipulator of molecular magnets
    Bhandary, S. and Ghosh, S. and Herper, H. and Wende, H. and Eriksson, O. and Sanyal, B.
    Physical Review Letters 107 (2011)
    One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S=1→S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.257202
  • Improved interfacial local structural ordering of epitaxial Fe 3Si(001) thin films on GaAs(001) by a MgO(001) tunneling barrier
    Makarov, S.I. and Krumme, B. and Stromberg, F. and Weis, C. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    Although the quasi-Heusler compound Fe3Si is a promising candidate for spintronics applications, its combination with the reactive GaAs surface is problematic, since it deteriorates its beneficial attributes due to a large amount of interdiffusion at the Fe3Si/GaAs interface. Here, we show the epitaxial growth of Fe3Si with low evaporation rates on GaAs(001) and report on improved local structural D03 ordering in epitaxial Fe3Si(001) films grown on GaAs(001) by inserting a MgO buffer layer. Conversion-electron Mssbauer spectroscopy with 57Fe3Si tracer layers reveals that the effect of thermally induced interdiffusion at the Fe3Si/GaAs(001) interface is dramatically reduced by inserting a 30 MgO tunneling barrier between the film and the substrate. The chemical order of Fe3Si is comparable to that of Fe3Si films which are grown directly on MgO(001) single crystals. It is proposed that this preparation method can be useful to achieve high-efficiency spin-polarized electron currents from ferromagnetic Fe 3Si into semiconducting GaAs(001). © 2011 American Institute of Physics.
    view abstract10.1063/1.3646390
  • Induced magnetic Cu moments and magnetic ordering in Cu2MnAl thin films on MgO(0 0 1) observed by XMCD
    Krumme, B. and Herper, H.C. and Erb, D. and Weis, C. and Antoniak, C. and Warland, A. and Westerholt, K. and Entel, P. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The disorder-order transition of a highly defective A2-ordered Cu 2MnAl film on MgO(0 0 1) upon annealing at 600 K was monitored by means of x-ray absorption spectroscopy (XAS) at the Cu and Mn L2,3 edges. Additionally, x-ray magnetic circular dichroism (XMCD) was employed to determine element-specific orbital and spin resolved magnetic moments of the Cu and Mn atoms. A small induced total magnetic moment of ≈0.04 0.01μB per atom was detected at the Cu sites, whereas a total magnetic moment of 3.57 0.52μB is carried by the Mn atoms. The experimental XAS and XMCD spectra of Cu agree reasonably with the results from ab initio calculations, magnetic moments derived by the sum rules are in accordance with the calculations. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/44/41/415004
  • Magnetocrystalline anisotropy and Gilbert damping in iron-rich Fe 1-xSix thin films
    Barsukov, I. and Mankovsky, S. and Rubacheva, A. and Meckenstock, R. and Spoddig, D. and Lindner, J. and Melnichak, N. and Krumme, B. and Makarov, S.I. and Wende, H. and Ebert, H. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The magnetocrystalline anisotropy of Fe1-xSix (0≤x≤0.4) epitaxial thin films on MgO(001) was studied by ferromagnetic resonance. The experimental results are in good agreement with theoretical predictions of ab initio electronic structure calculations using the fully relativistic Korringa-Kohn-Rostoker Green's function method within spin-density-functional theory. The Gilbert damping α is found to be isotropic by theory and experiment with a minimum at the composition x=0.2. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.180405
  • Many-body effects in X-ray absorption and magnetic circular dichroism spectra within the LSDA+DMFT framework
    Åipr, O. and Minár, J. and Scherz, A. and Wende, H. and Ebert, H.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    The theoretical description of photoemission spectra of transition metals was greatly improved recently by accounting for the correlations between the d electrons within the local spin-density approximation (LSDA) plus dynamical mean-field theory (DMFT). We assess the improvement of the LSDA+DMFT over the plain LSDA in x-ray absorption spectroscopy, which-unlike the photoemission spectroscopy-is probing unocccupied electronic states. By investigating the L2,3 edge x-ray absorption near-edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) of Fe, Co, and Ni, we find that the LSDA+DMFT improves the LSDA results, in particular concerning the asymmetry of the L 3 white line. Differences with respect to the experiment, nevertheless, remain-particularly concerning the ratio of the intensities of the L3 and L2 peaks. The changes in the XMCD peak intensities invoked by the use of the LSDA+DMFT are a consequence of the improved description of the orbital polarization and are consistent with the XMCD sum rules. Accounting for the core hole within the final-state approximation does not generally improve the results. This indicates that to get more accurate L2,3 edge XANES and XMCD spectra, one has to treat the core hole beyond the final-state approximation. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.115102
  • Multiple-scattering theory and interpretation of XMCD
    Rehr, J.J. and Wende, H.
    Linear and Chiral Dichroism in the Electron Microscope (2011)
    view abstract10.4032/9789814303170
  • Preparation and characterization of ultrathin stainless steel films
    Sahoo, B. and Schlage, K. and Major, J. and Von Hörsten, U. and Keune, W. and Wende, H. and Röhlsberger, R.
    AIP Conference Proceedings 1347 (2011)
    We report on the preparation of polycrystalline austenitic 310 ( 57Fe0.55Cr0.25Ni0.20) stainless steel (SS) thin films on Si substrates and the characterization of their residual magnetism via 57Fe conversion-electron Mössbauer spectroscopy (CEMS). The films were structurally characterized at room temperature by X-ray diffraction (XRD). The virgin films were found to be structurally disordered. Subsequent annealing at moderate temperatures in ultrahigh vacuum produces the ordered martensitic and austenitic SS phases. Further annealing at higher temperatures (up to temperatures where long-range diffusion into the substrate is still weak) transforms the films into the austenitic phase with no trace of a magnetic hyperfine interaction. However, when a 2 nm thick SS thin film is embedded between two carbon layers, the as prepared disordered SS film does not transform to the martensitic or austenitic SS phase irrespective of the annealing temperature, probably because the interdiffusion with C prohibits the formation of these phases. © 2011 American Institute of Physics.
    view abstract10.1063/1.3601785
  • Room temperature spin relaxation length in spin light-emitting diodes
    Soldat, H. and Li, M. and Gerhardt, N.C. and Hofmann, M.R. and Ludwig, Ar. and Ebbing, A. and Reuter, D. and Wieck, A.D. and Stromberg, F. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    We investigate the spin relaxation length in GaAs spin light-emitting diode devices under drift transport at room temperature. The spin-polarised electrons are injected through a MgO tunnel barrier from a Fe/Tb multilayer in magnetic remanence. The decrease in circular polarization with increasing injection path length is investigated and found to be exponential, supporting drift-based transport. The spin relaxation length in our samples is 26 nm, and a lower bound for the spin injection efficiency at the spin injector/GaAs interface is estimated to be 25±2%. © 2011 American Institute of Physics.
    view abstract10.1063/1.3622662
  • Synthesis and magnetic properties of cobalt ferrite nanoparticles
    Etier, M.F. and Shvartsman, V.V. and Stromberg, F. and Landers, J. and Wende, H. and Lupascu, D.C.
    Materials Research Society Symposium Proceedings 1398 (2011)
    Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.699
  • Textured growth of the high moment material Gd(0 0 0 1)/Cr(0 0 1)/Fe(0 0 1)
    Stromberg, F. and Antoniak, C. and Von Hörsten, U. and Keune, W. and Sanyal, B. and Eriksson, O. and Wende, H.
    Journal of Physics D: Applied Physics 44 (2011)
    By magnetic coupling of Fe and Gd via Cr interlayers, the large local moment of Gd can be combined with the high Curie temperature of Fe. The textured growth of a Gd film is studied here by preparing trilayer systems of Fe/Cr/Gd on MgO(1 0 0) substrates by molecular-beam epitaxy (MBE). The thickness of the Cr interlayer was varied between 3 and 5 monolayers. The structural quality of the samples was confirmed by in situ RHEED and ex situ XRD measurements. Epitaxial Cr(0 0 1)/Fe(0 0 1) growth was observed, as expected. By use of 57Fe-CEMS (conversion electron Mössbauer spectroscopy) in combination with the 57Fe tracer layer method the Fe/Cr interface could be examined on an atomic scale and well separated Fe/Gd layers for all Cr thicknesses were confirmed. The unusual Gd/Cr crystallographic relationship of Gd(0 0 0 1)∥Cr(0 0 1), with domains of the hexagonal Gd basal planes randomly oriented in the sample plane and not in registry with the underlying Cr(0 0 1) lattice, was found from combined RHEED and x-ray measurements. Annealing of the samples resulted in a remarkable improvement of the crystalline structure of the Gd layers. On the other hand, the appearance of a single line in the CEM spectrum leads to the conclusion that during annealing a small amount of Fe diffuses into the Cr layer. The electronic structure and magnetism of this system are investigated by first-principles theory. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/44/26/265004
  • Composition dependence of exchange stiffness in FexPt 1-x alloys
    Antoniak, C. and Lindner, J. and Fauth, K. and Thiele, J.-U. and Minár, J. and Mankovsky, S. and Ebert, H. and Wende, H. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The exchange stiffness constants of chemically disordered Fe xPt1-x films with thickness around 50 nm were determined by means of ferromagnetic resonance. It was found to increase with increasing Fe content from 6±4 pJ/m for x=0.27 to 15±4 pJ/m for x=0.67. Theoretical results from fully relativistic and scalar-relativistic band-structure calculations using the Korringa-Kohn-Rostoker method confirm the experimentally obtained values. In addition, determination of the magnetocrystalline anisotropy by angular-dependent measurements of the ferromagnetic resonance gave the possibility to estimate the exchange length that was found to be 40-50 nm for all compositions investigated in this work. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.064403
  • Electronic structure of Co-phthalocyanine calculated by GGA+U and hybrid functional methods
    Bhattacharjee, S. and Brena, B. and Banerjee, R. and Wende, H. and Eriksson, O. and Sanyal, B.
    Chemical Physics 377 (2010)
    Electronic structure calculations have been performed for the Co-phthalocyanine molecule using density functional theory (DFT) within the framework of Generalized Gradient Approximation (GGA). The electronic correlation in Co 3d orbitals is treated in terms of the GGA+U method in the framework of the Hubbard model. We find that for U = 6 eV, the calculated structural parameters as well as the spectral features are in good agreement with the experimental findings. From our calculation both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are dominated by the pyrrole carbon, with a HOMO-LUMO gap of about 1.4 eV. The GGA+U results obtained with U = 6 eV compare reasonably well with the calculations performed using Gaussian basis set and hybrid functionals in terms of ground state geometry, spin state and spectral features. The calculated valence band photoemission spectrum is in quite good agreement with the recently published experimental results. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.chemphys.2010.08.020
  • Epitaxial growth and interfacial magnetism of spin aligner for remanent spin injection: [Fe/Tb]n /Fe/MgO/GaAs -light emitting diode as a prototype system
    Schuster, E. and Brand, R.A. and Stromberg, F. and Lo, F.-Y. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Hövel, S. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W.
    Journal of Applied Physics 108 (2010)
    We have successfully grown and characterized [Fe/Tb]10 /Fe (001) / 57Fe (001) /MgO (001) multilayer contacts on a GaAs-based light emitting diode. Using 57Fe conversion-electron Mössbauer spectroscopy at room temperature (RT) and at 4.2 K, we provide atomistic proof of large perpendicular Fe spin components in zero external field at and below RT at the 57Fe (001) /MgO (001) interface. Further, indirect evidence of large interfacial Fe atomic moments is provided. Our contacts serve as a prototype spin aligner for remanent electrical spin injection at RT. © 2010 American Institute of Physics.
    view abstract10.1063/1.3476265
  • Forcing ferromagnetic coupling between rare-earth-metal and 3d ferromagnetic films
    Sanyal, B. and Antoniak, C. and Burkert, T. and Krumme, B. and Warland, A. and Stromberg, F. and Praetorius, C. and Fauth, K. and Wende, H. and Eriksson, O.
    Physical Review Letters 104 (2010)
    Using density functional calculations, we have studied the magnetic properties of nanocomposites composed of rare-earth-metal elements in contact with 3d transition metals (Fe and Cr). We demonstrate the possibility to obtain huge magnetic moments in such nanocomposites, of order 10μB/rare-earth-metal atom, with a potential to reach the maximum magnetic moment of Fe-Co alloys at the top of the so-called Slater-Pauling curve. A first experimental proof of concept is given by thin-film synthesis of Fe/Gd and Fe/Cr/Gd nanocomposites, in combination with x-ray magnetic circular dichroism. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.104.156402
  • Magnetic properties of ultrathin Fe3Si films on GaAs(001)
    Weis, C. and Krumme, B. and Herper, H.C. and Stromberg, F. and Antoniak, C. and Warland, A. and Entel, P. and Keune, W. and Wende, H.
    Journal of Physics: Conference Series 200 (2010)
    For a detailed understanding of the interface between Fe3Si and GaAs, we investigate Fe3Si films in the ultrathin limit down to a few monolayers and compare the results to Fe3Si/MgO(001) which serves as a reference in the present study. From X-ray magnetic circular dichroism measurements we determine averaged spin and orbital magnetic Fe moments. Further insight follows from SPR-KKR calculations. Conversion electron Mössbauer spectroscopy (CEMS) yields information on the chemical ordering and is able to distinguish inequivalent Fe lattice sites. The CEMS results indicate structural disorder which we attribute to an interdiffusion at the Fe3Si/GaAs interface. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/7/072105
  • Magnetocrystalline anisotropy of Fe-Si alloys on MgO(001)
    Zhang, Y.N. and Cao, J.X. and Barsukov, I. and Lindner, J. and Krumme, B. and Wende, H. and Wu, R.Q.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    Experimental investigations on magnetic anisotropy energies of epitaxial Fe100-x Six thin films on MgO(001) have been extended to low Si concentration. We separated different contributions and found that the strain-induced magnetocrystalline anisotropy term favors an in-plane easy axis for the Fe94.5 Si5.5 sample, but an out-of-plane easy axis for the Fe75 Si25 sample. First principles calculations using the highly precise full potential linearized augmented plane wave method indicated that this results from the sign change in magnetoelastic coupling coefficient in the composition range. Analysis in electronic structures provides clear insights for the understanding of magnetic anisotropy in these films. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.144418
  • Tailoring spin relaxation in thin films by tuning extrinsic relaxation channels
    Barsukov, I. and Meckenstock, R. and Lindner, J. and Moller, M. and Hassel, C. and Posth, O. and Farle, M. and Wende, H.
    IEEE Transactions on Magnetics 46 (2010)
    The importance of extrinsic spin relaxation processes such as magnonmagnon scattering for the overall damping in ferromagnetic thin films has been shown for low relaxation rate systems. Due to its anisotropic behavior, it offers an opportunity for controlling and tailoring the spin relaxation. In this paper, a ferromagnetic resonance study of a system with pure Gilbert damping Fe 94.5Si5.5/MgO(001) is shown. Fe3 Si/MgO(001) systems with native magnonmagnon scattering are discussed. Possibilities for inducing magnonmagnon scattering by volume and surface defects in these systems are presented, offering a method for controlled tailoring of the overall damping in thin films. © 2006 IEEE.
    view abstract10.1109/TMAG.2010.2044482
  • X-ray absorption measurements on nanoparticle systems: Self-assembled arrays and dispersions
    Antoniak, C. and Warland, A. and Darbandi, M. and Spasova, M. and Trunova, A. and Fauth, K. and Aziz, E.F. and Farle, M. and Wende, H.
    Journal of Physics D: Applied Physics 43 (2010)
    X-ray absorption spectroscopy methods are presented as a useful tool to determine local structure, composition and magnetic moments as well as to estimate the effective anisotropy of substrate supported self-assembled arrays of wet-chemically synthesized FePt nanoparticles. A compositional inhomogeneity within the nanoparticles yields reduced magnetic moments with respect to the corresponding bulk material and may also hinder the formation of the chemically ordered L10 phase in FePt nanoparticles. The latter is indicated by a reduced effective anisotropy, which is one order of magnitude smaller than expected from the known value of the corresponding bulk material. As a new approach, measurements of the x-ray absorption near-edge structure of Fe-oxide nanoparticles in dispersion are presented and ageing effects are discussed on the basis of multiplet calculations. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/43/47/474007
  • X-Ray Magnetic Dichroism
    Wende, H. and Antoniak, C.
    Magnetism and Synchrotron Radiation: New Trends 133 (2010)
    An introduction is given to the X-ray magnetic dichroisrn focussing on X-ray magnetic circular dichroism (XMCD). The standard analysis of XMCD spectra by using the sum rules is elucidated. Additionally, aspects of the experimental realization and the data analysis are presented. By means of experimental examples of light 3d metal films, rare earth single crystals, and Fe-porphyrin molecules, the assets and drawbacks of the XMCD technique are illustrated. It is shown that the comparison of ab initio calculated spectra to the experimental results can provide the magnetic properties of the samples if the standard analysis fails.
    view abstract10.1007/978-3-642-04498-4_5
  • magnetic coupling phenomena in nanostructures

  • magnetism

  • nanostructures

  • spintronics

  • surfaces

  • thin films

  • x-ray absorption

  • x-ray magnetic circular dichroism

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