Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

Below, you can either scroll through the complete list of our annually published material, or search for a specific author or term via the free text search to get to know our research strengths. You can also review the publication record of every Materials Chain member via his or her personal member’s page.

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  • 2022 • 214 A mechanically strong and ductile soft magnet with extremely low coercivity
    Han, L. and Maccari, F. and Souza Filho, I.R. and Peter, N.J. and Wei, Y. and Gault, B. and Gutfleisch, O. and Li, Z. and Raabe, D.
    Nature 608 310-316 (2022)
    Soft magnetic materials (SMMs) serve in electrical applications and sustainable energy supply, allowing magnetic flux variation in response to changes in applied magnetic field, at low energy loss1. The electrification of transport, households and manufacturing leads to an increase in energy consumption owing to hysteresis losses2. Therefore, minimizing coercivity, which scales these losses, is crucial3. Yet meeting this target alone is not enough: SMMs in electrical engines must withstand severe mechanical loads; that is, the alloys need high strength and ductility4. This is a fundamental design challenge, as most methods that enhance strength introduce stress fields that can pin magnetic domains, thus increasing coercivity and hysteresis losses5. Here we introduce an approach to overcome this dilemma. We have designed a Fe–Co–Ni–Ta–Al multicomponent alloy (MCA) with ferromagnetic matrix and paramagnetic coherent nanoparticles (about 91 nm in size and around 55% volume fraction). They impede dislocation motion, enhancing strength and ductility. Their small size, low coherency stress and small magnetostatic energy create an interaction volume below the magnetic domain wall width, leading to minimal domain wall pinning, thus maintaining the soft magnetic properties. The alloy has a tensile strength of 1,336 MPa at 54% tensile elongation, extremely low coercivity of 78 A m−1 (less than 1 Oe), moderate saturation magnetization of 100 A m2 kg−1 and high electrical resistivity of 103 μΩ cm. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41586-022-04935-3
  • 2022 • 213 Ab initio calculation of the magnetic Gibbs free energy of materials using magnetically constrained supercells
    Mendive-Tapia, E. and Neugebauer, J. and Hickel, T.
    Physical Review B 105 (2022)
    We present a first-principles approach for the computation of the magnetic Gibbs free energy of materials using magnetically constrained supercell calculations. Our approach is based on an adiabatic approximation of slowly varying local moment orientations, the so-called finite-temperature disordered local moment picture. It describes magnetic phase transitions and how electronic and/or magnetostructural mechanisms generate a discontinuous (first-order) character. We demonstrate that the statistical mechanics of the local moment orientations can be described by an affordable number of supercell calculations containing noncollinear magnetic configurations. The applicability of our approach is illustrated by firstly studying the ferromagnetic state in bcc Fe. We then investigate the temperature-dependent properties of a triangular antiferromagnetic state stabilizing in two antiperovskite systems Mn3AN (A=Ga, Ni). Our calculations provide the negative thermal expansion of these materials as well as the ab initio origin of the discontinuous character of the phase transitions, electronic and/or magnetostructural, in good agreement with experiment. © 2022 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the ""Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society.
    view abstractdoi: 10.1103/PhysRevB.105.064425
  • 2022 • 212 Machine learning–enabled high-entropy alloy discovery
    Rao, Z. and Tung, P.-Y. and Xie, R. and Wei, Y. and Zhang, H. and Ferrari, A. and Klaver, T.P.C. and Körmann, F. and Sukumar, P.T. and da Silva, A.K. and Chen, Y. and Li, Z. and Ponge, D. and Neugebauer, J. and Gutfleisch, O. and...
    Science 378 (2022)
    High-entropy alloys are solid solutions of multiple principal elements that are capable of reaching composition and property regimes inaccessible for dilute materials. Discovering those with valuable properties, however, too often relies on serendipity, because thermodynamic alloy design rules alone often fail in high-dimensional composition spaces. We propose an active learning strategy to accelerate the design of high-entropy Invar alloys in a practically infinite compositional space based on very sparse data. Our approach works as a closed-loop, integrating machine learning with density-functional theory, thermodynamic calculations, and experiments. After processing and characterizing 17 new alloys out of millions of possible compositions, we identified two high-entropy Invar alloys with extremely low thermal expansion coefficients around 2 × 10−6 per degree kelvin at 300 kelvin. We believe this to be a suitable pathway for the fast and automated discovery of high-entropy alloys with optimal thermal, magnetic, and electrical properties. Copyright © 2022 The Authors, some rights reserved.
    view abstractdoi: 10.1126/science.abo4940
  • 2022 • 211 Simultaneous Multi-Property Probing During Magneto-Structural Phase Transitions: An Element-Specific and Macroscopic Hysteresis Characterization at ID12 of the ESRF
    Aubert, A. and Skokov, K. and Gomez, G. and Chirkova, A. and Radulov, I. and Wilhelm, F. and Rogalev, A. and Wende, H. and Gutfleisch, O. and Ollefs, K.
    IEEE Transactions on Instrumentation and Measurement 71 (2022)
    We present a new instrument for advanced magnetic studies based on the high field X-ray magnetic circular dichroism (XMCD) end-station developed at the beamline ID12 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It offers a unique possibility to measure simultaneously element-specific and macroscopic properties related to magnetic, electronic, and structural degrees of freedom of magnetic materials. Under strictly the same experimental conditions, one can measure the XMCD response, macroscopic magnetization, volume changes, and caloric properties of a magnetic material as a function of magnetic field (up to 17 T) and temperature (5-325 K). To illustrate the performance of this new instrument, we present a case study of an equiatomic FeRh alloy across the first-order magneto-structural transition. This development is the first step toward a new fully dedicated end-station based on a 7 T split-coil superconducting magnet with an additional capability to perform X-ray diffraction experiments. © 1963-2012 IEEE.
    view abstractdoi: 10.1109/TIM.2022.3157001
  • 2022 • 210 Spanning Fermi arcs in a two-dimensional magnet
    Chen, Y.-J. and Hanke, J.-P. and Hoffmann, M. and Bihlmayer, G. and Mokrousov, Y. and Blügel, S. and Schneider, C.M. and Tusche, C.
    Nature Communications 13 (2022)
    The discovery of topological states of matter has led to a revolution in materials research. When external or intrinsic parameters break symmetries, global properties of topological materials change drastically. A paramount example is the emergence of Weyl nodes under broken inversion symmetry. While a rich variety of non-trivial quantum phases could in principle also originate from broken time-reversal symmetry, realizing systems that combine magnetism with complex topological properties is remarkably elusive. Here, we demonstrate that giant open Fermi arcs are created at the surface of ultrathin hybrid magnets where the Fermi-surface topology is substantially modified by hybridization with a heavy-metal substrate. The interplay between magnetism and topology allows us to control the shape and the location of the Fermi arcs by tuning the magnetization direction. The hybridization points in the Fermi surface can be attributed to a non-trivial mixed topology and induce hot-spots in the Berry curvature, dominating spin and charge transport as well as magneto-electric coupling effects. © 2022, The Author(s).
    view abstractdoi: 10.1038/s41467-022-32948-z
  • 2022 • 209 Tailoring magnetic anisotropy by graphene-induced selective skyhook effect on 4f-metals
    Herman, A. and Kraus, S. and Tsukamoto, S. and Spieker, L. and Caciuc, V. and Lojewski, T. and Günzing, D. and Dreiser, J. and Delley, B. and Ollefs, K. and Michely, T. and Atodiresei, N. and Wende, H.
    Nanoscale 14 7682-7691 (2022)
    From macroscopic heavy-duty permanent magnets to nanodevices, the precise control of the magnetic properties in rare-earth metals is crucial for many applications used in our daily life. Therefore, a detailed understanding and manipulation of the 4f-metals’ magnetic properties are key to further boosting the functionalization and efficiency of future applications. We present a proof-of-concept approach consisting of a dysprosium-iridium surface alloy in which graphene adsorption allows us to tailor its magnetic properties. By adsorbing graphene onto a long-range ordered two-dimensional dysprosium-iridium surface alloy, the magnetic 4f-metal atoms are selectively lifted from the surface alloy. This selective skyhook effect introduces a giant magnetic anisotropy in dysprosium atoms as a result of manipulating its geometrical structure within the surface alloy. Introducing and proving this concept by our combined theoretical and experimental approach provides an easy and unambiguous understanding of its underlying mechanism. Our study sets the ground for an alternative path on how to modify the crystal field around 4f-atoms and therefore their magnetic anisotropies. © 2022 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d2nr01458k
  • 2022 • 208 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 3140-3150 (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 abstractdoi: 10.1021/acs.jpcc.1c10625
  • 2021 • 207 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 K...
    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 abstractdoi: 10.1038/s41467-021-27094-x
  • 2021 • 206 Ab initio based models for temperature-dependent magnetochemical interplay in bcc Fe-Mn alloys
    Schneider, A. and Fu, C.-C. and Waseda, O. and Barreteau, C. and Hickel, T.
    Physical Review B 103 (2021)
    Body-centered cubic (bcc) Fe-Mn systems are known to exhibit a complex and atypical magnetic behavior from both experiments and 0 K electronic-structure calculations, which is due to the half-filled 3d band of Mn. We propose effective interaction models for these alloys, which contain both atomic-spin and chemical variables. They were parameterized on a set of key density functional theory (DFT) data, with the inclusion of noncollinear magnetic configurations being indispensable. Two distinct approaches, namely a knowledge-driven and a machine-learning approach have been employed for the fitting. Employing these models in atomic Monte Carlo simulations enables the prediction of magnetic and thermodynamic properties of the Fe-Mn alloys, and their coupling, as functions of temperature. This includes the decrease of Curie temperature with increasing Mn concentration, the temperature evolution of the mixing enthalpy, and its correlation with the alloy magnetization. Also, going beyond the defect-free systems, we determined the binding free energy between a vacancy and a Mn atom, which is a key parameter controlling the atomic transport in Fe-Mn alloys. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.024421
  • 2021 • 205 Ab initio study of the structural response to magnetic disorder and van der Waals interactions in FeSe
    Lochner, F. and Eremin, I.M. and Hickel, T. and Neugebauer, J.
    Physical Review B 103 (2021)
    The electronic structure in unconventional superconductors holds a key to understanding the momentum-dependent pairing interactions and the resulting superconducting gap function. In superconducting Fe-based chalcogenides, there have been controversial results regarding the importance of the kz dependence of the electronic dispersion, the gap structure, and the pairing mechanisms. Here, we use density functional theory to investigate the underlying structural properties in combination with a sophisticated real-space treatment of magnetic disorder for the prototype system FeSe. Our calculations demonstrate that interlayer and intralayer interactions need to be considered and that charge-driven van der Waals interactions between Se atoms instead of magnetic coupling effects drive the interlayer binding. The methodological advances and physical insights are important for upcoming investigations of the three-dimensional effects, including nontrivial topology, of FeSe1-xTex and FeSe1-xSx systems. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.054506
  • 2021 • 204 Bendable Polycrystalline and Magnetic CoFe2O4Membranes by Chemical Methods
    Salles, P. and Guzmán, R. and Zanders, D. and Quintana, A. and Fina, I. and Sánchez, F. and Zhou, W. and Devi, A. and Coll, M.
    ACS Applied Materials and Interfaces (2021)
    The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe2O4 (CFO) membranes by means of the Sr3Al2O6 (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid high-temperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties. © 2022 The Authors. Published by American Chemical Society.
    view abstractdoi: 10.1021/acsami.1c24450
  • 2021 • 203 Discovery and Implications of Hidden Atomic-Scale Structure in a Metallic Meteorite
    Kovács, A. and Lewis, L.H. and Palanisamy, D. and Denneulin, T. and Schwedt, A. and Scott, E.R.D. and Gault, B. and Raabe, D. and Dunin-Borkowski, R.E. and Charilaou, M.
    Nano Letters 21 8135-8142 (2021)
    Iron and its alloys have made modern civilization possible, with metallic meteorites providing one of the human's earliest sources of usable iron as well as providing a window into our solar system's billion-year history. Here highest-resolution tools reveal the existence of a previously hidden FeNi nanophase within the extremely slowly cooled metallic meteorite NWA 6259. This new nanophase exists alongside Ni-poor and Ni-rich nanoprecipitates within a matrix of tetrataenite, the uniaxial, chemically ordered form of FeNi. The ferromagnetic nature of the nanoprecipitates combined with the antiferromagnetic character of the FeNi nanophases gives rise to a complex magnetic state that evolves dramatically with temperature. These observations extend and possibly alter our understanding of celestial metallurgy, provide new knowledge concerning the archetypal Fe-Ni phase diagram and supply new information for the development of new types of sustainable, technologically critical high-energy magnets. ©
    view abstractdoi: 10.1021/acs.nanolett.1c02573
  • 2021 • 202 Finite temperature fluctuation-induced order and responses in magnetic topological insulators
    Scholten, M. and Facio, J.I. and Ray, R. and Eremin, I.M. and van den Brink, J. and Nogueira, F.S.
    Physical Review Research 3 (2021)
    We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for  and for EuS- heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For we predict an upwards Néel temperature shift at the surface up to , while the EuS- interface exhibits a smaller relative shift. The effective theory also predicts induced Dzyaloshinskii-Moriya interactions and a topological magnetoelectric effect, both of which feature a finite temperature and chemical potential dependence. © 2021 Published by the American Physical Society
    view abstractdoi: 10.1103/PhysRevResearch.3.L032014
  • 2021 • 201 Forming of Parts with Locally Defined Mechanical and Ferromagnetic Properties by Flow-Forming
    Wiens, E. and Homberg, W. and Arian, B. and Möhring, K. and Walther, F.
    Minerals, Metals and Materials Series 1913-1924 (2021)
    To generate the highly efficient use of material resources for formed parts, local adaptation of the required strength and integrated functions is advised. When using austenitic steel, the volume fraction of deformation-induced α′-martensite, which has an influence on the strength and the magnetic permeability of the material, is highly dependent on the degree of deformation and the workpiece temperature in the deformation zone. Through selective adjustment of the process parameters of wall thickness reduction ∆s and deformation temperature Td it proved possible to produce locally restricted areas with an α′-martensite volume from almost negligible to 80% at an identical stage of deformation, using a spinning or flow-forming process. In this way, axially graded and locally varying mechanical and sensory properties can be produced, such as for a magnetic displacement sensor. The aim of this ongoing work is the closed-loop control of properties through the use of micro-magnetic sensors during spinning processes. © 2021, The Minerals, Metals & Materials Society.
    view abstractdoi: 10.1007/978-3-030-75381-8_160
  • 2021 • 200 High-dimensional neural network potentials for magnetic systems using spin-dependent atom-centered symmetry functions
    Eckhoff, M. and Behler, J.
    npj Computational Materials 7 (2021)
    Machine learning potentials have emerged as a powerful tool to extend the time and length scales of first-principles quality simulations. Still, most machine learning potentials cannot distinguish different electronic spin arrangements and thus are not applicable to materials in different magnetic states. Here we propose spin-dependent atom-centered symmetry functions as a type of descriptor taking the atomic spin degrees of freedom into account. When used as an input for a high-dimensional neural network potential (HDNNP), accurate potential energy surfaces of multicomponent systems can be constructed, describing multiple collinear magnetic states. We demonstrate the performance of these magnetic HDNNPs for the case of manganese oxide, MnO. The method predicts the magnetically distorted rhombohedral structure in excellent agreement with density functional theory and experiment. Its efficiency allows to determine the Néel temperature considering structural fluctuations, entropic effects, and defects. The method is general and is expected to be useful also for other types of systems such as oligonuclear transition metal complexes. © 2021, The Author(s).
    view abstractdoi: 10.1038/s41524-021-00636-z
  • 2021 • 199 Impact of magnetic transition on Mn diffusion in α -iron: Correlative state-of-the-art theoretical and experimental study
    Hegde, O. and Kulitckii, V. and Schneider, A. and Soisson, F. and Hickel, T. and Neugebauer, J. and Wilde, G. and Divinski, S. and Fu, C.-C.
    Physical Review B 104 (2021)
    An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic excitations and magnetic transitions. We investigate the diffusion of Mn in bcc Fe using an effective interaction model and first-principles based spin-space averaged relaxations in magnetically disordered systems. The theoretical results are compared with the dedicated radiotracer measurements of Mn54 diffusion in a wide temperature range of 773 to 1173 K, performed by combining the precision grinding (higher temperatures) and ion-beam sputtering (low temperatures) sectioning techniques. The temperature evolution of Mn diffusion coefficients in bcc iron in theory and experiment agree very well and consistently reveal a reduced acceleration of Mn solute diffusion around the Curie point. By analyzing the temperature dependencies of the ratio of Mn diffusion coefficients to self-diffusion coefficients we observe a dominance of magnetic disorder over chemical effects on high-temperature diffusion. Therefore, the missing acceleration mainly reflects an anomalous behavior of the Mn solute in the magnetically ordered low-temperature state of the Fe host, as compared to other transition metals. © 2021 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.104.184107
  • 2021 • 198 Magnetic ordering and structural distortion in a PrFeAsO single crystal studied by neutron and X-ray scattering
    Kim, M.G. and Ratcliff, W. and Pajerowski, D.M. and Kim, J.-W. and Yan, J.-Q. and Lynn, J.W. and Goldman, A.I. and Kreyssig, A.
    Physical Review B 103 (2021)
    We report the magnetic ordering and structural distortion in PrFeAsO crystals, the basis compound for one of the oxypnictide superconductors, using high-resolution X-ray diffraction, neutron diffraction, and X-ray resonant magnetic scattering (XRMS). We find the structural tetragonal-to-orthorhombic phase transition at TS=147K, the AFM phase transition of the Fe moments at TFe=72K, and the Pr AFM phase transition at TPr=21K. Combined high-resolution neutron diffraction and XRMS show unambiguously that the Pr moments point parallel to the longer orthorhombic a axis and order antiferromagnetically along the a axis but ferromagnetically along the b and c directions in the stripelike AFM order. The temperature-dependent magnetic order parameter of the Pr moments shows no evidence for a reorientation of moments. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.103.174405
  • 2021 • 197 Multiscale modeling of cancellous bone considering full coupling of mechanical, electric and magnetic effects
    Blaszczyk, M. and Hackl, K.
    Biomechanics and Modeling in Mechanobiology (2021)
    Modeling of cancellous bone has important applications in the detection and treatment of fatigue fractures and diseases like osteoporosis. In this paper, we present a fully coupled multiscale approach considering mechanical, electric and magnetic effects by using the multiscale finite element method and a two-phase material model on the microscale. We show numerical results for both scales, including calculations for a femur bone, comparing a healthy bone to ones affected by different stages of osteoporosis. Here, the magnetic field strength resulting from a small mechanical impact decreases drastically for later stages of the disease, confirming experimental research. © 2021, The Author(s).
    view abstractdoi: 10.1007/s10237-021-01525-6
  • 2021 • 196 Optical and magneto-optical properties of epitaxial Mn2GaC MAX phase thin film
    Lyaschenko, S. and Maximova, O. and Shevtsov, D. and Varnakov, S. and Tarasov, I. and Wiedwald, U. and Rosen, J. and Ovchinnikov, S. and Farle, M.
    Journal of Magnetism and Magnetic Materials 528 (2021)
    We report measurements of the dielectric permittivity, optical conductivity and magnetic circular dichroism (MCD) of the epitaxial Mn2GaC MAX-phase thin film in an external magnetic field of up to 200 mT, at temperatures of 296 and 140 K and 1.4 to 3.5 eV. The optical conductivity and MCD spectra show absorption peaks which are consistent with the interband electronic transitions for different positions of Mn, Ga, and C ions as confirmed by theoretical calculations of the spin-dependent density of electronic states. The well-known structural phase transition at 214 K is also seen in the changes of optical, magneto-optical and surface magnetic properties of Mn2GaC in our experiment. © 2021 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2021.167803
  • 2021 • 195 Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi4Te7 and MnBi6Te10
    Vidal, R.C. and Bentmann, H. and Facio, J.I. and Heider, T. and Kagerer, P. and Fornari, C.I. and Peixoto, T.R.F. and Figgemeier, T. and Jung, S. and Cacho, C. and Büchner, B. and Van Den Brink, J. and Schneider, C.M. and Plucins...
    Physical Review Letters 126 (2021)
    Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi4Te7 and MnBi6Te10, the n=1 and 2 members of a modular (Bi2Te3)n(MnBi2Te4) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi2Te3-terminated surfaces but remains preserved for MnBi2Te4-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination. © 2021 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.126.176403
  • 2021 • 194 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 7565-7584 (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 abstractdoi: 10.1039/d0sm02104k
  • 2021 • 193 Structure determination and magnetic properties of the Mn-doped MAX phase Cr2GaC
    Siebert, J.P. and Mallett, S. and Juelsholt, M. and Pazniak, H. and Wiedwald, U. and Page, K. and Birkel, C.S.
    Materials Chemistry Frontiers 5 6082-6091 (2021)
    Introducing magnetic elements into the structure of layered ternary transition metal-based carbides that belong to the family of MAX phases has led to various intriguing phenomena, such as magnetic ordering close to or even above room temperature and structural changes accompanying magnetic transitions. However, synthesizing manganese-or even iron-containing-MAX phases has proven to be extremely challenging as a result of the intrinsic structural instability at higher electron counts of the later transition metals as well as the favored formation of thermodynamically stable competing phases. Owing to the available kinetic control over the reaction product coupled with (atomically) precise growth techniques, the thin film community has taken the lead in the synthesis of MAX phases that exhibit magnetic ordering. Producing bulk samples of sufficient quality to study the complex magnetic properties of Mn-containing MAX phase compounds poses a major obstacle, particularly if conventional high-temperature methods are used that promote the formation of stable side phases. Using a milder wet chemical-based approach, we have synthesized Mn-containing solid solutions of MAX phase Cr2GaC with Mn amounts ranging from 2 to 20 at% in the M-layers. The resulting (Cr1-xMnx)2GaC (x = 0.02-0.2) particles are structurally characterized using X-ray and neutron powder diffractometry, as well as scanning transmission electron microscopy to enable detailed magnetometry studies. We demonstrate that low amounts of Mn on the Cr site do not induce magnetic ordering, and a sample with a Mn content of x = 0.20 is also predominantly paramagnetic. Taking all side phases into account, locally ordered parts of the MAX phase could explain the magnetic order we observe at elevated temperatures. © 2021 the Partner Organisations.
    view abstractdoi: 10.1039/d1qm00454a
  • 2021 • 192 Synthesis, phase purification and magnetic characterization of the (Cr1-: X, Mnx)2AlC MAX-phase
    Sobolev, K. and Pazniak, H. and Farle, M. and Rodionova, V. and Wiedwald, U.
    Journal of Materials Chemistry C 9 16516-16522 (2021)
    The Cr2AlC MAX phase is a promising parent compound to introduce magnetism to nano-laminated ternary carbides by doping with Mn. Here, we demonstrate that Mn doping of bulk Cr2AlC powder synthesized by arc melting results in incorporation up to 16 at% Mn in the M-layers of the MAX phase. Simultaneously, the relative amount of secondary phases is overall low, however, increases with Mn doping. We successfully applied chemical treatment in dilute hydrochloric acid to eliminate secondary phases and studied the magnetic properties before and after treatment by magnetometry between 3 K and 800 K. All MAX-phases show a paramagnetic response. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d1tc03092b
  • 2021 • 191 Three-dimensional magnetic induction tomography: Practical implementation for imaging throughout the depth of a low conductive and voluminous body
    Klein, M. and Erni, D. and Rueter, D.
    Sensors 21 (2021)
    Magnetic induction tomography (MIT) is a contactless, low-energy method used to visual-ize the conductivity distribution inside a body under examination. A particularly demanding task is the three-dimensional (3D) imaging of voluminous bodies in the biomedical impedance regime. While successful MIT simulations have been reported for this regime, practical demonstration over the entire depth of weakly conductive bodies is technically difficult and has not yet been reported, particularly in terms of more realistic requirements. Poor sensitivity in the central regions critically affects the measurements. However, a recently simulated MIT scanner with a sinusoidal excitation field topology promises improved sensitivity (>20 dB) from the interior. On this basis, a large and fast 3D MIT scanner was practically realized in this study. Close agreement between theoretical forward calculations and experimental measurements underline the technical performance of the sensor system, and the previously only simulated progress is hereby confirmed. This allows 3D reconstructions from practical measurements to be presented over the entire depth of a voluminous body phantom with tissue-like conductivity and dimensions similar to a human torso. This feasibility demonstration takes MIT a step further toward the quick 3D mapping of a low conductive and voluminous object, for example, for rapid, harmless and contactless thorax or lung diagnostics. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstractdoi: 10.3390/s21227725
  • 2020 • 190 Beyond Solid Solution High-Entropy Alloys: Tailoring Magnetic Properties via Spinodal Decomposition
    Rao, Z. and Dutta, B. and Körmann, F. and Lu, W. and Zhou, X. and Liu, C. and da Silva, A.K. and Wiedwald, U. and Spasova, M. and Farle, M. and Ponge, D. and Gault, B. and Neugebauer, J. and Raabe, D. and Li, Z.
    Advanced Functional Materials (2020)
    Since its first emergence in 2004, the high-entropy alloy (HEA) concept has aimed at stabilizing single- or dual-phase multi-element solid solutions through high mixing entropy. Here, this strategy is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe-Co enriched regions which have an expanded volume (relative to unconstrained Fe-Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single-phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
    view abstractdoi: 10.1002/adfm.202007668
  • 2020 • 189 Composition and structure of magnetic high-temperature-phase, stable Fe-Au core-shell nanoparticles with zero-valent bcc Fe core
    Kamp, M. and Tymoczko, A. and Popescu, R. and Schürmann, U. and Nadarajah, R. and Gökce, B. and Rehbock, C. and Gerthsen, D. and Barcikowski, S. and Kienle, L.
    Nanoscale Advances 2 3912-3920 (2020)
    Advanced quantitative TEM/EDXS methods were used to characterize different ultrastructures of magnetic Fe-Au core-shell nanoparticles formed by laser ablation in liquids. The findings demonstrate the presence of Au-rich alloy shells with varying composition in all structures and elemental bcc Fe cores. The identified structures are metastable phases interpreted by analogy to the bulk phase diagram. Based on this, we propose a formation mechanism of these complex ultrastructures. To show the magnetic response of these magnetic core nanoparticles protected by a noble metal shell, we demonstrate the formation of nanostrands in the presence of an external magnetic field. We find that it is possible to control the lengths of these strands by the iron content within the alloy nanoparticles. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0na00514b
  • 2020 • 188 Dynamics of chiral state transitions and relaxations in an FeGe thin plate: Via in situ Lorentz microscopy
    Chai, K. and Li, Z.-A. and Liu, R. and Zou, B. and Farle, M. and Li, J.
    Nanoscale 12 14919-14925 (2020)
    Studying the magnetic transition between different topological spin textures in noncentrosymmetric magnets under external stimuli is an important topic in chiral magnetism. Here, using in situ Lorentz transmission electron microscopy (LTEM) we directly visualize the thermal-driven magnetic transitions and dynamic characteristics in FeGe thin plates. A novel protocol-dependent phase diagram of FeGe thin plates was obtained via pulsed laser excitation. Moreover, by setting the appropriate specimen temperature, the relaxation of chiral magnetic states in FeGe specimens was recorded and analyzed with an Arrhenius-type relaxation mechanism. We present the field-dependent activation energy barriers for chiral state transitions and the magnetic transition pathways of these spin textures for FeGe thin plates. Our results unveil the effects of thermal excitation on the topological spin texture transitions and provide useful information about magnetic dynamics of chiral magnetic state relaxation. © 2020 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr03278f
  • 2020 • 187 Electro-discharge sintering of nanocrystalline NdFeB magnets: process parameters, microstructure, and the resulting magnetic properties
    Leich, L. and Röttger, A. and Kuchenbecker, R. and Theisen, W.
    Journal of Materials Science: Materials in Electronics 31 20431-20443 (2020)
    This study investigates the compaction of nanocrystalline NdFeB magnet powder by electro-discharge sintering (EDS). On this account, process parameters, microstructure, and the associated magnetic properties of the EDS-densified nanocrystalline NdFeB specimens were investigated by varying the discharge energy EEDS and compression load pEDS. Although optimized process parameters could be evaluated, three different microstructures (fully densified zone, insufficiently densified zone, and melted zone) are present in the EDS-compacted specimens. Thereby, volume fractions of these formed three different microstructures determine the resulting mechanical and magnetic properties of the specimens. For all specimens, the intrinsic coercivity Hc,J deteriorates with increasing discharge energy, as the generated Joule heat leads to microstructural changes (grain growth, dissolution of magnetic phases), which reduces the magnetic properties. The compression load has less influence on the coercivity Hc,J, as it only affects the initial resistance of the pre-compacted powder loose. The residual induction Br deteriorates with increasing the discharge energy due to microstructural changes. An increase in the compression load pEDS results in an increase in the specimens’ density and thus promotes the residual induction Br. © 2020, The Author(s).
    view abstractdoi: 10.1007/s10854-020-04562-6
  • 2020 • 186 Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2 O4
    Kudlacik, D. and Ivanov, V.Y. and Yakovlev, D.R. and Sapega, V.F. and Schindler, J.J. and Debus, J. and Bayer, M. and Pisarev, R.V.
    Physical Review B 102 (2020)
    Copper metaborate CuB2O4 crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below TN1=20 K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic Cu2+ (3d9, S=1/2) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the Cu2+ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split d-d electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range 1.7-40 K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.035128
  • 2020 • 185 Joining dissimilar thin-walled tubes by Magnetic Pulse Welding
    Lueg-Althoff, J. and Bellmann, J. and Hahn, M. and Schulze, S. and Gies, S. and Tekkaya, A.E. and Beyer, E.
    Journal of Materials Processing Technology 279 (2020)
    Welding dissimilar metal tubes attracts interest for a wide range of automotive, aeronautical, and plant engineering applications as well as other consumables. Hybrid driveshafts or structural elements can meet mechanical requirements at a reduced weight. However, joining materials with strongly different thermo-physical properties is a challenge for conventional fusion welding processes. In Magnetic Pulse Welding (MPW), the weld formation is based on the high-velocity collision between the joining partners, without additional heat input. This allows for the fabrication of sound “cold” welds. MPW of tubular parts is usually realized by the radial electromagnetic compression of the outer “flyer” part and the subsequent impact on the inner “parent” part. This impact represents a harsh loading for the parent, which therefore is usually designed as a thick-walled or solid part to avoid damage or unwanted deformations. To further increase the lightweight potential, the objective of the present manuscript is the comprehensive analysis of MPW with thin-walled parent parts. Experimental and analytical investigations are presented, which enable to reduce the parent thickness without affecting the joint strength. The approaches comprise the observation of the impact and deformation behavior by inline laser-based measurement technology as well as the development of adequate, re-usable mandrels to support the parent parts. The focus is on aluminum flyer parts, which are welded to steel and copper parent parts. Critical values for the parent wall thickness are deduced and recommendations for the process design of MPW with thin-walled tubes are given. © 2019 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2019.116562
  • 2020 • 184 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 abstractdoi: 10.1063/5.0004125
  • 2020 • 183 Micro-magnetic damage characterization of bent and cold forged parts
    Samfaß, L. and Baak, N. and Meya, R. and Hering, O. and Tekkaya, A.E. and Walther, F.
    Production Engineering 14 77-85 (2020)
    Damage can have a strong impact on the fatigue performance of bulk formed parts for example produced by cold forging and sheet metal formed parts for example produced by bending. One suitable method to detect damage non-destructively in a time-efficient way is the micro-magnetic material characterization. In this paper, the suitability of harmonic analysis of the tangential magnetic field strength for the detection of damage in bent DP800-parts and cold forged 16MnCrS5-parts is discussed. For differently formed parts a correlation between the magnitude of damage and the behavior of the upper harmonics parameters is shown. © 2019, German Academic Society for Production Engineering (WGP).
    view abstractdoi: 10.1007/s11740-019-00934-y
  • 2020 • 182 Optically detected magnetic resonance in CdSe/CdMnS nanoplatelets
    Tolmachev, D.O. and Ivanov, V.Y. and Yakovlev, D.R. and Shornikova, E.V. and Witkowski, B. and Shendre, S. and Isik, F. and Delikani, S. and Demir, H.V. and Bayer, M.
    Nanoscale 12 21932-21939 (2020)
    Core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets containing magnetic Mn2+ ions are investigated by the optically detected magnetic resonance technique, combining 60 GHz microwave excitation and photoluminescence detection. Resonant heating of the Mn spin system is observed. We identify two mechanisms of optical detection, via variation of either the photoluminescence polarization or its intensity in an external magnetic field. The spin-lattice relaxation dynamics of the Mn spin system is measured and used for evaluation of the Mn concentration. In CdSe/(Cd,Zn,Mn)S nanoplatelets the addition of Zn in the shells significantly broadens the magnetic resonance, evidencing local strain. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/d0nr05633b
  • 2020 • 181 Performance of the standard exchange-correlation functionals in predicting melting properties fully from first principles: Application to Al and magnetic Ni
    Zhu, L.-F. and Körmann, F. and Ruban, A.V. and Neugebauer, J. and Grabowski, B.
    Physical Review B 101 (2020)
    We apply the efficient two-optimized references thermodynamic integration using Langevin dynamics method [Phys. Rev. B 96, 224202 (2017)2469-995010.1103/PhysRevB.96.224202] to calculate highly accurate melting properties of Al and magnetic Ni from first principles. For Ni we carefully investigate the impact of magnetism on the liquid and solid free energies including longitudinal spin fluctuations and the reverse influence of atomic vibrations on magnetic properties. We show that magnetic fluctuations are effectively canceling out for both phases and are thus not altering the predicted melting temperature. For both elements, the generalized gradient approximation (GGA) and the local-density approximation (LDA) are used for the exchange-correlation functional revealing a reliable ab initio confidence interval capturing the respective experimental melting point, enthalpy of fusion, and entropy of fusion. © 2020 authors. Published by the American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.101.144108
  • 2020 • 180 Size-selective optical printing of silicon nanoparticles through their dipolar magnetic resonance
    Zaza, C. and Violi, I.L. and Gargiulo, J. and Chiarelli, G. and Schumacher, L. and Jakobi, J. and Olmos-Trigo, J. and Cortes, E. and König, M. and Barcikowski, S. and Schlücker, S. and Saénz, J.J. and Maier, S.A. and Stefani, F.D.
    Proceedings of SPIE - The International Society for Optical Engineering 11297 (2020)
    Surfactant-free silicon nanoparticles of a predefined and narrow (σ < 10 nm) size range can be selectively immobilized on a substrate by optical printing from a polydisperse colloidal suspension by tuning the light wavelength to their size-dependent magnetic dipolar resonance. © 2020 SPIE.
    view abstractdoi: 10.1117/12.2539265
  • 2020 • 179 Structural stability of Co–V intermetallic phases and thermodynamic description of the Co–V system
    Wang, P. and Hammerschmidt, T. and Kattner, U.R. and Olson, G.B.
    Calphad: Computer Coupling of Phase Diagrams and Thermochemistry 68 (2020)
    The Co–V system has been reviewed. Density functional theory (DFT) calculations using the generalized gradient approximation (GGA) were used to obtain the energies for the end-members for all three intermediate phases, Co3V, σ and CoV3. Results from DFT calculations considering spin polarization were used to evaluate the CALPHAD (Calculation of phase diagrams) model parameters. The method to evaluate the contribution of the magnetism to the energies of Co-rich compounds that was introduced in our previous work is presented in more detail in the present work. For the description of the σ phase, the magnetic part of the total energy is included in the description of the pure Co end-member compound resulting in a non-linear description of the magnetic contribution over composition. The calculated phase diagram obtained from the present CALPHAD description is in good agreement with the experimental data. The metastable FCC-L12 phase diagram was calculated and compared with experimental data. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.calphad.2019.101729
  • 2020 • 178 Superconductivity with broken time-reversal symmetry inside a superconducting s-wave state
    Grinenko, V. and Sarkar, R. and Kihou, K. and Lee, C.H. and Morozov, I. and Aswartham, S. and Büchner, B. and Chekhonin, P. and Skrotzki, W. and Nenkov, K. and Hühne, R. and Nielsch, K. and Drechsler, S.-L. and Vadimov, V.L. and...
    Nature Physics 16 789-794 (2020)
    In general, magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors in which superconductivity coexists with magnetism, and a few examples are known where the superconductivity itself induces spontaneous magnetism. The best known of these compounds are Sr2RuO4 and some non-centrosymmetric superconductors. Here, we report the finding of a narrow dome of an s+ is′ superconducting phase with apparent broken time-reversal symmetry (BTRS) inside the broad s-wave superconducting region of the centrosymmetric multiband superconductor Ba1 − xKxFe2As2 (0.7 ≲ x ≲ 0.85). We observe spontaneous magnetic fields inside this dome using the muon spin relaxation (μSR) technique. Furthermore, our detailed specific heat study reveals that the BTRS dome appears very close to a change in the topology of the Fermi surface. With this, we experimentally demonstrate the likely emergence of a novel quantum state due to topological changes of the electronic system. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstractdoi: 10.1038/s41567-020-0886-9
  • 2020 • 177 Temperature-controlled magnetic nanoparticles hyperthermia inhibits primary tumor growth and metastases dissemination
    Garanina, A.S. and Naumenko, V.A. and Nikitin, A.A. and Myrovali, E. and Petukhova, A.Y. and Klimyuk, S.V. and Nalench, Y.A. and Ilyasov, A.R. and Vodopyanov, S.S. and Erofeev, A.S. and Gorelkin, P.V. and Angelakeris, M. and Savch...
    Nanomedicine: Nanotechnology, Biology, and Medicine 25 (2020)
    Magnetic hyperthermia (MHT) is a promising approach for cancer therapy. However, a systematic MHT characterization as function of temperature on the therapeutic efficiency is barely analyzed. Here, we first perform comparative temperature-dependent analysis of the cobalt ferrite nanoparticles-mediated MHT effectiveness in two murine tumors models – breast (4T1) and colon (CT26) cancer in vitro and in vivo. The overall MHT killing capacity in vitro increased with the temperature and CT26 cells were more sensitive than 4T1 when heated to 43 °C. Well in line with the in vitro data, such heating cured non-metastatic CT26 tumors in vivo, while only inhibiting metastatic 4T1 tumor growth without improving the overall survival. High-temperature MHT (>47 °C) resulted in complete 4T1 primary tumor clearance, 25–40% long-term survival rates, and, importantly, more effective prevention of metastasis comparing to surgical extraction. Thus, the specific MHT temperature must be defined for each tumor individually to ensure a successful antitumor therapy. © 2020 Elsevier Inc.
    view abstractdoi: 10.1016/j.nano.2020.102171
  • 2020 • 176 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 19-24 (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 abstractdoi: 10.1007/978-981-15-3719-6_3
  • 2020 • 175 Topological superconductivity induced by a triple- q magnetic structure
    Bedow, J. and Mascot, E. and Posske, T. and Uhrig, G.S. and Wiesendanger, R. and Rachel, S. and Morr, D.K.
    Physical Review B 102 (2020)
    We demonstrate that the recently discovered triple-q (3q) magnetic structure, when embedded in a magnet-superconductor hybrid (MSH) system, gives rise to the emergence of topological superconductivity. We investigate the structure of chiral Majorana edge modes at domain walls, and show that they can be distinguished from trivial in-gap modes through the spatial distribution of the induced supercurrents. Finally, we show that topological superconductivity in 3q MSH systems is a robust phenomenon that does not depend on the relative alignment of the magnetic and superconducting layers, or on the presence of electronic degrees of freedom in the magnetic layer. © 2020 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.102.180504
  • 2019 • 174 A scanning reflection X-ray microscope for magnetic imaging in the EUV range
    Schümmer, A. and Mertins, H.-C. and Schneider, C.M. and Adam, R. and Trellenkamp, S. and Borowski, R. and Bürgler, D.E. and Juschkin, L. and Berges, U.
    Journal of Synchrotron Radiation 26 2040-2049 (2019)
    The mechanical setup of a novel scanning reflection X-ray microscope is presented. It is based on zone plate optics optimized for reflection mode in the EUV spectral range. The microscope can operate at synchrotron radiation beamlines as well as at laboratory-based plasma light sources. In contrast to established X-ray transmission microscopes that use thin foil samples, the new microscope design presented here allows the investigation of any type of bulk materials. Importantly, this permits the investigation of magnetic materials by employing experimental techniques based on X-ray magnetic circular dichroism, X-ray linear magnetic dichroism or the transversal magneto-optical Kerr effect (T-MOKE). The reliable functionality of the new microscope design has been demonstrated by T-MOKE microscopy spectra of Fe/Cr-wedge/Fe trilayer samples. The spectra were recorded at various photon energies across the Fe 3p edge revealing the orientation of magnetic domains in the sample. © International Union of Crystallography 2019.
    view abstractdoi: 10.1107/S1600577519012219
  • 2019 • 173 Ab initio phase stabilities of Ce-based hard magnetic materials and comparison with experimental phase diagrams
    Sözen, H.Ä. and Ener, S. and MacCari, F. and Skokov, K.P. and Gutfleisch, O. and Körmann, F. and Neugebauer, J. and Hickel, T.
    Physical Review Materials 3 (2019)
    Recent developments in electrical transportation and renewable energies have significantly increased the demand of hard magnetic materials with a reduced critical rare-earth content, but with properties comparable to (Nd,Dy)-Fe-B permanent magnets. Though promising alternative compositions have been identified in high-throughput screenings, the thermodynamic stability of these phases against decomposition into structures with much less favorable magnetic properties is often unclear. In the case of Ce-Fe-Ti alloys, we have used finite temperature ab initio methods to provide this missing information. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, F(T,V), is calculated for the desired hard magnetic CeFe11Ti phase and all relevant competing phases. The latter have been confirmed experimentally by employing reactive crucible melting (RCM) and energy-dispersive x-ray spectroscopy (EDS). Our ab initio based free energy calculations reveal that the presence of the CeFe2 Laves phase suppresses the formation of CeFe11Ti up to 700 K. The result is in agreement with RCM, in which CeFe11Ti is only observed above 1000 K, while the CeFe2 and Ce2Fe17 phases are stable at lower temperatures. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.084407
  • 2019 • 172 Accelerating spin-space sampling by auxiliary spin dynamics and temperature-dependent spin-cluster expansion
    Wang, N. and Hammerschmidt, T. and Rogal, J. and Drautz, R.
    Physical Review B 99 (2019)
    Atomistic simulations of the thermodynamic properties of magnetic materials rely on an accurate modeling of magnetic interactions and an efficient sampling of the high-dimensional spin space. Recent years have seen significant progress with a clear trend from model systems to material-specific simulations that are usually based on electronic-structure methods. Here we develop a Hamiltonian Monte Carlo framework that makes use of auxiliary spin dynamics and an auxiliary effective model, the temperature-dependent spin-cluster expansion, in order to efficiently sample the spin space. Our method does not require a specific form of the model and is suitable for simulations based on electronic-structure methods. We demonstrate fast warm-up and a reasonably small dynamical critical exponent of our sampler for the classical Heisenberg model. We further present an application of our method to the magnetic phase transition in bcc iron using magnetic bond-order potentials. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.094402
  • 2019 • 171 Atomically Layered and Ordered Rare-Earth i-MAX Phases: A New Class of Magnetic Quaternary Compounds
    Tao, Q. and Lu, J. and Dahlqvist, M. and Mockute, A. and Calder, S. and Petruhins, A. and Meshkian, R. and Rivin, O. and Potashnikov, D. and Caspi, E.N. and Shaked, H. and Hoser, A. and Opagiste, C. and Galera, R.-M. and Salikhov,...
    Chemistry of Materials 31 2476-2485 (2019)
    In 2017, we discovered quaternary i-MAX phases - atomically layered solids, where M is an early transition metal, A is an A group element, and X is C - with a (M12/3M21/3)2AC chemistry, where the M1 and M2 atoms are in-plane ordered. Herein, we report the discovery of a class of magnetic i-MAX phases in which bilayers of a quasi-2D magnetic frustrated triangular lattice overlay a Mo honeycomb arrangement and an Al Kagomé lattice. The chemistry of this family is (Mo2/3RE1/3)2AlC, and the rare-earth, RE, elements are Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. The magnetic properties were characterized and found to display a plethora of ground states, resulting from an interplay of competing magnetic interactions in the presence of magnetocrystalline anisotropy. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.8b05298
  • 2019 • 170 Bulk Superconductivity and Role of Fluctuations in the Iron-Based Superconductor FeSe at High Pressures
    Gati, E. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C.
    Physical Review Letters 123 (2019)
    The iron-based superconductor FeSe offers a unique possibility to study the interplay of superconductivity with purely nematic as well magnetic-nematic order by pressure (p) tuning. By measuring specific heat under p up to 2.36 GPa, we study the multiple phases in FeSe using a thermodynamic probe. We conclude that superconductivity is bulk across the entire p range and competes with magnetism. In addition, whenever magnetism is present, fluctuations exist over a wide temperature range above both the bulk superconducting and the magnetic transitions. Whereas the magnetic fluctuations are likely temporal, the superconducting fluctuations may be either temporal or spatial. These observations highlight similarities between FeSe and underdoped cuprate superconductors. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.123.167002
  • 2019 • 169 Effect of the wall thickness on the forming behavior and welding result during magnetic pulse welding [Einfluss der Wandstärke auf das Umformverhalten und das Schweißergebnis beim Magnetpulsschweißen]
    Bellmann, J. and Lueg-Althoff, J. and Schulze, S. and Hahn, M. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    Materialwissenschaft und Werkstofftechnik 50 883-892 (2019)
    Magnetic pulse welding is a promising technology for the joining of dissimilar metals. Since the input of thermal energy is significantly reduced compared to conventional fusion welding technologies, critical intermetallic phases can largely be avoided. Therefore, proper collision conditions are necessary. Those require a careful adjustment of the energetic and geometric parameters at the impact welding setup. The thickness of the accelerated joining partner (flyer) determines the necessary energy input for a successful weld. However, at the same time, it has an effect on the weld formation. This study utilizes a novel optical measurement system to explain these findings and to gain insights into the forming behavior of the flyer parts. It is shown that the collision angle depends on the flyer tube thickness and, thus, directly has an effect on the welding result. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/mawe.201900024
  • 2019 • 168 Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices
    Königshofen, S. and Matthes, F. and Bürgler, D.E. and Schneider, C.M. and Dirksen, E. and Müller, T.J.J.
    Thin Solid Films 680 67-74 (2019)
    The growing field of molecular spintronics is an auspicious route to future concepts of data storage and processing. It has been reported that the hybridization of the electronic structures of non-magnetic organic molecules and ferromagnetic transition-metal (FM) surfaces can form new magnetic units, so-called hybrid molecular magnets, with distinct magnetic properties, which promise molecular spintronic devices with extremely high information density and low energy consumption. The investigation and profound understanding of these device concepts require the formation of clean and epitaxial interfaces between the surface of a FM bottom electrode and molecular thin films. This can only be realized under ultra-high vacuum conditions. In addition, the FM electrodes must be grown on an insulating substrate to electrically separate neighboring devices. Here, we report on procedures to realize an entirely in-situ preparation of mesoscopic test devices featuring structurally and chemically well-defined interfaces. Au(111)-buffered Co(0001) electrodes are deposited by molecular-beam epitaxy onto sapphire or mica substrates using a shadow-mask to define the geometry. The surface quality is subsequently characterized by scanning tunneling microscopy (STM) and other surface science analysis tools. 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), which serves as an exemplary molecule, is sublimed through another shadow-mask, and the interface formation in the monolayer regime is also studied by STM. Finally, we deposit a Cu top electrode through yet another shadow-mask to complete a mesoscopic (200 × 200 μm2) test device, which reveals in ex-situ transport measurements for the Co/BNTCDI/Cu junction non-metallic behavior and a resistance-area product of 24 MΩ·μm2 at 10 K. © 2019
    view abstractdoi: 10.1016/j.tsf.2019.04.021
  • 2019 • 167 Influence of process parameters, surface topography and corrosion condition on the fatigue behavior of steel/aluminum hybrid joints produced by magnetic pulse welding
    Mrzljak, S. and Gelinski, N. and Hülsbusch, D. and Schumacher, E. and Boehm, S. and Walther, F.
    Key Engineering Materials 809 KEM 197-202 (2019)
    In this study, magnetic pulse welded steel/aluminum hybrid joints are investigated with the aim of optimizing the process parameters regarding the fatigue behavior. Changes in discharge current, acceleration distance, welding geometry as well as influences of surface topography and corrosion, are examined regarding fatigue life and damage mechanisms. Instrumented multiple amplitude tests combined with constant amplitude tests are carried out for assessing structure-property-relations in a resource-efficient manner. Stress-induced change in strain and alternating current potential drop measurement are well suited for reliable detection of damage initiation and estimation of the fatigue limit. Results reveal that the fatigue properties primarily depend on the imperfections of the weld seam, which are affected mostly by the discharge current and the surface topography. Corrosion shows to be a relevant factor since it decreases fatigue performance. Suitable process parameters are achieved when the fatigue strength of the weld seam lies above the weaker hybrid joint (aluminum). For S235JR and EN AW-1050A-H14 (Al99.5) a suitable discharge current was found to be 349 kA at an acceleration distance of 1.5 mm. © 2019 Trans Tech Publications Ltd, Switzerland.
    view abstractdoi: 10.4028/
  • 2019 • 166 Magnetic properties of a 17.6 Mn-TRIP steel: Study of strain-induced martensite formation, austenite reversion, and athermal α′-formation
    Souza Filho, I.R. and Sandim, M.J.R. and Cohen, R. and Nagamine, L.C.C.M. and Sandim, H.R.Z. and Raabe, D.
    Journal of Magnetism and Magnetic Materials 473 109-118 (2019)
    Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior ε-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal ε-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for ε-formation (Ms ε). Results showed that ε-martensite is indeed expected to form before α′ which was confirmed in all cases by means of EBSD. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.10.034
  • 2019 • 165 Martensite to austenite reversion in a high-Mn steel: Partitioning-dependent two-stage kinetics revealed by atom probe tomography, in-situ magnetic measurements and simulation
    Souza Filho, I.R. and Kwiatkowski da Silva, A. and Sandim, M.J.R. and Ponge, D. and Gault, B. and Sandim, H.R.Z. and Raabe, D.
    Acta Materialia 166 178-191 (2019)
    Austenite (γ) reversion in a cold-rolled 17.6 wt.% Mn steel was tracked by means of dilatometry and in-situ magnetic measurements during slow continuous annealing. A splitting of the γ-reversion into two stages was observed to be a result of strong elemental partitioning between γ and α′-martensite during the low temperature stage between 390 and 575 °C. Atom probe tomography (APT) results enable the characterization of the Mn-enriched reversed-γ and the Mn-depleted remaining α′-martensite. Because of its lower Mn content, the reversion of the remaining α′-martensite into austenite takes place at a higher temperature range between 600 and 685 °C. APT results agree with partitioning predictions made by thermo-kinetic simulations of the continuous annealing process. The critical composition for γ-nucleation was predicted by thermodynamic calculations (Thermo-Calc) and a good agreement was found with the APT data. Additional thermo-kinetic simulations were conducted to evaluate partitioning-governed γ-growth during isothermal annealing at 500 °C and 600 °C. Si partitioning to γ was predicted by DICTRA and confirmed by APT. Si accumulates near the moving interface during γ-growth and homogenizes over time. We used the chemical composition of the remaining α′-martensite from APT data to calculate its Curie temperature (TCurie) and found good agreement with magnetic measurements. These results indicate that elemental partitioning strongly influences not only γ-reversion but also the TCurie of this steel. The results are important to better understand the thermodynamics and kinetics of austenite reversion for a wide range of Mn containing steels and its effect on magnetic properties. © 2018 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2018.12.046
  • 2019 • 164 OpenMolcas: From Source Code to Insight
    Fdez. Galván, I. and Vacher, M. and Alavi, A. and Angeli, C. and Aquilante, F. and Autschbach, J. and Bao, J.J. and Bokarev, S.I. and Bogdanov, N.A. and Carlson, R.K. and Chibotaru, L.F. and Creutzberg, J. and Dattani, N. and Del...
    Journal of Chemical Theory and Computation 15 5925-5964 (2019)
    In this Article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics, and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the Article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism, and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with postcalculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory, and new electronic and muonic basis sets. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.jctc.9b00532
  • 2019 • 163 Optical orientation of acceptor-bound hole magnetic polarons in bulk (Cd,Mn)Te
    Zhukov, E.A. and Kusrayev, Y.G. and Kirstein, E. and Thomann, A. and Salewski, M. and Kozyrev, N.V. and Yakovlev, D.R. and Bayer, M.
    Physical Review B 99 (2019)
    The optically induced long-lived spin polarization in the bulk diluted magnetic semiconductor (Cd,Mn)Te with small manganese concentration is studied by picosecond pump-probe Kerr rotation. At temperatures below 6 K and in transversal magnetic field the Kerr rotation signal contains three components: two oscillating components, corresponding to the Larmor precession of manganese spins and spins of photoexcited electrons, and a long-lived (up to 15 ns) nonoscillating component. The latter one is provided by optical orientation of equilibrium hole magnetic polarons involving holes bound to acceptors. The origin of the anisotropy controlling the orientation and the spin dynamics of the acceptor-bound hole magnetic polaron in bulk (Cd,Mn)Te is discussed. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.99.115204
  • 2019 • 162 Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO3
    Lömker, P. and Müller, M.
    Physical Review Materials 3 (2019)
    We demonstrate a route to prepare thin films of the ferromagnetic insulator europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO3 as the supplier of oxygen. The process allows tuning the electronic, magnetic, and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis. © 2019 American Physical Society.
    view abstractdoi: 10.1103/PhysRevMaterials.3.061401
  • 2019 • 161 Simulation of magnetised microstructure evolution based on a micromagnetics-inspired FE framework: application to magnetic shape memory behaviour
    Buckmann, K. and Kiefer, B. and Bartel, T. and Menzel, A.
    Archive of Applied Mechanics 89 1085-1102 (2019)
    Microstructure evolution in magnetic materials is typically a non-local effect, in the sense that the behaviour at a material point depends on the magnetostatic energy stored within the demagnetisation field in the entire domain. To account for this, we propose a finite element framework in which the internal state variables parameterising the magnetic and crystallographic microstructure are treated as global fields, optimising a global potential. Contrary to conventional micromagnetics, however, the microscale is not spatially resolved and exchange energy terms are neglected in this approach. The influence of microstructure evolution is rather incorporated in an effective manner, which allows the computation of meso- and macroscale problems. This approach necessitates the development and implementation of novel mixed finite element formulations. It further requires the enforcement of inequality constraints at the global level. To handle the latter, we employ Fischer–Burmeister complementarity functions and introduce the associated Lagrange multipliers as additional nodal degrees-of-freedom. As a particular application of this general methodology, a recently established energy-relaxation-based model for magnetic shape memory behaviour is implemented and tested. Special cases—including ellipsoidal specimen geometries—are used to verify the magnetisation and field-induced strain responses obtained from finite element simulations by comparison to calculations based on the demagnetisation factor concept. © 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstractdoi: 10.1007/s00419-018-1482-7
  • 2019 • 160 Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2018.2856461
  • 2018 • 159 Densification of nanocrystalline NdFeB magnets processed by electro-discharge sintering – Microstructure, magnetic, and mechanical properties
    Leich, L. and Röttger, A. and Theisen, W. and Krengel, M.
    Journal of Magnetism and Magnetic Materials 460 454-460 (2018)
    This work investigates the densification process of nanocrystalline NdFeB powder by electro-discharge sintering (EDS) and the associated magnetic properties. The EDS technique is used as a fast and energy-saving compaction process for metal powders. A large current is discharged from capacitors into a pre-compacted loose powder, thus resulting in complete compaction. In this study, the microstructure, magnetic, and mechanical properties of the compacted, hard magnetic NdFeB specimens were investigated under variation of the energy EEDS and compression load pEDS. For all specimens, the intrinsic coercivity HcJ decreases on increasing the discharge energy. However, the compaction load has apparently no influence on the coercivity HcJ, whereas the residual induction Br decreases only with increasing discharge energy. An increase in the compression load pEDS causes an increase in the specimens’ density and thus promotes residual induction Br. The applied EDS parameters led to the formation of three different microstructures (insufficiently densified zone, fully densified zone, and remelted zone) along the cross-section of the EDS-densified specimens. Volume fractions of the three different microstructures that form during the EDS process determine the resulting mechanical and magnetic properties of the specimens. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2018.04.035
  • 2018 • 158 Enhanced electronic and magnetic properties by functionalization of monolayer GaS via substitutional doping and adsorption
    Ur Rahman, A. and Rahman, G. and Kratzer, P.
    Journal of Physics Condensed Matter 30 (2018)
    The structural, electronic, and magnetic properties of two-dimensional (2D) GaS are investigated using density functional theory (DFT). After confirming that the pristine 2D GaS is a non-magnetic, indirect band gap semiconductor, we consider N and F as substitutional dopants or adsorbed atoms. Except for N substituting for Ga (NGa), all considered cases are found to possess a magnetic moment. Fluorine, both in its atomic and molecular form, undergoes a highly exothermic reaction with GaS. Its site preference (FS or FGa) as substitutional dopant depends on Ga-rich or S-rich conditions. Both for FGa and F adsorption at the Ga site, a strong F-Ga bond is formed, resulting in broken bonds within the GaS monolayer. As a result, FGa induces p-type conductivity in GaS, whereas FS induces a dispersive, partly occupied impurity band about 0.5 e below the conduction band edge of GaS. Substitutional doping with N at both the S and the Ga site is exothermic when using N atoms, whereas only the more favourable site under the prevailing conditions can be accessed by the less reactive N2 molecules. While NGa induces a deep level occupied by one electron at 0.5 eV above the valence band, non-magnetic NS impurities in sufficiently high concentrations modify the band structure such that a direct transition between N-induced states becomes possible. This effect can be exploited to render monolayer GaS a direct-band gap semiconductor for optoelectronic applications. Moreover, functionalization by N or F adsorption on GaS leads to in-gap states with characteristic transition energies that can be used to tune light absorption and emission. These results suggest that GaS is a good candidate for design and construction of 2D optoelectronic and spintronics devices. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aab8b8
  • 2018 • 157 Genetically Controlled Lysosomal Entrapment of Superparamagnetic Ferritin for Multimodal and Multiscale Imaging and Actuation with Low Tissue Attenuation
    Massner, C. and Sigmund, F. and Pettinger, S. and Seeger, M. and Hartmann, C. and Ivleva, N.P. and Niessner, R. and Fuchs, H. and de Angelis, M.H. and Stelzl, A. and Koonakampully, N.L. and Rolbieski, H. and Wiedwald, U. and Spaso...
    Advanced Functional Materials 28 (2018)
    Nanomaterials are of enormous value for biomedical applications because of their customizable features. However, the material properties of nanomaterials can be altered substantially by interactions with tissue thus making it important to assess them in the specific biological context to understand and tailor their effects. Here, a genetically controlled system is optimized for cellular uptake of superparamagnetic ferritin and subsequent trafficking to lysosomes. High local concentrations of photoabsorbing magnetoferritin give robust contrast in optoacoustic imaging and allow for selective photoablation of cells overexpressing ferritin receptors. Genetically controlled uptake of the biomagnetic nanoparticles also strongly enhances third-harmonic generation due to the change of refractive index caused by the magnetite–protein interface of ferritins entrapped in lysosomes. Selective uptake of magnetoferritin furthermore enables sensitive detection of receptor-expressing cells by magnetic resonance imaging, as well as efficient magnetic cell sorting and manipulation. Surprisingly, a substantial increase in the blocking temperature of lysosomally entrapped magnetoferritin is observed, which allows for specific ablation of genetically defined cell populations by local magnetic hyperthermia. The subcellular confinement of superparamagnetic ferritins thus enhances their physical properties to empower genetically controlled interrogation of cellular processes with deep tissue penetration. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201706793
  • 2018 • 156 Heisenberg model analysis on inelastic powder neutron scattering data using parent and K doped BaMn2As2 samples
    Ramazanoglu, M. and Sapkota, A. and Pandey, A. and Lamsal, J. and Abernathy, D.L. and Niedziela, J.L. and Stone, M.B. and Salci, R. and Acar, D.A. and Oztirpan, F.O. and Ozonder, S. and Kreyssig, A. and Goldman, A.I. and Johnston,...
    Physica B: Condensed Matter 551 51-59 (2018)
    Low temperature powder inelastic neutron scattering measurements were performed on three different powder samples; parent BaMn2As2,12.5% K-doped Ba0.875K0.125Mn2As2 and 25% K-doped Ba(0.75)K0.25Mn2As2. The Heisenberg Model involving J1‐J2‐Jz coupling constants were compared to the data by a powder integration routine using Monte Carlo integration methods. The best magnetic parameters were selected using a chi-square test where model intensities were compared to the full (q,E) dependence of magnetic scattering. A key step to this analysis is the characterization of the background which is formed mostly by phonon scattering intensities along with other sources including the magnetic impurity scattering events. The calculated powder magnetic intensities added to the estimated background obtained from the non-magnetic high momentum transfer region. The agreement between the simulated and the raw data enabled us to perform quantitative analysis of the unreacted MnO impurities. Overall, this is another confirmation along with earlier studies using this technique, that magnetic exchange constants can be calculated within an acceptable range with a very quick inelastic neutron powder experiment without need for a single crystal sample. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.physb.2017.11.003
  • 2018 • 155 Magnetic properties and structural characterization of layered (Cr 0.5 Mn 0.5 ) 2 AuC synthesized by thermally induced substitutional reaction in (Cr 0.5 Mn 0.5 ) 2 GaC
    Lai, C.-C. and Tao, Q. and Fashandi, H. and Wiedwald, U. and Salikhov, R. and Farle, M. and Petruhins, A. and Lu, J. and Hultman, L. and Eklund, P. and Rosen, J.
    APL Materials 6 (2018)
    The magnetic properties of the new phase (Cr 0.5 Mn 0.5 ) 2 AuC are compared to the known MAX-phase (Cr 0.5 Mn 0.5 ) 2 GaC, where the former was synthesized by thermally induced substitution reaction of Au for Ga in (Cr 0.5 Mn 0.5 ) 2 GaC. The reaction introduced a lattice expansion of ~3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr 0.5 Mn 0.5 ) 2 AuC displays similar features in the magnetic field- and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr 0.5 Mn 0.5 ) 2 GaC and (Mo 0.5 Mn 0.5 ) 2 GaC. The work suggests a pathway for tuning the magnetic properties of MAX phases. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5006304
  • 2018 • 154 Magnetic properties and structural characterization of layered (Cr0.5Mn0.5)2AuC synthesized by thermally induced substitutional reaction in (Cr0.5Mn0.5)2GaC
    Lai, C.-C. and Tao, Q. and Fashandi, H. and Wiedwald, U. and Salikhov, R. and Farle, M. and Petruhins, A. and Lu, J. and Hultman, L. and Eklund, P. and Rosen, J.
    APL Materials 6 (2018)
    The magnetic properties of the new phase (Cr0.5Mn0.5)2AuC are compared to the known MAX-phase (Cr0.5Mn0.5)2GaC, where the former was synthesized by thermally induced substitution reaction of Au for Ga in (Cr0.5Mn0.5)2GaC. The reaction introduced a lattice expansion of ~3% along the c-axis, an enhancement of the coercive field from 30 mT to 140 mT, and a reduction of the Curie temperature and the saturation magnetization. Still, (Cr0.5Mn0.5)2AuC displays similar features in the magnetic field- and temperature-dependent magnetization curves as previously reported magnetic MAX phases, e.g., (Cr0.5Mn0.5)2GaC and (Mo0.5Mn0.5)2GaC. The work suggests a pathway for tuning the magnetic properties of MAX phases. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5006304
  • 2018 • 153 Nematicity, magnetism and superconductivity in FeSe
    Böhmer, A.E. and Kreisel, A.
    Journal of Physics Condensed Matter 30 (2018)
    Iron-based superconductors are well known for their complex interplay between structure, magnetism and superconductivity. FeSe offers a particularly fascinating example. This material has been intensely discussed because of its extended nematic phase, whose relationship with magnetism is not obvious. Superconductivity in FeSe is highly tunable, with the superconducting transition temperature, T c, ranging from 8 K in bulk single crystals at ambient pressure to almost 40 K under pressure or in intercalated systems, and to even higher temperatures in thin films. In this topical review, we present an overview of nematicity, magnetism and superconductivity, and discuss the interplay of these phases in FeSe. We focus on bulk FeSe and the effects of physical pressure and chemical substitutions as tuning parameters. The experimental results are discussed in the context of the well-studied iron-pnictide superconductors and interpretations from theoretical approaches are presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa9caa
  • 2018 • 152 Nonlocal electron correlations in an itinerant ferromagnet
    Tusche, C. and Ellguth, M. and Feyer, V. and Krasyuk, A. and Wiemann, C. and Henk, J. and Schneider, C.M. and Kirschner, J.
    Nature Communications 9 (2018)
    Our understanding of the properties of ferromagnetic materials, widely used in spintronic devices, is fundamentally based on their electronic band structure. However, even for the most simple elemental ferromagnets, electron correlations are prevalent, requiring descriptions of their electronic structure beyond the simple picture of independent quasi-particles. Here, we give evidence that in itinerant ferromagnets like cobalt these electron correlations are of nonlocal origin, manifested in a complex self-energy Σσ(E,k) that disperses as function of spin σ, energy E, and momentum vector k. Together with one-step photoemission calculations, our experiments allow us to quantify the dispersive behaviour of the complex self-energy over the whole Brillouin zone. At the same time we observe regions of anomalously large “waterfall”-like band renormalization, previously only attributed to strong electron correlations in high-TC superconductors, making itinerant ferromagnets a paradigmatic test case for the interplay between band structure, magnetism, and many-body correlations. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-05960-5
  • 2018 • 151 Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO3/SrTiO3 heterostructures
    Veit, M.J. and Arras, R. and Ramshaw, B.J. and Pentcheva, R. and Suzuki, Y.
    Nature Communications 9 (2018)
    The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields. However, there is another promising strategy to achieve spin splitting using spatial symmetry breaking without the use of a magnetic field, known as Rashba-type splitting. Here we report evidence for a giant Rashba-type splitting at the interface of LaTiO3 and SrTiO3. Analysis of the magnetotransport reveals anisotropic magnetoresistance, weak anti-localization and quantum oscillation behavior consistent with a large Rashba-type splitting. It is surprising to find a large Rashba-type splitting in 3d transition metal oxide-based systems such as the LaTiO3/SrTiO3 interface, but it is promising for the development of a new kind of oxide-based spintronics. © 2018 The Author(s).
    view abstractdoi: 10.1038/s41467-018-04014-0
  • 2018 • 150 Origin of the low magnetic moment in Fe2AlTi: An Ab initio study
    Friák, M. and Slávik, A. and Miháliková, I. and Holec, D. and Všianská, M. and Šob, M. and Palm, M. and Neugebauer, J.
    Materials 11 (2018)
    The intermetallic compound Fe2AlTi (alternatively Fe2TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally measured ones. To unravel the source of this discrepancy, we analyze how various mechanisms present in realistic materials such as residual strain effects or deviations from stoichiometry affect magnetism. Since in spin-unconstrained calculations the system always evolves to the spin configuration which represents a local or global minimum in the total energy surface, finite temperature spin effects are not well described. We therefore turn the investigation around and use constrained spin calculations, fixing the global magnetic moment. This approach provides direct insight into local and global energy minima (reflecting metastable and stable spin phases) as well as the curvature of the energy surface, which correlates with the magnetic entropy and thus the magnetic configuration space accessible at finite temperatures. Based on this approach, we show that deviations from stoichiometry have a huge impact on the local magnetic moment and can explain the experimentally observed low magnetic moments. © 2018 by the authors.
    view abstractdoi: 10.3390/ma11091732
  • 2018 • 149 Parameter identification for magnetic pulse welding applications
    Bellmann, J. and Joern, L.-A. and Schulze, S. and Gies, S. and Beyer, E. and Tekkaya, A.E.
    Key Engineering Materials 767 KEM 431-438 (2018)
    Magnetic pulse welding (MPW) is a promising technology to join dissimilar metals and to produce multi-material structures, e.g. to fulfill lightweight requirements. During this impact welding process, proper collision conditions between both joining partners are essential for a sound weld formation. Controlling these conditions is difficult due to a huge number of influencing and interacting factors. Many of them are related to the pulse welding setup and the material properties of the moving part, the so-called flyer. In this paper, a new measurement system is applied that takes advantage of the high velocity impact flash. The flash is a side effect of the MPW process and its intensity depends on the impact velocity of the flyer. Thus, the intensity level can be used as a welding criterion. A procedure is described that enables the user to realize a fast parameter development with only a few experiments. The minimum energy level and the optimum distance between the parts to be joined can be identified. This is of importance since a low energy input decreases the thermal and mechanical shock loading on the tool coil and thus increases its lifetime. In a second step, the axial position of the flyer in the tool coil is adjusted to ensure a proper collision angle and a circumferential weld seam. © 2018 Trans Tech Publications, Switzerland.
    view abstractdoi: 10.4028/
  • 2018 • 148 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...
    Nanoscale 10 277-283 (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 abstractdoi: 10.1039/c7nr06610d
  • 2018 • 147 Production, deformation and mechanical investigation of magnetic alginate capsules
    Zwar, E. and Kemna, A. and Richter, L. and Degen, P. and Rehage, H.
    Journal of Physics Condensed Matter 30 (2018)
    In this article we investigated the deformation of alginate capsules in magnetic fields. The sensitivity to magnetic forces was realised by encapsulating an oil in water emulsion, where the oil droplets contained dispersed magnetic nanoparticles. We solved calcium ions in the aqueous emulsion phase, which act as crosslinking compounds for forming thin layers of alginate membranes. This encapsulating technique allows the production of flexible capsules with an emulsion as the capsule core. It is important to mention that the magnetic nanoparticles were stable and dispersed throughout the complete process, which is an important difference to most magnetic alginate-based materials. In a series of experiments, we used spinning drop techniques, capsule squeezing experiments and interfacial shear rheology in order to determine the surface Young moduli, the surface Poisson ratios and the surface shear moduli of the magnetically sensitive alginate capsules. In additional experiments, we analysed the capsule deformation in magnetic fields. In spinning drop and capsule squeezing experiments, water droplets were pressed out of the capsules at elevated values of the mechanical load. This phenomenon might be used for the mechanically triggered release of water-soluble ingredients. After drying the emulsion-filled capsules, we produced capsules, which only contained a homogeneous oil phase with stable suspended magnetic nanoparticles (organic ferrofluid). In the dried state, the thin alginate membranes of these particles were rather rigid. These dehydrated capsules could be stored at ambient conditions for several months without changing their properties. After exposure to water, the alginate membranes rehydrated and became flexible and deformable again. During this swelling process, water diffused back in the capsule. This long-term stability and rehydration offers a great spectrum of different applications as sensors, soft actuators, artificial muscles or drug delivery systems. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aaa6f5
  • 2018 • 146 Sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets
    Muckel, F. and Delikanli, S. and Hernández-Martínez, P.L. and Priesner, T. and Lorenz, S. and Ackermann, J. and Sharma, M. and Demir, H.V. and Bacher, G.
    Nano Letters 18 2047-2053 (2018)
    In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.8b00060
  • 2017 • 145 Combinatorial study of Fe-Co-V hard magnetic thin films
    Fackler, S.W. and Alexandrakis, V. and König, D. and Kusne, A.G. and Gao, T. and Kramer, M.J. and Stasak, D. and Lopez, K. and Zayac, B. and Mehta, A. and Ludwig, Al. and Takeuchi, I.
    Science and Technology of Advanced Materials 18 231-238 (2017)
    Thin film libraries of Fe-Co-V were fabricated by combinatorial sputtering to study magnetic and structural properties over wide ranges of composition and thickness by high-throughput methods: synchrotron X-ray diffraction, magnetometry, composition, and thickness were measured across the Fe-Co-V libraries. In-plane magnetic hysteresis loops were shown to have a coercive field of 23.9 kA m–1 (300 G) and magnetization of 1000 kA m–1. The out-of-plane direction revealed enhanced coercive fields of 207 kA m–1 (2.6 kG) which was attributed to the shape anisotropy of column grains observed with electron microscopy. Angular dependence of the switching field showed that the magnetization reversal mechanism is governed by 180° domain wall pinning. In the thickness-dependent combinatorial study, co-sputtered composition spreads had a thickness ranging from 50 to 500 nm and (Fe70Co30)100-xVx compositions of x = 2–80. Comparison of high-throughput magneto-optical Kerr effect and traditional vibrating sample magnetometer measurements show agreement of trends in coercive fields across large composition and thickness regions. © 2017 The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis.
    view abstractdoi: 10.1080/14686996.2017.1287520
  • 2017 • 144 Communication: Heterogeneous water dynamics on a clathrate hydrate lattice detected by multidimensional oxygen nuclear magnetic resonance
    Adjei-Acheamfour, M. and Storek, M. and Böhmer, R.
    Journal of Chemical Physics 146 (2017)
    Previous deuteron nuclear magnetic resonance studies revealed conflicting evidence regarding the possible motional heterogeneity of the water dynamics on the hydrate lattice of an ice-like crystal. Using oxygen-17 nuclei as a sensitive quadrupolar probe, the reorientational two-time correlation function displays a clear nonexponentiality. To check whether this dispersive behavior is a consequence of dynamic heterogeneity or rather of an intrinsic nonexponentiality, a multidimensional, four-time magnetic resonance experiment was devised that is generally applicable to strongly quadrupolarly perturbed half-integer nuclei such as oxygen-17. Measurements of an appropriate four-time function demonstrate that it is possible to select a subensemble of slow water molecules. Its mean time scale is compared to theoretical predictions and evidence for significant motional heterogeneity is found. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4983043
  • 2017 • 143 Current-Induced Magnetic Polarons in a Colloidal Quantum-Dot Device
    Muckel, F. and Barrows, C.J. and Graf, A. and Schmitz, A. and Erickson, C.S. and Gamelin, D.R. and Bacher, G.
    Nano Letters 17 4768-4773 (2017)
    Electrical spin manipulation remains a central challenge for the realization of diverse spin-based information processing technologies. Motivated by the demonstration of confinement-enhanced sp-d exchange interactions in colloidal diluted magnetic semiconductor (DMS) quantum dots (QDs), such materials are considered promising candidates for future spintronic or spin-photonic applications. Despite intense research into DMS QDs, electrical control of their magnetic and magneto-optical properties remains a daunting goal. Here, we report the first demonstration of electrically induced magnetic polaron formation in any DMS, achieved by embedding Mn2+-doped CdSe/CdS core/shell QDs as the active layer in an electrical light-emitting device. Tracing the electroluminescence from cryogenic to room temperatures reveals an anomalous energy shift that reflects current-induced magnetization of the Mn2+ spin sublattice, that is, excitonic magnetic polaron formation. These electrically induced magnetic polarons exhibit an energy gain comparable to their optically excited counterparts, demonstrating that magnetic polaron formation is achievable by current injection in a solid-state device. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b01496
  • 2017 • 142 Dichotomy between in-plane magnetic susceptibility and resistivity anisotropies in extremely strained BaFe2As2
    He, M. and Wang, L. and Ahn, F. and Hardy, F. and Wolf, T. and Adelmann, P. and Schmalian, J. and Eremin, I. and Meingast, C.
    Nature Communications 8 (2017)
    High-temperature superconductivity in the Fe-based materials emerges when the antiferromagnetism of the parent compounds is suppressed by either doping or pressure. Closely connected to the antiferromagnetic state are entangled orbital, lattice, and nematic degrees of freedom, and one of the major goals in this field has been to determine the hierarchy of these interactions. Here we present the direct measurements and the calculations of the in-plane uniform magnetic susceptibility anisotropy of BaFe2As2, which help in determining the above hierarchy. The magnetization measurements are made possible by utilizing a simple method for applying a large symmetry-breaking strain, based on differential thermal expansion. In strong contrast to the large resistivity anisotropy above the antiferromagnetic transition at T N, the anisotropy of the in-plane magnetic susceptibility develops largely below T N. Our results imply that lattice and orbital degrees of freedom play a subdominant role in these materials. © 2017 The Author(s).
    view abstractdoi: 10.1038/s41467-017-00712-3
  • 2017 • 141 Doubling of the magnetic energy product in ferromagnetic nanowires at ambient temperature by capping their tips with an antiferromagnet
    Wang, F.Z. and Salikhov, R. and Spasova, M. and Liébana-Viñas, S. and Bran, C. and Chen, Y.-S. and Vazquez, M. and Farle, M. and Wiedwald, U.
    Nanotechnology 28 (2017)
    We present an approach to prepare free-standing tips of micrometer-long nanowires electrodeposited in anodic aluminum oxide nanopores. Such open tips can be further processed, e.g. for vertical interconnects of functional layers or for tailoring the magnetization reversal of ferromagnetic nanowires. The magnetic switching of nanowires is usually initiated by vortex or domain formation at the nanowire tips. We show that coating the tips of Fe30Co70 nanowires (diameter 40 nm, length 16 μm) with thin antiferromagnetic Fe50Mn50 capping layers (thickness ≈10 nm) leads to magnetic hardening with a more than doubled energy product at ambient temperature. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa77b7
  • 2017 • 140 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 abstractdoi: 10.1088/1361-6463/aa8e7c
  • 2017 • 139 Enhanced spin-orbit coupling in tetragonally strained Fe-Co-B films
    Salikhov, R. and Reichel, L. and Zingsem, B. and Abrudan, R. and Edström, A. and Thonig, D. and Rusz, J. and Eriksson, O. and Schultz, L. and Fähler, S. and Farle, M. and Wiedwald, U.
    Journal of Physics Condensed Matter 29 (2017)
    Tetragonally strained interstitial Fe-Co-B alloys were synthesized as epitaxial films grown on a 20 nm thick Au0.55Cu0.45 buffer layer. Different ratios of the perpendicular to in-plane lattice constant c/a = 1.013, 1.034 and 1.02 were stabilized by adding interstitial boron with different concentrations 0, 4, and 10 at.%, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the total orbital magnetic moment significantly increases with increasing c/a ratio, indicating that reduced crystal symmetry and interstitial B leads to a noticeable enhancement of the effect of spin-orbit coupling (SOC) in the Fe-Co-B alloys. First-principles calculations reveal that the increase in orbital magnetic moment mainly originates from B impurities in octahedral position and the reduced symmetry around B atoms. These findings offer the possibility to enhance SOC phenomena - namely the magnetocrystalline anisotropy and the orbital moment - by stabilizing anisotropic strain by doping 4 at.% B. Results on the influence of B doping on the Fe-Co film microstructure, their coercive field and magnetic relaxation are also presented. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-648X/aa7498
  • 2017 • 138 Improving B1 Efficiency and Signal-to-Noise-Ratio of a Surface Coil by a High-Impedance-Surface RF Shield for 7-T Magnetic Resonance Imaging
    Chen, Z. and Solbach, K. and Erni, D. and Rennings, A.
    IEEE Transactions on Microwave Theory and Techniques 65 988-997 (2017)
    In this paper, we present a fundamental investigation to improve the B₁ efficiency and the signal-to-noise ratio (SNR) of a radio frequency (RF) surface coil for ultrahigh-field magnetic resonance imaging (MRI) by utilizing a high-impedance surface (HIS) as the RF shield. An analytical investigation indicates that a circular loop backed by a perfect magnetic conductor (PMC), which is the ideal case of an HIS, suggests an improved magnetic field compared with the case of a perfect electric conductor (PEC) and the case without any shield. This improvement is verified by a full-wave simulation, where the surface coil is modeled by an ideal impressed current model with azimuthal component (Jsurf,α = 1 A/m). The electromagnetic field is effectively shielded out behind the PEC and PMC shields compared with the case without any shield. Furthermore, the surface coil with uniform current distribution and the PMC shield is realized by a series resonant loop structure and a 2-D HIS structure, respectively. Since the normal component of the magnetic field is supported at the surface of an HIS, whereas suppressed by a conventional PEC, the B₁ field in the vicinity of the HIS shield is enhanced compared with the case with a PEC shield. Hence, an improvement on SNR and B₁ efficiency is achieved by utilizing an HIS shield, especially in the regions adjacent to the surface coil. It has been found that the improvement of B₁ efficiency is more prominent than the improvement of SNR due to different normalizations. The difference of peak SAR between considered shields, which is used for B₁ efficiency normalization, is considerably larger than the difference of the power loss within the phantom, which is used for the SNR normalization. The proposed approach is validated by full-wave finite-element method simulations and near-field measurements, which reveal good agreement with each other.
    view abstractdoi: 10.1109/TMTT.2016.2631169
  • 2017 • 137 Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films with High Anisotropy Grown from an Alloy Mn55Bi45 Target
    Sabet, S. and Hildebrandt, E. and Römer, F.M. and Radulov, I. and Zhang, H. and Farle, M. and Alff, L.
    IEEE Transactions on Magnetics 53 (2017)
    Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 eμcm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of \sim 1.86 {\cdot 10{7}} erg/cm3 (1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2016.2636817
  • 2017 • 136 Magnetic fluctuations and superconducting properties of CaKFe4As4 studied by As 75 NMR
    Cui, J. and Ding, Q.-P. and Meier, W.R. and Böhmer, A.E. and Kong, T. and Borisov, V. and Lee, Y. and Bud'Ko, S.L. and Valentí, R. and Canfield, P.C. and Furukawa, Y.
    Physical Review B 96 (2017)
    We report As75 nuclear magnetic resonance (NMR) studies on a new iron-based superconductor, CaKFe4As4, with Tc=35 K. As75 NMR spectra show two distinct lines corresponding to the As(1) and As(2) sites close to the K and Ca layers, respectively, revealing that K and Ca layers are well ordered without site inversions. We found that nuclear quadrupole frequencies νQ of the As(1) and As(2) sites show an opposite temperature T dependence. Nearly T independent behavior of the Knight shifts K is observed in the normal state, and a sudden decrease in K in the superconducting (SC) state suggests spin-singlet Cooper pairs. As75 spin-lattice relaxation rates 1/T1 show a power-law T dependence with different exponents for the two As sites. The isotropic antiferromagnetic spin fluctuations characterized by the wave vector q=(π,0) or (0,π) in the single-iron Brillouin zone notation are revealed by 1/T1T and K measurements. Such magnetic fluctuations are necessary to explain the observed temperature dependence of the As75 quadrupole frequencies, as evidenced by our first-principles calculations. In the SC state, 1/T1 shows a rapid decrease below Tc without a Hebel-Slichter peak and decreases exponentially at low T, consistent with an s± nodeless two-gap superconductor. © 2017 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.96.104512
  • 2017 • 135 Magnetic properties of nanolaminated (Mo0.5Mn0.5)2GaC MAX phase
    Salikhov, R. and Meshkian, R. and Weller, D. and Zingsem, B. and Spoddig, D. and Lu, J. and Ingason, A.S. and Zhang, H. and Rosen, J. and Wiedwald, U. and Farle, M.
    Journal of Applied Physics 121 (2017)
    The magnetic properties of hexagonal (Mo0.5Mn0.5)2GaC MAX phase synthesized as epitaxial films on MgO (111) substrates with the c-axis perpendicular to the film plane are presented. The analysis of temperature-dependent ferromagnetic resonance (FMR) and magnetometry data reveals a ferro- to paramagnetic phase transition at 220 K. The electrical transport measurements at 5 K show a negative magnetoresistance of 6% in a magnetic field of 9 T. Further analysis confirms the spin-dependent scattering of charge carriers in this layered material. A small perpendicular (c-axis) magnetocrystalline anisotropy energy density (MAE) of 4.5 kJ/m3 at 100 K was found using FMR. Accordingly, (Mo0.5Mn0.5)2GaC behaves similar to the (Cr0.5Mn0.5)2GaC MAX phase as a soft magnetic material. The density functional theory calculations reveal that the sign and the amplitude of the MAE can be very sensitive to (Mo0.5Mn0.5)2GaC lattice parameters, which may explain the measured soft magnetic properties. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4982197
  • 2017 • 134 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 Pushkare...
    Acta Physica Polonica A 131 1069-1071 (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 abstractdoi: 10.12693/APhysPolA.131.1069
  • 2017 • 133 Magnetic Skyrmion Formation at Lattice Defects and Grain Boundaries Studied by Quantitative Off-Axis Electron Holography
    Li, Z.-A. and Zheng, F. and Tavabi, A.H. and Caron, J. and Jin, C. and Du, H. and Kovács, A. and Tian, M. and Farle, M. and Dunin-Borkowski, R.E.
    Nano Letters 17 1395-1401 (2017)
    We use in situ Lorentz microscopy and off-axis electron holography to investigate the formation and characteristics of skyrmion lattice defects and their relationship to the underlying crystallographic structure of a B20 FeGe thin film. We obtain experimental measurements of spin configurations at grain boundaries, which reveal inversions of crystallographic and magnetic chirality across adjacent grains, resulting in the formation of interface spin stripes at the grain boundaries. In the absence of material defects, we observe that skyrmions lattices possess dislocations and domain boundaries, in analogy to atomic crystals. Moreover, the distorted skyrmions can flexibly change their size and shape to accommodate local geometry, especially at sites of dislocations in the skyrmion lattice. Our findings provide a detailed understanding of the elasticity of topologically protected skyrmions and their correlation with underlying material defects. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b04280
  • 2017 • 132 Magnetic subunits within a single molecule-surface hybrid
    Heß, V. and Friedrich, R. and Matthes, F. and Caciuc, V. and Atodiresei, N. and Bürgler, D.E. and Blügel, S. and Schneider, C.M.
    New Journal of Physics 19 (2017)
    Magnetic molecule-surface hybrids are ideal building blocks for molecular spintronic devices due to their appealing tailorable magnetic properties and nanoscale size. So far, assemblies of interacting molecular-surface hybrids needed for spintronic functionality were generated by depositing aromatic molecules onto transition-metal surfaces, resulting in a random arrangement of hybrid magnets due to the inherent and strong hybridization. Here, we demonstrate the formation of multiple intramolecular subunits within a single molecule-surface hybrid by means of spin-polarized scanning tunneling microscopy experiments and ab initio density functional theory calculations. This novel effect is realized by depositing a polycyclic aromatic molecule on a magnetic surface. A highly asymmetric chiral adsorption position induces different structural, electronic, and magnetic properties in each aromatic ring of the molecule. In particular, the induced molecular spin polarization near the Fermi energy varies among the rings due to site- and spin-dependent molecule-surface hybridization. Our results showcase a possible organic chemistry route of tailoring geometrically well-defined assemblies of magnetically distinguishable subunits in molecule-surface hybrids. © 2017 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/aa6ece
  • 2017 • 131 Magnon accumulation by clocked laser excitation as source of long-range spin waves in transparent magnetic films
    Jäckl, M. and Belotelov, V.I. and Akimov, I.A. and Savochkin, I.V. and Yakovlev, D.R. and Zvezdin, A.K. and Bayer, M.
    Physical Review X 7 (2017)
    Optical tools are promising for spin-wave generation because of the possibilities of ultrafast manipulation and local excitation. However, a single laser pulse can inject spin waves (SWs) only with a broad frequency spectrum, resulting in short propagation distances and low wave amplitudes. Here, we excite a magnetic garnet film by a train of fs-laser pulses with a 1-GHz repetition rate so that the pulse separation is shorter than the decay time of magnetic modes, which allows us to achieve a collective impact on the magnetization and establish a quasistationary source of spin waves, namely, a coherent accumulation of magnons ("magnon cloud"). This approach has several appealing features: (i) The magnon source is tunable, (ii) the SW amplitude can be significantly enhanced, (iii) the SW spectrum is quite narrow, providing long-distance propagation, (iv) the periodic pumping results in an almost constant-in-time SWamplitude for the distances larger than 20 μm away from the source, and (v) the SW emission shows pronounced directionality. These results expand the capabilities of ultrafast coherent optical control of magnetization and pave the way for applications in data processing, including the quantum regime. The quasistationary magnon accumulation might also be of interest for applications in magnon Bose-Einstein condensates.
    view abstractdoi: 10.1103/PhysRevX.7.021009
  • 2017 • 130 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 193 364-370 (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 abstractdoi: 10.1016/j.matchemphys.2017.02.046
  • 2017 • 129 Modifications of aluminum film caused by micro-plasmoids and plasma spots in the effluent of an argon non-equilibrium plasma jet
    Engelhardt, M. and Ries, S. and Hermanns, P. and Bibinov, N. and Awakowicz, P.
    Journal of Physics D: Applied Physics 50 (2017)
    A smooth layer of hard aluminium film is deposited onto a glass substrate with a multi-frequency CCP discharge and then treated in the effluent of a non-equilibrium atmospheric pressure plasma jet (N-APPJ) operated with Ar flow. A thin filament is formed in the argon N-APPJ through contraction of a diffuse feather-like discharge. The aluminium surface treated in the effluents of the N-APPJ is significantly modified. Erosion tracks of different forms and micro-balls composed of aluminium are observed on the treated surface. Based on CCD images of active plasma discharge channels, SEM images of the treated surface and current-voltage characteristics, these surface modifications are interpreted as traces of plasma spots and plasmoids. Plasma spots are focused plasma channels, which are characterized by an intense emission in CCD images at the contact point of a plasma channel with the treated metal surface and by deep short tracks on the aluminium surface, observed in SEM images. Plasmoids are plasma objects without contact to any power supply which can produce long, thin and shallow traces, as can be observed on the treated surface using electron microscopy. Based on observed traces and numerous transformations of plasma spots to plasmoids and vice versa, it is supposed that both types of plasma objects are formed by an extremely high axial magnetic field and differ from each other due to the existence or absence of contact to a power supply and the consequential transport of electric current. The reason for the magnetic field at the axis of these plasma objects is possibly a circular current of electron pairs in vortices, which are formed in plasma by the interaction of ionization waves with the substrate surface. The extremely high magnetic field of plasma spots and plasmoids leads to a local destruction of the metal film and top layer of the glass substrate and to an attraction of paramagnetic materials, namely aluminium and oxygen. The magnetic attraction of aluminium is a reason for the extraction of some pieces of metal and the formation of erosion tracks and holes in the metal film. In the absence of metal atomization, the extracted aluminium forms spherical micro-particles, which are distributed over the surface of the treated metal film by the gas flow. A thin (100 nm) gold (diamagnetic) layer on top of the aluminium film surface reduces the erosion rate of plasma spots and plasmoids drastically (more than three orders of magnitude). © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa802f
  • 2017 • 128 Nanoparticle atoms pinpointed
    Farle, M.
    Nature 542 35-36 (2017)
    doi: 10.1038/542035a
  • 2017 • 127 Nanoscale x-ray investigation of magnetic metallofullerene peapods
    Fritz, F. and Westerström, R. and Kostanyan, A. and Schlesier, C. and Dreiser, J. and Watts, B. and Houben, L. and Luysberg, M. and Avdoshenko, S.M. and Popov, A.A. and Schneider, C.M. and Meyer, C.
    Nanotechnology 28 (2017)
    Endohedral lanthanide ions packed inside carbon nanotubes (CNTs) in a one-dimensional assembly have been studied with a combination of high resolution transmission electron microscopy (HRTEM), scanning transmission x-ray microscopy (STXM), and x-ray magnetic circular dichroism (XMCD). By correlating HRTEM and STXM images we show that structures down to 30 nm are resolved with chemical contrast and record x-ray absorption spectra from endohedral lanthanide ions embedded in individual nanoscale CNT bundles. XMCD measurements of an Er3N@C80 bulk sample and a macroscopic assembly of filled CNTs indicate that the magnetic properties of the endohedral Er3+ ions are unchanged when encapsulated in CNTs. This study demonstrates the feasibility of local magnetic x-ray characterisation of low concentrations of lanthanide ions embedded in molecular nanostructures. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa8b4c
  • 2017 • 126 Negatively Charged and Dark Excitons in CsPbBr3 Perovskite Nanocrystals Revealed by High Magnetic Fields
    Canneson, D. and Shornikova, E.V. and Yakovlev, D.R. and Rogge, T. and Mitioglu, A.A. and Ballottin, M.V. and Christianen, P.C.M. and Lhuillier, E. and Bayer, M. and Biadala, L.
    Nano Letters 17 6177-6183 (2017)
    The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = -0.22. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b02827
  • 2017 • 125 Room-temperature electron spin dynamics of Ce3+ ions in a YAG crystal
    Liang, P. and Hu, R.R. and Chen, C. and Belykh, V.V. and Jia, T.Q. and Sun, Z.R. and Feng, D.H. and Yakovlev, D.R. and Bayer, M.
    Applied Physics Letters 110 (2017)
    Circularly polarized optical excitation generates electron spin polarization in the lowest 5d state of rare-earth Ce3+ ions in a YAG crystal. The 5d electron spin dynamics is investigated in transverse and longitudinal magnetic fields by time-resolved pump-probe Faraday rotation. Long lived electron spin coherence with a dephasing time of 2.5 ns is found at room temperature. In a transverse magnetic field of 1 T, the electron spin coherence shows a distinct beating-like amplitude modulation due to several slightly different Larmor frequencies corresponding to different electron g factors of magnetically inequivalent positions of the Ce3+ ions in the crystal lattice. Hyperfine coupling between the 5d electron of Ce3+ ions and environmental nuclear spins dominates the spin relaxation, which can be efficiently suppressed by a longitudinal magnetic field as small as 10 mT. The dependence of electron spin relaxation on both the transverse and longitudinal magnetic fields agrees well with the one predicted theoretically for the hyperfine coupling mechanism. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4984232
  • 2017 • 124 Scanning tunnelling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets
    Gastiasoro, M.N. and Eremin, I. and Fernandes, R.M. and Andersen, B.M.
    Nature Communications 8 (2017)
    The combination of electronic correlations and Fermi surfaces with multiple nesting vectors can lead to the appearance of complex multi-Q magnetic ground states, hosting unusual states such as chiral density waves and quantum Hall insulators. Distinguishing single-Q and multi-Q magnetic phases is however a notoriously difficult experimental problem. Here we propose theoretically that the local density of states (LDOS) near a magnetic impurity, whose orientation may be controlled by an external magnetic field, can be used to map out the detailed magnetic configuration of an itinerant system and distinguish unambiguously between single-Q and multi-Q phases. We demonstrate this concept by computing and contrasting the LDOS near a magnetic impurity embedded in three different magnetic ground states relevant to iron-based superconductors - one single-Q and two double-Q phases. Our results open a promising avenue to investigate the complex magnetic configurations in itinerant systems via standard scanning tunnelling spectroscopy, without requiring spin-resolved capability. © The Author(s) 2017.
    view abstractdoi: 10.1038/ncomms14317
  • 2017 • 123 Shell-ferromagnetic precipitation in martensitic off-stoichiometric Ni-Mn-In Heusler alloys produced by temper-annealing under magnetic field
    Çakır, A. and Acet, M. and Wiedwald, U. and Krenke, T. and Farle, M.
    Acta Materialia 127 117-123 (2017)
    The variety of the multifunctional properties of martensitic Ni-Mn based Heusler alloys are related to the presence of a magnetostructural transition. We report here on a new functionality based on a newly observed property. The observed property is that all off-stoichiometric Ni-Mn-based Heuslers, here in the form Ni50Mn50−xInx with 0< x< 25, decompose into predominantly cubic ferromagnetic Ni50Mn25In25 and tetragonal antiferromagnetic NiMn components when temper-annealed. The new functionality is based on magnetic field assisted temper-annealing of a compound with stoichiometry x=5, whereby precipitates of Ni50Mn25In25 with a ferromagnetic shell are formed with spins in the field direction, strongly pinned by the surrounding antiferromagnetic anisotropy, even at temperatures as high as 500 K. The remanent pinning at high temperatures survives any thermal cycling between lowest temperatures and the annealing temperature and any magnetic field cycling between −9 and +9 T. The resulting product can serve as a thermally stable, magnetic-field-proof memory. © 2017
    view abstractdoi: 10.1016/j.actamat.2017.01.027
  • 2017 • 122 The combination of micro-resonators with spatially resolved ferromagnetic resonance
    Schaffers, T. and Meckenstock, R. and Spoddig, D. and Feggeler, T. and Ollefs, K. and Schöppner, C. and Bonetti, S. and Ohldag, H. and Farle, M. and Ney, A.
    Review of Scientific Instruments 88 (2017)
    We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 μm × 1 μm) were grown and their FMR excitations were investigated using both setups. © 2017 Author(s).
    view abstractdoi: 10.1063/1.4996780
  • 2017 • 121 The effect of dynamical compressive and shear strain on magnetic anisotropy in a low symmetry ferromagnetic film
    Linnik, T.L. and Kats, V.N. and Jager, J. and Salasyuk, A.S. and Yakovlev, D.R. and Rushforth, A.W. and Akimov, A.V. and Kalashnikova, A.M. and Bayer, M. and Scherbakov, A.V.
    Physica Scripta 92 (2017)
    Dynamical strain generated upon excitation of a metallic film by a femtosecond laser pulse may become a versatile tool enabling control of the magnetic state of thin films and nanostructures via inverse magnetostriction on a picosecond time scale. Here, we explore two alternative approaches to manipulate magnetocrystalline anisotropy and excite magnetization precession in a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga), either by injecting a picosecond strain pulse into it from a substrate, or by generating dynamical strain of a complex temporal profile in the film directly. In the former case, we realize ultrafast excitation of magnetization dynamics solely by strain pulses. In the latter case, optically-generated strain emerging abruptly in the film modifies its magnetocrystalline anisotropy, competing with heat-induced change of anisotropy parameters. We demonstrate that the optically-generated strain remains efficient for launching magnetization precession, when the heat-induced changes of anisotropy parameters do not trigger the precession any more. We emphasize that in both approaches the ultrafast change of magnetic anisotropy mediating the precession excitation relies on the mixed, compressive, and shear character of the dynamical strain, which emerges due to low-symmetry of the metallic film under study. © 2017 The Royal Swedish Academy of Sciences.
    view abstractdoi: 10.1088/1402-4896/aa6943
  • 2017 • 120 The FMR Behaviour of Li–Ni Ferrite Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A. and Farle, M. and Salikhov, R.
    Journal of Superconductivity and Novel Magnetism 30 2575-2579 (2017)
    Magnetic hysteresis loop and ferromagnetic resonance (FMR) behaviour had been studied by considering the variation of the molar ratio of Li0.5−0.5xNixFe2.5−0.5xO4 ferrite nanostructure prepared by the hydrothermal method. The parameters of the ferrite nanostructure include coercivity (Hc), saturation magnetisation (Ms) and magnetic susceptibility. There was low coercivity at x = 0.3, high saturation magnetisation and high susceptibility (χi) at (x = 0.5). These parameters showed an improvement in their values compared with the samples prepared by conventional methods. The FMR analysis indicated that these samples were lossy materials due to having a broad line width ranging from 800 to 925 G. The FMR lines had asymmetric shapes due to anisotropic broadening and conduction mechanism. © 2017 Springer Science+Business Media New York
    view abstractdoi: 10.1007/s10948-017-4058-9
  • 2017 • 119 Tunable magneto-responsive mesoporous block copolymer membranes
    Tang, Y. and Lin, X. and Ito, K. and Hong, L. and Ishizone, T. and Yokoyama, H. and Ulbricht, M.
    Journal of Membrane Science 544 406-415 (2017)
    Mesoporous magneto-responsive membranes with remote-controllable step-wise tunable changes of sieving barrier pore size were successfully prepared. The membranes were composed of a thin mesoporous size-selective layer containing iron oxide nanoparticles (IONPs) embedded in poly(oligo(ethylene glycol) methyl ether methacrylate)-block-polystyrene-block-poly(oligo(ethylene glycol) methyl ether methacrylate) (PMEOnMA-b-PS-b-PMEOnMA), on top of a polyvinylidene fluoride (PVDF) macroporous support membrane. IONPs and PMEOnMA-b-PS-b-PMEOnMA mixture solutions were spun-cast on PVDF membranes, and then mesopores were introduced into the hydrophilic PMEOnMA domains by controlled selective swelling with a mixture of methanol and supercritical CO2. The loaded IONPs aggregated into small clusters (0.1–1 µm) and were finely dispersed in the PMEOnMA-b-PS-b-PMEOnMA mesoporous top layer. No defect was found in the membranes with IONPs based on water and dextran ultrafiltration performances, and scanning electron microscopy (SEM) images showed that their barrier pore sizes were almost identical to those of no IONPs containing membranes. Upon stimulation with alternating magnetic field with different input energy, the water permeability as well as the dextran rejection were tuned into different levels, indicating that the sieving barrier pore size distribution of these thin-film mixed matrix nanocomposite membranes was step-wise adjustable. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.memsci.2017.08.069
  • 2016 • 118 A higherature ferromagnetic topological insulating phase by proximity coupling
    Katmis, F. and Lauter, V. and Nogueira, F.S. and Assaf, B.A. and Jamer, M.E. and Wei, P. and Satpati, B. and Freeland, J.W. and Eremin, I. and Heiman, D. and Jarillo-Herrero, P. and Moodera, J.S.
    Nature 533 513-516 (2016)
    Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (17 K). The magnetism induced at the interface resulting from the large spin-orbit interaction and the spin-momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ∼2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies. © 2016 Macmillan Publishers Limited.
    view abstractdoi: 10.1038/nature17635
  • 2016 • 117 A versatile large-scale and green process for synthesizing magnetic nanoparticles with tunable magnetic hyperthermia features
    Simeonidis, K. and Liébana-Viñas, S. and Wiedwald, U. and Ma, Z. and Li, Z.-A. and Spasova, M. and Patsia, O. and Myrovali, E. and Makridis, A. and Sakellari, D. and Tsiaoussis, I. and Vourlias, G. and Farle, M. and Angelakeris, M.
    RSC Advances 6 53107-53117 (2016)
    This work proposes a large-scale synthesis methodology for engineered and functional magnetic nanoparticles (i.e. ferrites, sulfides) designed towards the principles of green and sustainable production combined with biomedical applicability. The experimental setup consists of a two-stage continuous-flow reactor in which single-crystalline nanoparticles are formed by the coprecipitation of metal salts in an aqueous environment. A series of optimized iron-based nanocrystals (Fe3O4, Fe3S4, CoFe2O4 and MnFe2O4) with diameters between 18 and 38 nm has been obtained. The samples were validated as potential magnetic hyperthermia agents by their heating efficiency as determined by specific loss power (SLP) in calorimetric experiments. In an effort to enhance colloidal stability and surface functionality, nanoparticles were coated by typical molecules of biomedical interest in a single step process. Finally, two-phase particle systems have been produced by a two-stage procedure to enhance the heating rate by the effective combination of different magnetic features. Results indicate relatively high SLP values for uncoated nanoparticles (420 W g-1 for Fe3O4) and a reduction of 20-60% in the heat dissipation rate upon covering by functional groups. Eventually, such effect was more than counterbalanced by the magnetic coupling of different phases in binary systems, since SLP was multiplied up to ∼1700 W g-1 for MnFe2O4/Fe3O4 suggesting a novel route to tune the efficiency of magnetic hyperthermia agents. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ra09362k
  • 2016 • 116 Adiabatic edge channel transport in a nanowire quantum point contact register
    Heedt, S. and Manolescu, A. and Nemnes, G.A. and Prost, W. and Schubert, J. and Grützmacher, D. and Schäpers, Th.
    Nano Letters 16 4569-4575 (2016)
    We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov-Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov-de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b01840
  • 2016 • 115 Analytical approach for magnetic pulse welding of sheet connections
    Hahn, M. and Weddeling, C. and Lueg-Althoff, J. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 131-142 (2016)
    An analytical model to calculate the acting forming pressure in magnetic pulse welding by determining the magnetic field strength between the flyer sheet and a one-turn coil was presented. By neglecting plastic deformation of the flyer, the model allows to calculate the transient velocity and displacement behavior, too. The electromagnetic acceleration of 5000-series aluminum alloy sheets was investigated under various experimental parameters. Utilizing Photon Doppler Velocimetry revealed that the analytical model appropriately describes the influence of current amplitude, coil geometry, and, especially, discharge frequency on the velocity-displacement curve of the flyer and hence on the impact velocity. The model introduced was applied to compute the impact velocity for the welding of long lap joints of 5000-series aluminum alloy sheets and 6000-series aluminum alloy hollow profiles. Through peel tests it was shown that the weld strength at least complied with the strength of the weaker base material as failure always happened in the flyer sheet. The wavy interface pattern typical for impact welding was identified with the help of metallography. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmatprotec.2015.11.021
  • 2016 • 114 Application of micro-magnetic testing systems for non-destructive analysis of wear progress in case-hardened 16MnCr5 gear wheels
    Tenkamp, J. and Haack, M. and Walther, F. and Weibring, M. and Tenberge, P.
    Materials Testing 58 709--716 (2016)
    Micro-magnetic testing methods are qualified for non-destructive quantification of hardness, hardness depth and residual stresses. Among others they are applied for detection of grinding burn in gear wheels, but an application for wear condition monitoring has not yet been published. In this paper, results of initial research of determination of wear condition in gear wheels by application of micro-magnetic testing systems are presented. For comparison of different wear conditions, gears were loaded for increasing numbers of cycles in a test rig based on FVA information sheet 54/7 and DIN ISO 14635 part 1. Operating conditions were altered by usage of different lubricants. Afterwards, wear conditions were determined by conventional techniques, i.e., measuring change in profile and loss of material. Four measurement principles were evaluated for change in micro-magnetic properties determination, magnetic Barkhausen noise analysis, permeability and eddy-current measurements, as well as harmonic analysis of tangential field strength. A general suitability of micro-magnetic testing approach for characterization of wear condition of gear wheels could be demonstrated by comparison of micro-magnetic properties with common wear indicators. Micro-magnetic properties were not solely influenced by wear condition, as the selected oil, and hence the tribochemical conditions in contact also showed a significant effect on measured values. Therefore, further survey is required for direct correlation of micro-magnetic properties with (micro-) structural material changes.
    view abstractdoi: 10.3139/120.110924
  • 2016 • 113 Atomic structure of the i- R -Cd quasicrystals and consequences for magnetism
    Yamada, T. and Takakura, H. and Kong, T. and Das, P. and Jayasekara, W.T. and Kreyssig, A. and Beutier, G. and Canfield, P.C. and De Boissieu, M. and Goldman, A.I.
    Physical Review B 94 (2016)
    We report on the six-dimensional (6D) structural refinement of three members of the i-R-Cd quasicrystals (R = Gd, Dy, Tm) via synchrotron x-ray diffraction from single-grain samples, and show that this series is isostructural to the i-YbCd5.7 quasicrystal. However, our refinements suggest that the R occupancy on the Yb icosahedron sites within the Tsai-type atomic cluster is approximately 80%, with the balance taken up by Cd. Similarities between the i-R-Cd series and i-ScZn7.33, and their differences with i-YbCd5.7 and i-Ca15Cd85, indicate that there are at least two subclasses of Tsai-type icosahedral quasicrystals. We further show from x-ray resonant magnetic scattering (XRMS) measurements on a set of closely related Tb1-xYxCd6 1/1 approximants that the dilution of the magnetic R ions on the icosahedron within the Tsai-type cluster by nonmagnetic Y disrupts the commensurate magnetic ordering in the approximant phase. © 2016 American Physical Society.
    view abstractdoi: 10.1103/PhysRevB.94.060103
  • 2016 • 112 Domain wall dynamics of periodic magnetic domain patterns in Co2MnGe-Heusler microstripes
    Gross, K. and Westerholt, K. and Zabel, H.
    New Journal of Physics 18 (2016)
    Highly symmetric periodic domain patterns were obtained in Co2MnGe-Heusler microstripes as a result of the competition between growth-induced in-plane magnetic anisotropy and shape anisotropy. Zero field magnetic configurations and magnetic field-induced domain wall (DW) motion were studied by magnetic force microscopy-image technique for two different cases: dominant uniaxial- and dominant cubic in-plane anisotropy. We implemented a magneto-optical Kerr effect susceptometer to investigate the DW dynamics of periodic domain structures by measuring the in-phase and out-of-phase components of the Kerr signal as a function of magnetic field frequency and amplitude. The DW dynamics for fields applied transversally to the long stripe axis was found to be dominated by viscous slide motion. We used the inherent symmetry/periodicity properties of the magnetic domain structure to fit the experimental results with a theoretical model allowing to extract the DW mobility for the case of transverse DWs (μ TDW = 1.1 m s-1 Oe-1) as well as for vortex-like DWs (μ VDW = 8.7 m s-1 Oe-1). Internal spin structure transformations may cause a reduction of DW mobility in TDWs as observed by OMMFF simulations. © 2016 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstractdoi: 10.1088/1367-2630/18/3/033007
  • 2016 • 111 Effect of Ni Content on Structural and Magnetic Properties of Li-Ni Ferrites Nanostructure Prepared by Hydrothermal Method
    Al-Shakarchi, E.K. and Lafta, S.H. and Musa, A.M. and Farle, M. and Salikov, R.
    Journal of Superconductivity and Novel Magnetism 29 923-929 (2016)
    Li-Ni ferrite with chemical formula Li0.5−0.5xNixFe2.5−0.5xO4 was prepared by hydrothermal method with different Ni contents (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0) using metal chlorides, ferrous sulfate and sodium hydroxide as oxidants. The hydrothermal treatment was accomplished at (155 ∘ C) for (3 h). The required analyses of XRD, FTIR, SEM, TEM, EDX and magnetic hysteresis loop were performed to characterize the complete behavior as a function of x. It was found that lattice constant has a small increase as x increased. Crystallite size had a minimum value of about 13 nm at x = 1.0 and maximum value of about 33 nm at x = 0.3. It was also found that XRD density increased as x was increased. The particle size distributions showed that the maximum value is around 22 nm. FTIR analysis showed the presence of two main peaks with some shifting. Nanospheres were the predominant particles beside the presence of low nanorod concentration. M-H loops had super paramagnetic shape. The coercivity had a minimum value at x = 0.5. The magnetic saturation had a maximum value at x = 0.3, and the initial susceptibility χi had a maximum value at x = 0.5. © 2015, Springer Science+Business Media New York.
    view abstractdoi: 10.1007/s10948-015-3334-9
  • 2016 • 110 Enhanced magnetocrystalline anisotropy of Fe30Co70 nanowires by Cu additives and annealing
    Palmero, E.M. and Salikhov, R. and Wiedwald, U. and Bran, C. and Spasova, M. and Vázquez, M. and Farle, M.
    Nanotechnology 27 (2016)
    The use of 3d transition metal-based magnetic nanowires (NWs) for permanent magnet applications requires large magnetocrystalline anisotropy energy (MAE), which in combination with the NWs' magnetic shape anisotropy yields magnetic hardening and an enhancement of the magnetic energy product. Here, we report on the significant increase in MAE by 125 kJ m-3 in Fe30Co70 NWs with diameters of 20-150 nm embedded in anodic aluminum oxide templates by adding 5 at.% Cu and subsequent annealing at 900 K. Ferromagnetic resonance (FMR) reveals that this enhancement of MAE is twice as large as the enhancement of MAE in annealed, but undoped NWs. X-ray diffraction (XRD) analysis suggests that upon annealing the immiscible Cu in FeCo NWs causes a crystal reorientation with respect to the NW axis with a considerable distortion of the bcc FeCo lattice. This strain is most likely the origin of the strongly enhanced MAE. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/27/36/365704
  • 2016 • 109 Fermi surface manipulation by external magnetic field demonstrated for a prototypical ferromagnet
    Mlynczak, E. and Eschbach, M. and Borek, S. and Minár, J. and Braun, J. and Aguilera, I. and Bihlmayer, G. and Döring, S. and Gehlmann, M. and Gospodaric, P. and Suga, S. and Plucinski, L. and Blügel, S. and Ebert, H. and Schne...
    Physical Review X 6 (2016)
    We consider the details of the near-surface electronic band structure of a prototypical ferromagnet, Fe(001). Using high-resolution angle-resolved photoemission spectroscopy, we demonstrate openings of the spin-orbit-induced electronic band gaps near the Fermi level. The band gaps, and thus the Fermi surface, can be manipulated by changing the remanent magnetization direction. The effect is of the order of ΔE = 100 meV and Δk = 0.1 Å-1. We show that the observed dispersions are dominated by the bulk band structure. First-principles calculations and one-step photoemission calculations suggest that the effect is related to changes in the electronic ground state and not caused by the photoemission process itself. The symmetry of the effect indicates that the observed electronic bulk states are influenced by the presence of the surface, which might be understood as related to a Rashba-type effect. By pinpointing the regions in the electronic band structure where the switchable band gaps occur, we demonstrate the significance of spinorbit interaction even for elements as light as 3d ferromagnets. These results set a new paradigm for the investigations of spin-orbit effects in the spintronic materials. The same methodology could be used in the bottom-up design of the devices based on the switching of spin-orbit gaps such as electric-field control of magnetic anisotropy or tunneling anisotropic magnetoresistance.
    view abstractdoi: 10.1103/PhysRevX.6.041048
  • 2016 • 108 Impact of CoFe buffer layers on the structural and electronic properties of the Co2MnSi/MgO interface
    Fetzer, R. and Liu, H.-X. and Stadtmüller, B. and Uemura, T. and Yamamoto, M. and Aeschlimann, M. and Cinchetti, M.
    49 (2016)
    The latest improvement of MgO-based magnetic tunnel junctions has been achieved by the combination of CoFe buffer layers and potentially half-metallic ultrathin Co2MnSi electrodes. By this, tunnel magnetoresistance ratios of almost 2000% could be obtained. However, a complete understanding of the underlying processes leading to this enhancement is not yet given. We present a comprehensive study regarding the structural and electronic spin properties of the CoFe(30 nm)-buffered Co2MnSi(3 nm)/MgO(2 nm) buried interface identical to the one formed in actual devices. Low energy electron diffraction experiments show that the ultrathin Co2MnSi layer adopts the lattice constant of the underlying CoFe buffer layer, leading to improved structural conditions at the interface to MgO. In contrast, the Co2MnSi/MgO interface spin polarization at the Fermi level is not affected by the magnetic CoFe buffer layer, as found by interface-sensitive spin-resolved extremely low energy photoemission spectroscopy. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/19/195002
  • 2016 • 107 Influence of magnetic excitations on the phase stability of metals and steels
    Körmann, F. and Hickel, T. and Neugebauer, J.
    Current Opinion in Solid State and Materials Science 20 77-84 (2016)
    Within this article we highlight recent advances in the modeling of magnetic contributions to the finite temperature phase stability of structural materials. A key quantity in this context is the specific heat capacity Cp, since it provides a sensitive link to thermophysical and calorimetric experiments and to established thermodynamic databases. For iron-based materials, the Heisenberg model and its extensions are used as an elegant way for coupling ground-state ab initio calculations with concepts of many-body theory to simulate the temperature dependence. Besides analytical concepts to derive the free energy of the Heisenberg model, our work is mainly devoted to numerical approaches such as Monte-Carlo methods. In particular, we highlight the need to go beyond a classical to a fully quantum-mechanical description of magnetic excitations. In order to achieve a quantitative description of Cp, also lattice and electronic degrees of freedom as well as their dependence on magnetism are addressed. Due to the large variety of experimental data, pure iron is best suited to discuss the method developments and to perform evaluations. Nevertheless, the application to other magnetic elements (e.g. Co, Ni) and Fe-based materials (e.g. Fe3C) will also be addressed. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.cossms.2015.06.001
  • 2016 • 106 Interface effects on the magnetic properties of layered Ni2MnGa/Ni2MnSn alloys: A first-principles investigation
    Dutta, B. and Opahle, I. and Hickel, T.
    Functional Materials Letters 9 (2016)
    The effect of interfaces on the magnetic properties of multilayers is analyzed forNi2MnGa/Ni2MnSn system using density functional theory. The Ni spin moments at the interface change by about 30% compared to the bulk value, whereas the effect on the Mn spin moments is much less pronounced. A similar strong effect is also observed for the Ni orbital moments at the interface. The magneto-crystalline anisotropy of the multilayer systems can be understood by the additive contribution of the respective values of strained bulk materials. © 2016 World Scientific Publishing Company.
    view abstractdoi: 10.1142/S1793604716420108
  • 2016 • 105 Magnetic anisotropy and relaxation of single Fe/FexOy core/shell- nanocubes: A ferromagnetic resonance investigation
    Terwey, A. and Meckenstock, R. and Zingsem, B.W. and Masur, S. and Derricks, C. and Römer, F.M. and Farle, M.
    AIP Advances 6 (2016)
    In this work a full angle dependent Ferromagnetic Resonance (FMR) investigation on a system of 20 separated Fe/FexOy nanocubes without dipolar coupling is reported. The angular magnetic field dependence of FMR spectra of 20 single particles and 2 dimers were recorded using a microresonator setup with a sensitivity of 106 μB at X-band frequencies. We determine an effective magnetocrystalline anisotropy field of 2K4,eff/M = 50 mT ± 5 mT for selected particles, which is smaller than the one of bulk Fe due to the core shell morphology of the particles. The FMR resonances have a linewidth of 4 mT ± 1 mT, corresponding to a magnetic effective damping parameter α = 0.0045 ± 0.0005 matching the values of high quality iron thin films. Numerical calculations taking into account the different angular orientations of the 24 particles with respect to the external magnetic field yield a good agreement to the experiment. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4944399
  • 2016 • 104 Magnetic field effects of Rydberg Excitons in Cu2O
    Thewes, J. and Heckötter, J. and Aßmann, M. and Fröhlich, D. and Grünwald, P. and Scheel, S. and Bayer, M.
    Proceedings of SPIE - The International Society for Optical Engineering 9749 (2016)
    Rydberg excitons are semiconductor analogues to Rydberg atoms, where one electron is promoted to an energy level of large principal quantum number η and which behave in a manner similar to hydrogen. Their huge spatial extent results in giant dipole moments and interaction effects, which can be used to create nonlinearities at the single excitation level. In contrast to hydrogen, the effective masses and Rydberg energies involved are moderately small, so that in contrast to Rydberg atoms the high field limit of Rydberg physics can be studied using fields strengths that can be realized in the lab. Here we investigate the effects of external magnetic fields of up to 7T on Rydberg excitons both in Faraday and Voigt geometry. In both cases complicated splitting patterns emerge. We investigate the differences between the two geometries and highlight spectroscopic features that are especially easy to access using them. We show that the large number of resonances in the spectrum renders a microscopic treatment of each individual resonance implausible. We instead demonstrate general effects introduced by the field like avoided crossings and discuss alternative approaches to the level structure in terms of collective descriptions. © 2016 SPIE.
    view abstractdoi: 10.1117/12.2218532
  • 2016 • 103 Magnetic Mesoporous Photonic Cellulose Films
    Giese, M. and Blusch, L.K. and Schlesinger, M. and Meseck, G.R. and Hamad, W.Y. and Arjmand, M. and Sundararaj, U. and MacLachlan, M.J.
    Langmuir 32 9329-9334 (2016)
    Novel hybrid materials of cellulose and magnetic nanoparticles (NPs) were synthesized and characterized. The materials combine the chiral nematic structural features of mesoporous photonic cellulose (MPC) with the magnetic properties of cobalt ferrite (CoFe2O4). The photonic, magnetic, and dielectric properties of the hybrid materials were investigated during the dynamic swelling and deswelling of the MPC films. It was observed that the dielectric properties of the generated MPC films increased tremendously following swelling in water, endorsing efficient swelling ability of the generated mesoporous films. The high magnetic permeability of the developed MPC films in conjunction with their superior dielectric properties, predominantly in the swollen state, makes them interesting for electromagnetic interference shielding applications. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.6b02974
  • 2016 • 102 Magnetic microstructure in a stress-annealed Fe73.5Si15.5B7Nb3Cu1 soft magnetic alloy observed using off-axis electron holography and Lorentz microscopy
    Kovács, A. and Pradeep, K.G. and Herzer, G. and Raabe, D. and Dunin-Borkowski, R.E.
    AIP Advances 6 (2016)
    Fe-Si-B-Nb-Cu alloys are attractive for high frequency applications due to their low coercivity and high saturation magnetization. Here, we study the effect of stress annealing on magnetic microstructure in Fe73.5Si15.5B7Nb3Cu1 using off-axis electron holography and the Fresnel mode of Lorentz transmission electron microscopy. A stress of 50 MPa was applied to selected samples during rapid annealing for 4 s, resulting in uniaxial anisotropy perpendicular to the stress direction. The examination of focused ion beam milled lamellae prepared from each sample revealed a random magnetic domain pattern in the sample that had been rapidly annealed in the absence of stress, whereas a highly regular domain pattern was observed in the stress-annealed sample. We also measured a decrease in domain wall width from ∼ 94 nm in the sample annealed without stress to ∼ 80 nm in the stress-annealed sample. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4942954
  • 2016 • 101 Magnetic proximity effect and shell-ferromagnetism in metastable Ni50Mn45Ga5
    Krenke, T. and Çakır, A. and Scheibel, F. and Acet, M. and Farle, M.
    Journal of Applied Physics 120 (2016)
    The present study on magnetic and structural properties of Ni50Mn45Ga5 confirms that structural metastability is an inherent property of Ni50Mn50- xXx Heusler alloys with X as In, Ga, and Sn. The ternary alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic L21, Ni50Mn25Ga25, and tetragonal L10 Ni50Mn50. Depending on the annealing temperature, the magnetic-proximity effect giving rise to shell-ferromagnetism has been observed when annealing is carried out under an external magnetic field. The upper and lower remanence values MR+ and MR− have the same sign even at high temperatures. Such alloys can be promising candidates for heat- and magnetic-field-resistant magnetic recording media. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4972480
  • 2016 • 100 Magnetic switching of nanoscale antidot lattices
    Wiedwald, U. and Gräfe, J. and Lebecki, K.M. and Skripnik, M. and Haering, F. and Schütz, G. and Ziemann, P. and Goering, E. and Nowak, U.
    Beilstein Journal of Nanotechnology 7 733-750 (2016)
    We investigate the rich magnetic switching properties of nanoscale antidot lattices in the 200 nm regime. In-plane magnetized Fe, Co, and Permalloy (Py) as well as out-of-plane magnetized GdFe antidot films are prepared by a modified nanosphere lithography allowing for non-close packed voids in a magnetic film. We present a magnetometry protocol based on magneto-optical Kerr microscopy elucidating the switching modes using first-order reversal curves. The combination of various magnetometry and magnetic microscopy techniques as well as micromagnetic simulations delivers a thorough understanding of the switching modes. While part of the investigations has been published before, we summarize these results and add significant new insights in the magnetism of exchange-coupled antidot lattices. © 2016 Wiedwald et al.
    view abstractdoi: 10.3762/bjnano.7.65
  • 2016 • 99 Magnetization and transport properties of single crystalline RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb)
    Drachuck, G. and Böhmer, A.E. and Bud'ko, S.L. and Canfield, P.C.
    Journal of Magnetism and Magnetic Materials 417 420-433 (2016)
    Single crystals of RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb) were grown out of a high temperature solution rich in Pd and P and characterized by room-temperature powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization and temperature-dependent in-plane resistivity measurements. In this series, YPd2P2 and LaPd2P2 YbPd2P2 (with Yb2+) are non-local-moment bearing. Furthermore, YPd2P2 and LaPd2P2 are found to be superconducting with Tc≃0.75 and 0.96 K respectively. CePd2P2 and PrPd2P2 magnetically order at low temperature with a ferromagnetic component along the crystallographic c-axis. The rest of the series manifest low temperature antiferromagnetic ordering. EuPd2P2 has Eu2+ ions and both EuPd2P2 and GdPd2P2 have isotropic paramagnetic susceptibilities consistent with L=0 and [formula presented] and exhibit multiple magnetic transitions. For R=Eu–Dy, there are multiple, T&gt;1.8K transitions in zero applied magnetic field and for R=Nd, Eu, Gd, Tb, and Dy there are clear metamagnetic transitions at T=2.0 K for H&lt;55kOe. Strong anisotropies arising mostly from crystal electric field (CEF) effects were observed for most magnetic rare earths with L≠0. The experimentally estimated CEF parameters B20 were calculated from the anisotropic paramagnetic θab and θc values and compared to theoretical trends across the rare earth series. The ordering temperatures as well as the polycrystalline averaged paramagnetic Curie–Weiss temperature, θave, were extracted from magnetization and resistivity measurements, and compared to the de-Gennes factor. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2016.05.089
  • 2016 • 98 Magnetoresponsive Poly(ether sulfone)-Based Iron Oxide cum Hydrogel Mixed Matrix Composite Membranes for Switchable Molecular Sieving
    Lin, X. and Nguyen Quoc, B. and Ulbricht, M.
    ACS Applied Materials and Interfaces 8 29001-29014 (2016)
    Stimuli-responsive membranes that can adjust mass transfer and interfacial properties "on demand" have drawn large interest over the last few decades. Here, we designed and prepared a novel magnetoresponsive separation membrane with remote switchable molecular sieving effect by simple one-step and scalable nonsolvent induced phase separation (NIPS) process. Specifically, poly(ether sulfone) (PES) as matrix for an anisotropic membrane, prefabricated poly(N-isopropylacrylamide) (PNIPAAm) nanogel (NG) particles as functional gates, and iron oxide magnetic nanoparticles (MNP) as localized heaters were combined in a synergistic way. Before membrane casting, the properties of the building blocks, including swelling property and size distribution for NG, and magnetic property and heating efficiency for MNP, were investigated. Further, to identify optimal film casting conditions for membrane preparation by NIPS, in-depth rheological study of the effects of composition and temperature on blend dope solutions was performed. At last, a composite membrane with 10% MNP and 10% NG blended in a porous PES matrix was obtained, which showed a large, reversible, and stable magneto-responsivity. It had 9 times higher water permeability at the "on" state of alternating magnetic field (AMF) than at the "off"-state. Moreover, the molecular weight cutoff of such membrane could be reversibly shifted from ∼70 to 1750 kDa by switching off or on the external AMF, as demonstrated in dextran ultrafiltration tests. Overall, it has been proved that the molecular sieving performance of the novel mixed matrix composite membrane can be controlled by the swollen/shrunken state of PNIPAAm NG embedded in the nanoporous barrier layer of a PES-based anisotropic porous matrix, via the heat generation of nearby MNP. And the structure of such membrane can be tailored by the NIPS process conditions. Such membrane has potential as enabling material for remote-controlled drug release systems or devices for tunable fractionations of biomacromolecule/-particle mixtures. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b09369
  • 2016 • 97 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 abstractdoi: 10.1098/rsta.2015.0308
  • 2016 • 96 On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
    Grünebohm, A. and Herper, H.C. and Entel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/39/395001
  • 2016 • 95 Optimum nanoscale design in ferrite based nanoparticles for magnetic particle hyperthermia
    Liébana-Viñas, S. and Simeonidis, K. and Wiedwald, U. and Li, Z.-A. and Ma, Zh. and Myrovali, E. and Makridis, A. and Sakellari, D. and Vourlias, G. and Spasova, M. and Farle, M. and Angelakeris, M.
    RSC Advances 6 72918-72925 (2016)
    The study demonstrates the multiplex enhancement of the magnetic hyperthermia response in ferrites by nanoscale design and tuning without sparing the biocompatibility of iron-oxide. We propose core/shell nanoparticles with a 7-9 nm ferrite core, either magnetically soft MnFe2O4 or hard CoFe2O4, encapsulated by a 2-3 nm Fe3O4 shell providing a core/shell interface. In this case, the exchange interaction between core and shell dramatically affects the macroscopic magnetic behavior and, at the same time, a biocompatible shell prevents interactions of the toxic cores with their environment. The tunable, yet superior, magnetic hyperthermia response is proven by an increase of the specific loss power by a factor of 24 for CoFe2O4-Fe3O4 core/shell particles. This gain is directly connected with the magnetic coupling strength at the core/shell interface and opens the possibility of further optimization. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ra17892h
  • 2016 • 94 Partitioning of Cr and Si between cementite and ferrite derived from first-principles thermodynamics
    Sawada, H. and Kawakami, K. and Körmann, F. and Grabowski, B. and Hickel, T. and Neugebauer, J.
    Acta Materialia 102 241-250 (2016)
    Partitioning of Cr and Si between cementite and ferrite was investigated by first-principles thermodynamics taking into account vibrational, electronic, and magnetic Gibbs energy contributions. At finite temperatures, these contributions lower the partitioning Gibbs energy and compete with the configurational entropy, which favors impurity segregation to ferrite due to its larger volume fraction compared to cementite. Due to the large positive partitioning enthalpy contribution of Si at T = 0 K, partitioning of Si to cementite is virtually absent in agreement with experiment. The situation is drastically different for Cr impurities. Incorporation of finite-temperature effects resolves the discrepancy between experimental observations and previous T = 0 K first-principles calculations. Cr strongly segregates to cementite due to the enhanced magnetic entropy of cementite above 400 K, i.e., near the Curie temperature of cementite. The increasing magnetic fluctuations in ferrite cause a strong reduction of the partitioning coefficient in the temperature range from 773 to 973 K in qualitative agreement with experiment. Quantitative agreement with calphad data and experimental data for equilibrium Cr concentrations in a wide range of alloy compositions is achieved by renormalizing the theoretical magnetic partitioning Gibbs energy by a constant scaling factor. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.09.010
  • 2016 • 93 Phase selection and nanocrystallization in Cu-free soft magnetic FeSiNbB amorphous alloy upon rapid annealing
    Morsdorf, L. and Pradeep, K.G. and Herzer, G. and Kovács, A. and Dunin-Borkowski, R.E. and Povstugar, I. and Konygin, G. and Choi, P. and Raabe, D.
    Journal of Applied Physics 119 (2016)
    Nucleation of soft magnetic Fe3Si nanocrystals in Cu-free Fe74.5Si15.5Nb3B7 alloy, upon rapid (10 s) and conventional (30 min) annealing, was investigated using x-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy, and atom probe tomography. By employing rapid annealing, preferential nucleation of Fe3Si nanocrystals was achieved, whereas otherwise there is simultaneous nucleation of both Fe3Si and undesired Fe-B compound phases. Analysis revealed that the enhanced Nb diffusivity, achieved during rapid annealing, facilitates homogeneous nucleation of Fe3Si nanocrystals while shifting the secondary Fe-B crystallization to higher temperatures resulting in pure soft magnetic nanocrystallization with very low coercivities of ∼10 A/m. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4944595
  • 2016 • 92 Precise synthesis of discrete and dispersible carbon-protected magnetic nanoparticles for efficient magnetic resonance imaging and photothermal therapy
    Lu, A.-H. and Zhang, X.-Q. and Sun, Q. and Zhang, Y. and Song, Q. and Schüth, F. and Chen, C. and Cheng, F.
    Nano Research 9 1460-1469 (2016)
    Carbon-protected magnetic nanoparticles exhibit long-term stability in acid or alkaline medium, good biocompatibility, and high saturation magnetization. As a result, they hold great promise for magnetic resonance imaging, photothermal therapy, etc. However, since pyrolysis, which is often required to convert the carbon precursors to carbon, typically leads to coalescence of the nanoparticles, the obtained carbon-protected magnetic nanoparticles are usually sintered as a non-dispersible aggregation. We have successfully synthesized discrete, dispersible, and uniform carbon-protected magnetic nanoparticles via a precise surface/interface nano-engineering approach. Remarkably, the nanoparticles possess excellent water-dispersibility, biocompatibility, a high T2 relaxivity coefficient (384 mM–1·s–1), and a high photothermal heating effect. Furthermore, they can be used as multifunctional core components suited for future extended investigation in early diagnosis, detection and therapy, catalysis, separation, and magnetism. [Figure not available: see fulltext.] © 2016, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s12274-016-1042-9
  • 2016 • 91 Quantization and anomalous structures in the conductance of Si/SiGe quantum point contacts
    Von Pock, J.F. and Salloch, D. and Qiao, G. and Wieser, U. and Hackbarth, T. and Kunze, U.
    Journal of Applied Physics 119 (2016)
    Quantum point contacts (QPCs) are fabricated on modulation-doped Si/SiGe heterostructures and ballistic transport is studied at low temperatures. We observe quantized conductance with subband separations up to 4 meV and anomalies in the first conductance plateau at 4e2/h. At a temperature of T = 22 mK in the linear transport regime, a weak anomalous kink structure arises close to 0.5(4e2/h), which develops into a distinct plateau-like structure as temperature is raised up to T = 4 K. Under magnetic field parallel to the wire up to B = 14 T, the anomaly evolves into the Zeeman spin-split level at 0.5(4e2/h), resembling the "0.7 anomaly" in GaAs/AlGaAs QPCs. Additionally, a zero-bias anomaly (ZBA) is observed in nonlinear transport spectroscopy. At T = 22 mK, a parallel magnetic field splits the ZBA peak up into two peaks. At B = 0, elevated temperatures lead to similar splitting, which differs from the behavior of ZBAs in GaAs/AlGaAs QPCs. Under finite dc bias, the differential resistance exhibits additional plateaus approximately at 0.8(4e2/h) and 0.2(4e2/h) known as "0.85 anomaly" and "0.25 anomaly" in GaAs/AlGaAs QPCs. Unlike the first regular plateau at 4e2/h, the 0.2(4e2/h) plateau is insensitive to dc bias voltage up to at least VDS = 80 mV, in-plane magnetic fields up to B = 15 T, and to elevated temperatures up to T = 25 K. We interpret this effect as due to pinching off one of the reservoirs close to the QPC. We do not see any indication of lifting of the valley degeneracy in our samples. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4945116
  • 2016 • 90 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 De...
    ACS Nano 10 9353-9360 (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 abstractdoi: 10.1021/acsnano.6b04107
  • 2016 • 89 Spin-hybrids: A single-molecule approach to spintronics
    Bürgler, D.E. and Heß, V. and Esat, T. and Fahrendorf, S. and Matthes, F. and Schneider, C. M. and Besson, C. and Monakhov, K.Y. and Kögerler, P. and Ghisolfi, A. and Braunstein, P. and Atodiresei, N. and Caciuc, V. and Blügel, S.
    e-Journal of Surface Science and Nanotechnology 14 17-22 (2016)
    Molecular spintronics aims at exploiting and controlling spin-dependent transport processes at the molecular level. Achieving this aim requires not only appropriate molecules, molecular structures and preparation procedures. Equally important is the understanding and engineering of the electronic and spin-dependent interactions between different molecular species, molecule and substrate, as well as molecule and electrodes. These interactions may not only determine the spin-dependent functionality of the molecular structures, but also their integrity on the substrate. Likewise, there may be also a modification of the surface properties below and in the vicinity of a molecule. We have investigated several molecules on different metallic surfaces, among them magnetic Nd doubledecker phthalocyanines, a cubane-type {Ni4} complex with single-molecule magnet properties, and a nonmagnetic triazine-based molecule. For NdPc2 molecules adsorbed on a Cu(100) surface, our scanning tunneling microscopy and spectroscopy studies show specific electronic states of the molecule-substrate complex. We find that the electric field between STM tip and sample must be taken into account to properly describe the electronic states associated with the upper Pc ligand. © 2016 The Surface Science Society of Japan.
    view abstractdoi: 10.1380/ejssnt.2016.17
  • 2016 • 88 Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe
    Kothapalli, K. and Böhmer, A.E. and Jayasekara, W.T. and Ueland, B.G. and Das, P. and Sapkota, A. and Taufour, V. and Xiao, Y. and Alp, E. and Bud'ko, S.L. and Canfield, P.C. and Kreyssig, A. and Goldman, A.I.
    Nature Communications 7 (2016)
    A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Mössbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures between 1.0 and 1.7 GPa and merge into a single first-order transition for pressures ≥31.7 GPa, reminiscent of what has been found for the evolution of these transitions in the prototypical system Ba(Fe1-xCox)2As2. Our results are consistent with a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.
    view abstractdoi: 10.1038/ncomms12728
  • 2016 • 87 Structural and multifunctional properties of magnetron-sputtered Fe-P(-Mn) thin films
    Decker, P. and Stein, H.S. and Salomon, S. and Brüssing, F. and Savan, A. and Hamann, S. and Ludwig, Al.
    Thin Solid Films 603 262-267 (2016)
    Structural and magnetic properties of magnetron-sputtered Fe-P(-Mn) thin films with compositions around the Fe2P single phase region are reported, revealing the compositional range of the Fe2P-type structure and the change of the magnetic properties within this composition spread. The structural analysis shows that in order to obtain crystalline Fe-P phases the P content must be higher than (Fe0.97Mn0.03)2.33P. A maximum phase fraction of the Fe2P-type structure is obtained in the examined (Fe0.97Mn0.03)1.78P sample. The hysteresis loops for the Fe2P(-Mn) thin films show a two-step magnetic reversal with one part belonging to an amorphous phase fraction and the other to the Fe2P(-Mn) phase. A maximum coercivity of 0.36 T was measured for the Fe2P(-Mn) phase fraction also at the composition of (Fe0.97Mn0.03)1.78P. Furthermore, electrochemical properties of FeP2(-Mn) thin films as hydrogen evolution catalysts (HER) are studied. FeP2(-Mn) shows a HER onset potential about 200 mV lower than that of Pt. Chronoamperometric testing at - 11.5 mA/cm2 for over 3500 s revealed no obvious decay in current density, suggesting good stability under typical working conditions in a photoelectrochemical device. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2016.02.023
  • 2016 • 86 The Chemical Shift Baseline for High-Pressure NMR Spectra of Proteins
    Frach, R. and Kibies, P. and Böttcher, S. and Pongratz, T. and Strohfeldt, S. and Kurrmann, S. and Koehler, J. and Hofmann, M. and Kremer, W. and Kalbitzer, H.R. and Reiser, O. and Horinek, D. and Kast, S.M.
    Angewandte Chemie - International Edition 55 8757-8760 (2016)
    High-pressure (HP) NMR spectroscopy is an important method for detecting rare functional states of proteins by analyzing the pressure response of chemical shifts. However, for the analysis of the shifts it is mandatory to understand the origin of the observed pressure dependence. Here we present experimental HP NMR data on the15N-enriched peptide bond model, N-methylacetamide (NMA), in water, combined with quantum-chemical computations of the magnetic parameters using a pressure-sensitive solvation model. Theoretical analysis of NMA and the experimentally used internal reference standard 4,4-dimethyl-4-silapentane-1-sulfonic (DSS) reveal that a substantial part of observed shifts can be attributed to purely solvent-induced electronic polarization of the backbone. DSS is only marginally responsive to pressure changes and is therefore a reliable sensor for variations in the local magnetic field caused by pressure-induced changes of the magnetic susceptibility of the solvent. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201602054
  • 2016 • 85 Thin film synthesis and characterization of a chemically ordered magnetic nanolaminate (V,Mn)3GaC2
    Tao, Q. and Salikhov, R. and Mockute, A. and Lu, J. and Farle, M. and Wiedwald, U. and Rosen, J.
    APL Materials 4 (2016)
    We report on synthesis and characterization of a new magnetic nanolaminate (V,Mn)3GaC2, which is the first magnetic MAX phase of a 312 stoichiometry. Atomically resolved energy dispersive X-ray mapping of epitaxial thin films reveals a tendency of alternate chemical ordering between V and Mn, with atomic layers composed of primarily one element only. Magnetometry measurements reveal a ferromagnetic response between 50 K and 300 K, with indication of a magnetic ordering temperature well above room temperature. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4961502
  • 2016 • 84 Tuning the magnetism of ferrite nanoparticles
    Viñas, S.L. and Simeonidis, K. and Li, Z.-A. and Ma, Z. and Myrovali, E. and Makridis, A. and Sakellari, D. and Angelakeris, M. and Wiedwald, U. and Spasova, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 415 20-23 (2016)
    The importance of magnetic interactions within an individual nanoparticle or between adjacent ones is crucial not only for the macroscopic collective magnetic behavior but for the AC magnetic heating efficiency as well. On this concept, single-(MFe2O4 where M=Fe, Co, Mn) and core-shell ferrite nanoparticles consisting of a magnetically softer (MnFe2O4) or magnetically harder (CoFe2O4) core and a magnetite (Fe3O4) shell with an overall size in the 10 nm range were synthesized and studied for their magnetic particle hyperthermia efficiency. Magnetic measurements indicate that the coating of the hard magnetic phase (CoFe2O4) by Fe3O4 provides a significant enhancement of hysteresis losses over the corresponding single-phase counterpart response, and thus results in a multiplication of the magnetic hyperthermia efficiency opening a novel pathway for high-performance, magnetic hyperthermia agents. At the same time, the existence of a biocompatible Fe3O4 outer shell, toxicologically renders these systems similar to iron-oxide ones with significantly milder side-effects. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2016.02.098
  • 2016 • 83 Vaporizing foil actuator welding as a competing technology to magnetic pulse welding
    Hahn, M. and Weddeling, C. and Taber, G. and Vivek, A. and Daehn, G.S. and Tekkaya, A.E.
    Journal of Materials Processing Technology 230 8-20 (2016)
    Photonic Doppler velocimetry was applied to compare magnetic pulse welding and vaporizing foil actuator welding against each other in the form of lap joints made of 5000 series aluminum alloy sheets under identical experimental conditions which are: charging energies of the pulse generator, specimen geometry, initial distances between flyer and target plate. Impact velocities resulting from rapidly vaporizing aluminum foils were up to three times higher than those of purely electromagnetically accelerated flyer plates. No magnetic pulse welds were achieved, while every vaporizing foil experiment yielded a strong weld in that failure always occurred in the joining partners instead of in the weld seam during tensile tests. An analytical model to calculate the transient flyer velocity is presented and compared to the measurements. The average deviation between model and experiment is about 11% with regard to the impact velocity. Hence, the model may be used for the process design of collision welds generated by vaporizing foil actuators. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmatprotec.2015.11.010
  • 2015 • 82 Ab initio thermodynamics of the CoCrFeMnNi high entropy alloy: Importance of entropy contributions beyond the configurational one
    Ma, D. and Grabowski, B. and Körmann, F. and Neugebauer, J. and Raabe, D.
    Acta Materialia 100 90-97 (2015)
    We investigate the thermodynamic properties of the prototype equi-atomic high entropy alloy (HEA) CoCrFeMnNi by using finite-temperature ab initio methods. All relevant free energy contributions are considered for the hcp, fcc, and bcc structures, including electronic, vibrational, and magnetic excitations. We predict the paramagnetic fcc phase to be most stable above room temperature in agreement with experiment. The corresponding thermal expansion and bulk modulus agree likewise well with experimental measurements. A careful analysis of the underlying entropy contributions allows us to identify that the originally postulated dominance of the configurational entropy is questionable. We show that vibrational, electronic, and magnetic entropy contributions must be considered on an equal footing to reliably predict phase stabilities in HEA systems. © 2015 Acta Materialia Inc.
    view abstractdoi: 10.1016/j.actamat.2015.08.050
  • 2015 • 81 An approach for transparent and electrically conducting coatings: A transparent plastic varnish with nanoparticulate magnetic additives
    Beck, G. and Barcikowski, S. and Chakravadhanula, V.S.K. and Comesaña-Hermo, M. and Deng, M. and Farle, M. and Hilgendorff, M. and Jakobi, J. and Janek, J. and Kienle, L. and Mogwitz, B. and Schubert, T. and Stiemke, F.
    Thin Solid Films 595 96-107 (2015)
    For the purpose of preparing TCCs (= transparent and electrical conducting coatings), metallic and ferromagnetic nano-additives were dispersed into a transparent varnish and the obtained dispersions were coated on transparent plastic substrates. During hardening of the dispersion the magnetic nano-additives were aligned by a magnetic field. The resulting coatings have electrical pathways along lines of nano-additive chains and are highly transparent in the areas between the lines. Therefore, the electrical conductivity is anisotropic, and it depends on the alignment of the nano-additives (i.e. on the distance between the nano-additives within the chains and the length of the lines) as well as on the thickness of an oxide and/or solvent shell around the nano-additives. The transparency depends also on the alignment and here especially on the thickness and the distance between the formed lines. The quality of the alignment in turn, depends on the magnetic properties and on the size of the particles. We used commercial plastic varnishes, which form electrically isolating (≥ 10− 12 S/m) and transparent (about 90% transparency) coatings, and the following magnetic additives: Co-, Fe-, CoPt3, CoPt3@Au- and Fe@Au-nanoparticles as well as CoNi-nanowires. Coatings with Fe@Au-nanoparticles show the best results in terms of the electrical conductivity (10− 5 S/m–10− 6 S/m) at transparencies above 70%. Furthermore, in addition to the magnetic nano-additives, transparent additives (Al2O3-particles) and non-magnetic, but better conducting additives (carbon-nanotubes) were added to the varnish to increase the transparency and the electrical conductivity, respectively. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2015.10.059
  • 2015 • 80 Characteristics of 5M modulated martensite in Ni-Mn-Ga magnetic shape memory alloys
    Çaklr, A. and Acet, M. and Righi, L. and Albertini, F. and Farle, M.
    AIP Advances 5 (2015)
    The applicability of the magnetic shape memory effect in Ni-Mn-based martensitic Heusler alloys is closely related to the nature of the crystallographically modulated martensite phase in these materials. We study the properties of modulated phases as a function of temperature and composition in three magnetic shape memory alloys Ni49.8Mn25.0Ga25.2, Ni49.8Mn27.1Ga23.1 and Ni49.5Mn28.6Ga21.9. The effect of substituting Ga for Mn leads to an anisotropic expansion of the lattice, where the b-parameter of the 5M modulated structure increases and the a and c-parameters decrease with increasing Ga concentration. The modulation vector is found to be both temperature and composition dependent. The size of the modulation vector corresponds to an incommensurate structure for Ni49.8Mn25.0Ga25.2 at all temperatures. For the other samples the modulation is incommensurate at low temperatures but reaches a commensurate value of q 0.400 close to room temperature. The results show that commensurateness of the 5M modulated structure is a special case of incommensurate 5M at a particular temperature. © 2015 Author(s).
    view abstractdoi: 10.1063/1.4932233
  • 2015 • 79 Correlation between structure and magnetic properties in CoxFe100-x nanowires: The roles of composition and wire diameter
    Bran, C. and Palmero, E.M. and Li, Z.-A. and Del Real, R.P. and Spasova, M. and Farle, M. and Vázquez, M.
    Journal of Physics D: Applied Physics 48 (2015)
    The structural and magnetic characteristics of CoxFe100-x (0 ≤ x ≤ 100) cylindrical nanowire arrays are investigated for two series of nanowires with diameters of 20 and 40 nm, respectively. The crystalline structure evolves with Co content from bcc Fe through a mixed bcc-fcc phase to a final polycrystalline fcc or hcp phase for 40 and 20 nm diameter Co nanowires, respectively. A monocrystalline structure is found only in a few nanowires with a 40 nm diameter. The magnetic characterization under axial magnetic field reveals an increase in coercivity and remanence for increasing Co content as the crystalline structure evolves from bcc Fe to fcc Co. These parameters decrease when hcp Co with a stronger magnetocrystalline anisotropy and nearly perpendicular 'c' axis is formed. Overall higher values are observed in nanowires when the nanowire diameter decreases from 40 to 20 nm. An increase of the total magnetic anisotropy energy density is found with decreasing temperature, especially for Co wires where the strong magnetocrystalline anisotropy plays the most significant role. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/14/145304
  • 2015 • 78 Domain Structure and Magnetoresistance in Co2MnGe Zigzag Structures
    Gross, K. and Westerholt, K. and Gómez, M.E. and Zabel, H.
    Physics Procedia 75 1072-1079 (2015)
    We report a clear manifestation of the negative contribution to the magnetoresistance due to domain walls in Co2MnGe-Heusler submicron zigzag wires in which the domain structure, domain size and domain wall density can be well controlled. The magnetic behavior of these systems results from the interplay between the intrinsic magneto-crystalline (K4) anisotropy, growth induced uniaxial (KU = 4.7x103 J/m3) anisotropy and shape anisotropy (KS), as observed by magnetic-force microscopy (MFM) and longitudinal Kerr hysteresis loop measurements. Magnetoresistance measurements were performed by the four-point method under a field applied in the plane of the wires at a temperature of 300 K. In these structures, domain wall-creation and annihilation occur in a coherent way. As a result, clear jumps of the resistance are detected during the transition from single-domain- to multi-domain states. At room temperature a value RDW = -2.5 mΩ was obtained; this result is the same order of magnitude as other experimental and theoretical findings. The negative resistive contribution due to the domain wall is also discussed and compared with the existing theoretical models. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.177
  • 2015 • 77 Enhanced biomedical heat-triggered carriers via nanomagnetism tuning in ferrite-based nanoparticles
    Angelakeris, M. and Li, Z.-A. and Hilgendorff, M. and Simeonidis, K. and Sakellari, D. and Filippousi, M. and Tian, H. and Van Tendeloo, G. and Spasova, M. and Acet, M. and Farle, M.
    Journal of Magnetism and Magnetic Materials 381 179-187 (2015)
    Biomedical nanomagnetic carriers are getting a higher impact in therapy and diagnosis schemes while their constraints and prerequisites are more and more successfully confronted. Such particles should possess a well-defined size with minimum agglomeration and they should be synthesized in a facile and reproducible high-yield way together with a controllable response to an applied static or dynamic field tailored for the specific application. Here, we attempt to enhance the heating efficiency in magnetic particle hyperthermia treatment through the proper adjustment of the core-shell morphology in ferrite particles, by controlling exchange and dipolar magnetic interactions at the nanoscale. Thus, core-shell nanoparticles with mutual coupling of magnetically hard (CoFe2O4) and soft (MnFe2O4) components are synthesized with facile synthetic controls resulting in uniform size and shell thickness as evidenced by high resolution transmission electron microscopy imaging, excellent crystallinity and size monodispersity. Such a magnetic coupling enables the fine tuning of magnetic anisotropy and magnetic interactions without sparing the good structural, chemical and colloidal stability. Consequently, the magnetic heating efficiency of CoFe2O4 and MnFe2O4 core-shell nanoparticles is distinctively different from that of their counterparts, even though all these nanocrystals were synthesized under similar conditions. For better understanding of the AC magnetic hyperthermia response and its correlation with magnetic-origin features we study the effect of the volume ratio of magnetic hard and soft phases in the bimagnetic core-shell nanocrystals. Eventually, such particles may be considered as novel heating carriers that under further biomedical functionalization may become adaptable multifunctional heat-triggered nanoplatforms. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2014.12.069
  • 2015 • 76 Impact of local order and stoichiometry on the ultrafast magnetization dynamics of Heusler compounds
    Steil, D. and Schmitt, O. and Fetzer, R. and Kubota, T. and Naganuma, H. and Oogane, M. and Ando, Y. and Rodan, S. and Blum, C.G.F. and Balke, B. and Wurmehl, S. and Aeschlimann, M. and Cinchetti, M.
    48 (2015)
    Nowadays, a wealth of information on ultrafast magnetization dynamics of thin ferromagnetic films exists in the literature. Information is, however, scarce on bulk single crystals, which may be especially important for the case of multi-sublattice systems. In Heusler compounds, representing prominent examples for such multi-sublattice systems, off-stoichiometry and degree of order can significantly change the magnetic properties of thin films, while bulk single crystals may be generally produced with a much more well-defined stoichiometry and a higher degree of ordering. A careful characterization of the local structure of thin films versus bulk single crystals combined with ultrafast demagnetization studies can, thus, help to understand the impact of stoichiometry and order on ultrafast spin dynamics. Here, we present a comparative study of the structural ordering and magnetization dynamics for thin films and bulk single crystals of the family of Heusler alloys with composition Co2Fe1 - xMnxSi. The local ordering is studied by 59Co nuclear magnetic resonance (NMR) spectroscopy, while the time-resolved magneto-optical Kerr effect gives access to the ultrafast magnetization dynamics. In the NMR studies we find significant differences between bulk single crystals and thin films, both regarding local ordering and stoichiometry. The ultrafast magnetization dynamics, on the other hand, turns out to be mostly unaffected by the observed structural differences, especially on the time scale of some hundreds of femtoseconds. These results confirm hole-mediated spin-flip processes as the main mechanism for ultrafast demagnetization and the robustness of this demagnetization channel against defect states in the minority band gap as well as against the energetic position of the band gap with respect to the Fermi energy. The very small differences observed in the magnetization dynamics on the picosecond time-scale, on the other hand, can be explained by considering the differences in the electronic structure at the Fermi energy and in the heat diffusion of thin films and bulk crystals. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/48/16/164016
  • 2015 • 75 Influence of alkylphosphonic acid grafting on the electronic and magnetic properties of La2/3Sr1/3MnO3 surfaces
    Galbiati, M. and Tatay, S. and Delprat, S. and Barraud, C. and Cros, V. and Jacquet, E. and Coloma, F. and Choueikani, F. and Otero, E. and Ohresser, P. and Haag, N. and Cinchetti, M. and Aeschlimann, M. and Seneor, P. and Mattana...
    353 24-28 (2015)
    Self-Assembled monolayers (SAMs) are highly promising materials for molecular engineering of electronic and spintronics devices thanks to their surface functionalization properties. In this direction, alkylphosphonic acids have been used to functionalize the most common ferromagnetic electrode in organic spintronics: La<inf>2/3</inf>Sr<inf>1/3</inf>MnO<inf>3</inf> (LSMO). However, a study on the influence of SAMs grafting on LSMO electronic and magnetic properties is still missing. In this letter, we probe the influence of alkylphosphonic acids-based SAMs on the electronic and magnetic properties of the LSMO surface using different spectroscopies. We observe by X-ray photoemission and X-ray absorption that the grafting of the molecules on the LSMO surface induces a reduction of the Mn oxidation state. Ultraviolet photoelectron spectroscopy measurements also show that the LSMO work function can be modified by surface dipoles opening the door to both tune the charge and spin injection efficiencies in organic devices such as organic light-emitting diodes. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.apsusc.2015.06.051
  • 2015 • 74 Intermartensitic transitions and phase stability in Ni50Mn50-xSnx Heusler alloys
    Çakir, A. and Righi, L. and Albertini, F. and Acet, M. and Farle, M.
    Acta Materialia 99 140-149 (2015)
    Ni-Mn based Heusler alloys are of considerable interest due to their multifunctional properties such as magnetic shape memory, magnetocaloric effect and spintronics. The reason for these multifunctional properties is the presence of a first order martensitic transition and its strong coupling to the magnetization. In this work, one of the outstanding class of martensitic Heuslers, Ni-Mn-Sn, is investigated in relation to magneto-structural phase transitions and the stability of the various crystallographic structures under varying temperature. Temperature-dependent X-ray diffraction, resistance and magnetization measurements on Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> alloys are performed in a broad valence electron concentration range 7.91 ≤ (e/a) ≤ 8.34 (5.1 ≤ x ≤ 20.3at.%). The results reveal that in addition to the austenite-martensite transition, further intermartensitic transitions take place with decreasing temperature. Depending on the composition, we observe that the parent martensite phase tends to transform to L1<inf>0</inf> martensite as the ground state phase when the temperature is lowered. A phase diagram of Ni<inf>50</inf>Mn<inf>50-x</inf>Sn<inf>x</inf> is constructed to include intermartensitic phase transition boundaries. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.actamat.2015.07.072
  • 2015 • 73 Itinerant ferromagnetism in the as 4p conduction band of Ba0.6 K0.4Mn2As2 identified by X-Ray magnetic circular dichroism
    Ueland, B.G. and Pandey, A. and Lee, Y. and Sapkota, A. and Choi, Y. and Haskel, D. and Rosenberg, R.A. and Lang, J.C. and Harmon, B.N. and Johnston, D.C. and Kreyssig, A. and Goldman, A.I.
    Physical Review Letters 114 (2015)
    X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba0.6K0.4Mn2As2 show that the ferromagnetism below TC≈100K arises in the As 4p conduction band. No XMCD signal is observed at the Mn x-ray absorption edges. Below TC, however, a clear XMCD signal is found at the As K edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic c axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As 4p conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.217001
  • 2015 • 72 Kinetic arrest in magnetically inhomogeneous C-deficient Mn3GaC
    Cakir, O. and Acet, M. and Farle, M. and Dias, E. and Priolkar, K.
    Journal of Magnetism and Magnetic Materials 390 96--99 (2015)
    Thermal broadening of the first order magnetostructural transition and enhancement in ferromagnetic exchange occurs in carbon deficient Mn3GaC0.9. We show from temperature and field-dependent magnetization measurements that this leads to inhomogeneous magnetism and causes frustration and kinetic arrest effects. The arrested state is deactivated and the system returns to its ground state when the cooling-field is removed. This causes open hysteresis loops with which we study the kinetic arrest effect in this system. (C) 2015 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2015.04.084
  • 2015 • 71 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 abstractdoi: 10.1088/0022-3727/48/30/305006
  • 2015 • 70 Magnetic hardening of Fe30Co70nanowires
    Liébana Viñas, S. and Salikhov, R. and Bran, C. and Palmero, E.M. and Vazquez, M. and Arvan, B. and Yao, X. and Toson, P. and Fidler, J. and Spasova, M. and Wiedwald, U. and Farle, M.
    Nanotechnology 26 (2015)
    3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of Fe30Co70nanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/26/41/415704
  • 2015 • 69 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 9794-9801 (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 abstractdoi: 10.1039/c5tc02011e
  • 2015 • 68 Magnetic phase transitions in Ta/CoFeB/MgO multilayers
    Barsukov, I. and Fu, Y. and Safranski, C. and Chen, Y.-J. and Youngblood, B. and Gonçalves, A.M. and Spasova, M. and Farle, M. and Katine, J.A. and Kuo, C.C. and Krivorotov, I.N.
    Applied Physics Letters 106 (2015)
    We study thin films and magnetic tunnel junction nanopillars based on Ta/Co<inf>20</inf>Fe<inf>60</inf>B<inf>20</inf>/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co<inf>20</inf>Fe<inf>60</inf>B<inf>20</inf>/MgO interface. At approximately 160K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4921306
  • 2015 • 67 Magnetic States of the Ni1.75Co0.25Mn1.25Cr0.25In0.5 Heusler Alloy
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The equilibrium magnetic and structural reference states of Co- and Cr-doped Ni2Mn1.5In0.5 Heusler alloy are investigated by means of the first-principles method using a supercell approach. Three different ferrimagnetic and one ferromagnetic (FM) spin configurations, as well as two supercells with different distributions of excess Mn and In atoms, were considered. It is found that for supercell #1, the FM spin state in austenite is stable, where the martensite with different spin configurations is unstable, while in the case of supercell #2, a ferrimagnetic configuration for both austenite and martensite is favorable. The different trends for martensitic transformation were studied by c/a calculations for the tetragonal and orthorombic distortions of supercells along the z -axis and the y -axis, showing martensitic variants for supercell #2 at a ratio c/a &gt; 1 and c/a < 1. © 1965-2012 IEEE.
    view abstractdoi: 10.1109/TMAG.2015.2438953
  • 2015 • 66 Magneto-responsive alginate capsules
    Degen, P. and Zwar, E. and Schulz, I. and Rehage, H.
    Journal of Physics Condensed Matter 27 (2015)
    Upon incorporation of magnetic nanoparticles (mNPs) into gels, composite materials called ferrogels are obtained. These magneto-responsive systems have a wide range of potential applications including switches and sensors as well as drug delivery systems. In this article, we focus on the properties of calcium alginate capsules, which are widely used as carrier systems in medicine and technology. We studied the incorporation of different kinds of mNPs in matrix capsules and in the core and the shell of hollow particles. We found out that not all particle-alginate or particle-CaCl<inf>2</inf> solution combinations were suitable for a successful capsule preparation on grounds of a destabilization of the nanoparticles or the polymer. For those systems allowing the preparation of switchable beads or capsules, we systematically studied the size and microscopic structure of the capsules, their magnetic behavior and mechanical resistance. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/19/194105
  • 2015 • 65 Magnetocaloric effect in (La1-xSmx)0.67Pb0.33MnO3 (0 ≤ x ≤ 0.3) manganites near room temperature
    Çetin, S.K. and Acet, M. and Güneş, M. and Ekicibil, A. and Farle, M.
    Journal of Alloys and Compounds 650 285-294 (2015)
    Abstract We present an investigation on structure, magnetic and magnetocaloric properties of the perovskite manganites (La<inf>1-x</inf>Sm<inf>x</inf>)<inf>0.67</inf>Pb<inf>0.33</inf>MnO<inf>3</inf> (x = 0, 0.1, 0.2, 0.3) synthesized by sol-gel technique. The XRD patterns show that all synthesized samples have reflections typical of the perovskite structure with orthorhombic symmetry. Thermomagnetic measurements showed that all the samples display a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature decreases with increasing Sm content from 358 K for x = 0-286 K for x = 0.3. We determined the isothermal magnetic entropy changes of all the samples from the magnetization measurements and furthermore measured the adiabatic temperature change of the sample for x = 0.3 directly. The results showed that the adiabatic temperature change, determined using together the entropy change and the specific heat and the value obtained by direct temperature change measurements both give 1.3 K for a magnetic field change of 3 T at about 280 K. We also measured the cyclic adiabatic temperature-change of the sample and the results indicate that the sample undergoes a reversible temperature-change on field-cycling which is essential for magnetic refrigeration. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.jallcom.2015.07.217
  • 2015 • 64 Magnetoconductance of a magnetic double barrier in a quantum wire
    Schüler, B. and Cerchez, M. and Xu, H. and Heinzel, T. and Reuter, D. and Wieck, A.D.
    Superlattices and Microstructures 79 54-62 (2015)
    Magnetotransport measurements of a ballistic quantum wire exposed to two magnetic barriers of opposite polarity in series are reported. We find two types of conductance resonances with quite different characteristic magnetic fields. Numerical simulations show that the first type, characterized by a larger fluctuation period of the magnetoconductance, originates from bound states localized at the magnetic barrier oriented in the same direction with the perpendicular component of the external magnetic field, and the second type of conductance fluctuation can be traced back to states that reside close to the second magnetic barrier with antiparallel alignment. The simulations furthermore show that the confinement mechanism for these states can be understood in terms of spatially varying diamagnetic shifts of the one-dimensional wire modes. © 2014 Elsevier Ltd
    view abstractdoi: 10.1016/j.spmi.2014.12.017
  • 2015 • 63 Magnetodielectric effect in relaxor/ferrimagnetic composites
    Naveed Ul-Haq, M. and Yunus, T. and Mumtaz, A. and Shvartsman, V.V. and Lupascu, D.C.
    Journal of Alloys and Compounds 640 462-467 (2015)
    Abstract We report on the effect of an applied static magnetic field on the dielectric properties of ferroelectric relaxor/ferrimagnetic composites consisting of [Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf>]<inf>0.8</inf>-[CoFe<inf>2</inf>O<inf>4</inf>]<inf>0.2</inf> (BST<inf>0.8</inf>-CFO<inf>0.2</inf>). The pure Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf> (BST30) as well as the composites, were synthesized by solid state reaction method. The X-ray diffraction analysis confirmed that BST30 and CFO coexist in the composite without any secondary phase. The real and the imaginary part of the dielectric permittivity were studied as a function of temperature, with and without static magnetic field, respectively. Relaxor characteristics such as dielectric permittivity and its peak temperature are observed to vary with the magnetic field. This is explained in the context that the applied magnetic field creates magnetostriction in the ferrite phase which is transferred to the relaxor phase via the interface coupling. The strain in the relaxor phase results in the reorientation of local polarization entities, polar nano regions (PNRs), which alters the dielectric characteristics of the sample. This effect is explained in relation to local order parameter q(T) which is found to increase in a certain temperature range above the typical ferroelectric temperature regime with the application of magnetic field. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jallcom.2015.03.215
  • 2015 • 62 Magnetostatic nearest neighbor interactions in a Co48Fe52 nanowire array probed by in-field magnetic force microscopy
    Vock, S. and Tschulik, K. and Uhlemann, M. and Hengst, C. and Fähler, S. and Schultz, L. and Neu, V.
    Journal of Applied Physics 118 (2015)
    The magnetization behavior of nanowires embedded in an array is influenced by the sum of the dipolar fields produced by all surrounding nanowires. These magnetostatic interactions largely modify the array properties and thus complicate the reconstruction of the ensemble averaged behavior of the individual nanowires, such as the intrinsic switching field distribution. Simply correcting the shearing of the hysteresis in a mean-field approach does not account for the locally fluctuating demagnetizing field, which originates from the individual magnetization configuration in the close surrounding of each nanowire. We present an in-field Magnetic Force Microscopy study of electrochemically produced Co48Fe52 nanowires, in which the influence of the magnetic nearest neighbor configuration on the switching behavior of the individual embedded nanowires is clearly detected. Based on this finding, a statistical evaluation method of nearest neighbor histograms is proposed, which potentially allows to judge the strength of the local magnetostatic interactions against the magnitude of the intrinsic switching field distribution. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4937275
  • 2015 • 61 Microscopic analysis of the composition driven spin-reorientation transition in NixPd1-x/Cu(001)
    Gottlob, D.M. and Doğanay, H. and Nickel, F. and Cramm, S. and Krug, I.P. and Nemšák, S. and Schneider, C.M.
    Ultramicroscopy 159 503-507 (2015)
    The spin-reorientation transition (SRT) in epitaxial NixPd1-x/Cu(001) is studied by photoemission microscopy utilizing the X-ray magnetic circular dichroism effect at the Ni L2,3 edge. In a composition/thickness wedged geometry, a composition driven SRT could be observed between 37ML and 60ML, and 0 and 38% of Pd. Microspectroscopy in combination with azimuthal sample rotation confirms a magnetization preference changing from the [001] to an in-plane easy axis. At this increased thickness, the domain patterns arrange comparable to SRTs in ultrathin films. The images document domains equivalent to a canted state SRT, at which an additional effect of in-plane anisotropies could be identified. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.ultramic.2015.05.022
  • 2015 • 60 Non-collinear magnetism with analytic Bond-Order Potentials
    Ford, M.E. and Pettifor, D.G. and Drautz, R.
    Journal of Physics Condensed Matter 27 (2015)
    The theory of analytic Bond-Order Potentials as applied to non-collinear magnetic structures of transition metals is extended to take into account explicit rotations of Hamiltonian and local moment matrix elements between locally and globally defined spin-coordinate systems. Expressions for the gradients of the energy with respect to the Hamiltonian matrix elements, the interatomic forces and the magnetic torques are derived. The method is applied to simulations of the rotation of magnetic moments in α iron, as well as α and β manganese, based on d-valent orthogonal tight-binding parametrizations of the electronic structure. A new weighted-average terminator is introduced to improve the convergence of the Bond-Order Potential energies and torques with respect to tight-binding reference values, although the general behavior is qualitatively correct for low-moment expansions. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/27/8/086002
  • 2015 • 59 Numerical energy relaxation to model microstructure evolution in functional magnetic materials
    Kiefer, B. and Buckmann, K. and Bartel, T.
    GAMM Mitteilungen 38 171-196 (2015)
    This paper proposes energy relaxation-based approaches for the modeling of magnetostriction, with a particular focus on single crystalline magnetic shape memory alloy response. The theoretical development relies on concepts of energy relaxation in the context of nonconvex free energy landscapes whose wells define preferred states of spontaneous straining and magnetization. The constrained theory of magnetoelasticity developed by DeSimone and James [1] represents the point of departure for the model development, and its capabilities, but also limitations, are demonstrated by means of representative numerical examples. The key features that characterize the extended approach are (i) the incorporation of elastic deformations, whose distribution among the individual phases occurs in an energy minimizing fashion, (ii) a finite magnetocrystalline anisotropy energy, that allows magnetization rotations away from easy axes, and (iii) dissipative effects, that are accounted for in an incremental variational setting for standard dissipative materials. In the context of introducing elastic strain energy, two different relaxation concepts, the convexification approach and the rank-one relaxation with respect to first-order laminates, are considered. In this manner, important additional response features, e.g. the hysteretic nature, the linear magnetization response in the pre-variant reorientation regime, and the stress dependence of the maximum field induced strain, can be captured, which are prohibited by the inherent assumptions of the constrained theory. The enhanced modeling capabilities of the extended approach are demonstrated by several representative response simulations and comparison to experimental results taken from literature. These examples particularly focus on the response of single crystals under cyclic magnetic field loading at constant stress and cyclic mechanical loading at constant magnetic field. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/gamm.201510009
  • 2015 • 58 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 abstractdoi: 10.1063/1.4919014
  • 2015 • 57 Perpendicular magnetisation from in-plane fields in nano-scaled antidot lattices
    Gräfe, J. and Haering, F. and Tietze, T. and Audehm, P. and Weigand, M. and Wiedwald, U. and Ziemann, P. and Gawroński, P. and Schütz, G. and Goering, E.J.
    Nanotechnology 26 (2015)
    Investigations of geometric frustrations in magnetic antidot lattices have led to the observation of interesting phenomena like spin-ice and magnetic monopoles. By using highly focused magneto-optical Kerr effect measurements and x-ray microscopy with magnetic contrast we deduce that geometrical frustration in these nanostructured thin film systems also leads to an out-of-plane magnetization from a purely in-plane applied magnetic field. For certain orientations of the antidot lattice, formation of perpendicular magnetic domains has been found with a size of several μm that may be used for an in-plane/out-of-plane transducer. © 2015 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/26/22/225203
  • 2015 • 56 Predictions of a Large Magnetocaloric Effect in Co- and Cr-Substituted Heusler Alloys Using First-Principles and Monte Carlo Approaches
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Zagrebin, M.A. and Grünebohm, A. and Entel, P.
    Physics Procedia 75 1381-1388 (2015)
    The effect of Co- and Cr-doping on magnetic and magnetocaloric poperties of Ni-Mn-(In, Ga, Sn, and Al) Heusler alloys has been theoretically studied by combining first principles with Monte Carlo approaches. The magnetic and magnetocaloric properties are obtained as a function of temperature and magnetic field using a mixed type of Potts and Blume-Emery-Griffiths model where the model parameters are obtained from ab initio calculations. The Monte Carlo calculations allowed to make predictions of a giant inverse magnetocaloric effect in partially new hypothetical magnetic Heusler alloys across the martensitic transformation. © 2015 The Authors. Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.phpro.2015.12.155
  • 2015 • 55 Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters
    Yang, J. and Fainblat, R. and Kwon, S.G. and Muckel, F. and Yu, J.H. and Terlinden, H. and Kim, B.H. and Iavarone, D. and Choi, M.K. and Kim, I.Y. and Park, I. and Hong, H.-K. and Lee, J. and Son, J.S. and Lee, Z. and Kang, K. and...
    Journal of the American Chemical Society 137 12776-12779 (2015)
    Doping semiconductor nanocrystals with magnetic transition-metal ions has attracted fundamental interest to obtain a nanoscale dilute magnetic semiconductor, which has unique spin exchange interaction between magnetic spin and exciton. So far, the study on the doped semiconductor NCs has usually been conducted with NCs with larger than 2 nm because of synthetic challenges. Herein, we report the synthesis and characterization of Mn2+-doped (CdSe)13 clusters, the smallest doped semiconductors. In this study, single-sized doped clusters are produced in large scale. Despite their small size, these clusters have semiconductor band structure instead of that of molecules. Surprisingly, the clusters show multiple excitonic transitions with different magneto-optical activities, which can be attributed to the fine structure splitting. Magneto-optically active states exhibit giant Zeeman splittings up to elevated temperatures (128 K) with large g-factors of 81(±8) at 4 K. Our results present a new synthetic method for doped clusters and facilitate the understanding of doped semiconductor at the boundary of molecules and quantum nanostructure. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/jacs.5b07888
  • 2015 • 54 Self-assembly of smallest magnetic particles
    Taheri, S.M. and Michaelis, M. and Friedrich, T. and Förster, B. and Drechsler, M. and Römer, F.M. and Bösecke, P. and Narayanan, T. and Weber, B. and Rehberg, I. and Rosenfeldt, S. and Förster, S.
    Proceedings of the National Academy of Sciences of the United States of America 112 14484-14489 (2015)
    The assembly of tiny magnetic particles in external magnetic fields is important for many applications ranging from data storage to medical technologies. The development of ever smaller magnetic structures is restricted by a size limit, where the particles are just barely magnetic. For such particles we report the discovery of a kind of solution assembly hitherto unobserved, to our knowledge. The fact that the assembly occurs in solution is very relevant for applications, where magnetic nanoparticles are either solutionprocessed or are used in liquid biological environments. Induced by an external magnetic field, nanocubes spontaneously assemble into 1D chains, 2D monolayer sheets, and large 3D cuboids with almost perfect internal ordering. The self-assembly of the nanocubes can be elucidated considering the dipole-dipole interaction of small superparamagnetic particles. Complex 3D geometrical arrangements of the nanodipoles are obtained under the assumption that the orientation of magnetization is freely adjustable within the superlattice and tends to minimize the binding energy. On that basis the magnetic moment of the cuboids can be explained.
    view abstractdoi: 10.1073/pnas.1511443112
  • 2015 • 53 Solid solution magnetic FeNi nanostrand-polymer composites by connecting-coarsening assembly
    Barcikowski, S. and Baranowski, T. and Durmus, Y. and Wiedwald, U. and Gökce, B.
    Journal of Materials Chemistry C 3 10699-10704 (2015)
    An approach to assemble high aspect ratio nanostrands consisting of magnetic nanowires and their incorporation in a polymer with the aim of tailoring transparent FeNi nanostrand-PMMA-composites is presented. These nanostrands are controllable in length (<600 μm) and width (<12 μm) via process parameters and have an ultra-high aspect ratio (∼160). This rapid and universal method provides flexible and transparent magnetic materials with tunable transparency and magnetic attraction force by adjusting the density of nanoparticles in the composite. These composites can be used as a window coating for shielding radio frequency electromagnetic waves while being transparent in the optical range. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5tc02160j
  • 2015 • 52 Structural and magnetic phase transitions near optimal superconductivity in BaFe2 (As1-x Px)2
    Hu, D. and Lu, X. and Zhang, W. and Luo, H. and Li, S. and Wang, P. and Chen, G. and Han, F. and Banjara, S.R. and Sapkota, A. and Kreyssig, A. and Goldman, A.I. and Yamani, Z. and Niedermayer, C. and Skoulatos, M. and Georgii, R....
    Physical Review Letters 114 (2015)
    We use nuclear magnetic resonance (NMR), high-resolution x-ray, and neutron scattering studies to study structural and magnetic phase transitions in phosphorus-doped BaFe2(As1-xPx)2. Previous transport, NMR, specific heat, and magnetic penetration depth measurements have provided compelling evidence for the presence of a quantum critical point (QCP) near optimal superconductivity at x=0.3. However, we show that the tetragonal-to-orthorhombic structural (Ts) and paramagnetic to antiferromagnetic (AF, TN) transitions in BaFe2(As1-xPx)2 are always coupled and approach TN≈Ts≥Tc (≈29K) for x=0.29 before vanishing abruptly for x≥0.3. These results suggest that AF order in BaFe2(As1-xPx)2 disappears in a weakly first-order fashion near optimal superconductivity, much like the electron-doped iron pnictides with an avoided QCP. © 2015 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.114.157002
  • 2015 • 51 Structural stability of Fe-based topologically close-packed phases
    Ladines, A.N. and Hammerschmidt, T. and Drautz, R.
    Intermetallics 59 59-67 (2015)
    Precipitates of topologically close-packed (TCP) phases play an important role in hardening mechanisms of high-performance steels. We analyze the influence of atomic size, electron count, magnetism and external stress on TCP phase stability in Fe-based binary transition metal alloys. Our density-functional theory calculations of structural stability are complemented by an analysis with an empirical structure map for TCP phases. The structural stability and lattice parameters of the Fe-Nb/Mo/V compounds are in good agreement with experiment. The average magnetic moments follow the Slater-Pauling relation to the average number of valence-electrons and can be rationalized in terms of the electronic density of states. The stabilizing effect of the magnetic energy, estimated by additional non-magnetic calculations, increases as the magnetic moment increases with band filling for the binary systems of Fe and early transition metals. For the case of Fe2Nb, we demonstrate that the influence of magnetism and external stress is sufficiently large to alter the energetic ordering of the closely competing Laves phases C14, C15 and C36. We find that the A15 phase is not stabilized by atomic-size differences, while the stability of C14 is increasing with increasing difference in atomic size. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2014.12.009
  • 2015 • 50 Structure-Correlated Exchange Anisotropy in Oxidized Co80Ni20 Nanorods
    Liébana-Viñas, S. and Wiedwald, U. and Elsukova, A. and Perl, J. and Zingsem, B. and Semisalova, A.S. and Salgueiriño, V. and Spasova, M. and Farle, M.
    Chemistry of Materials 27 4015-4022 (2015)
    Rare earth-free permanent magnets for applications in electro-magnetic devices promise better sustainability and availability and lower prices. Exploiting the combination of shape, magnetocrystalline and exchange anisotropy in 3D-metals can pave the way to practical application of nanomagnets. In this context, we study the structural and magnetic properties of Co<inf>80</inf>Ni<inf>20</inf> nanorods with a mean diameter of 6.5 nm and a mean length of 52.5 nm, prepared by polyol reduction of mixed cobalt and nickel acetates. Structural analysis shows crystalline rods with the crystallographic c-axis of the hexagonal close-packed (hcp) phase parallel to the long axis of the Co<inf>80</inf>Ni<inf>20</inf> alloy rods, which appear covered by a thin oxidized face-centered cubic (fcc) shell. The temperature dependence of the surprisingly high coercive field and the exchange bias effect caused by the antiferromagnetic surface oxide indicate a strong magnetic hardening due to alignment of anisotropy axes. We identify a temperature dependent local maximum of the coercive field at T = 250 K, which originates from noncollinear spin orientations in the ferromagnetic core and the antiferromagnetic shell. This might be useful for building four way magnetic switches as a function of temperature. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.5b00976
  • 2015 • 49 The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 193-197 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jmmm.2015.03.003
  • 2014 • 48 Convergence of an analytic bond-order potential for collinear magnetism in Fe
    Ford, M.E. and Drautz, R. and Hammerschmidt, T. and Pettifor, D.G.
    Modelling and Simulation in Materials Science and Engineering 22 (2014)
    Analytic bond-order potentials (BOPs) for magnetic transition metals are applied for pure iron as described by an orthogonal d-valent tight-binding (TB) model. Explicit analytic equations for the gradients of the binding energy with respect to the Hamiltonian on-site levels are presented, and are then used to minimize the energy with respect to the magnetic moments, which is equivalent to a TB self-consistency scheme. These gradients are also used to calculate the exact forces, consistent with the energy, necessary for efficient relaxations and molecular dynamics. The Jackson kernel is used to remove unphysical negative densities of states, and approximations for the asymptotic recursion coefficients are examined. BOP, TB and density functional theory results are compared for a range of bulk and defect magnetic structures. The BOP energies and magnetic moments for bulk structures are shown to converge with increasing numbers of moments, with nine moments sufficient for a quantitative comparison of structural energy differences. The formation energies of simple defects such as the monovacancies and divacancies also converge rapidly. Other physical quantities, such as the position of the high-spin to low-spin transition in ferromagnetic fcc (face centred cubic) iron, surface peaks in the local density of states, the elastic constants and the formation energies of the self-interstitial atom defects, require higher moments for convergence. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/22/3/034005
  • 2014 • 47 Lambda transitions in materials science: Recent advances in CALPHAD and first-principles modelling
    Körmann, F. and Breidi, A.A.H. and Dudarev, S.L. and Dupin, N. and Ghosh, G. and Hickel, T. and Korzhavyi, P. and Muñoz, J.A. and Ohnuma, I.
    Physica Status Solidi (B) Basic Research 251 53-80 (2014)
    This paper provides a comprehensive overview of state-of-the-art computational techniques to thermodynamically model magnetic and chemical order-disorder transitions. Recent advances as well as limitations of various approaches to these so-called lambda transitions are examined in detail, focussing on calphad models and first-principles methods based on density functional theory (DFT). On the one hand empirical implementations -based on the Inden-Hillert-Jarl formalism -are investigated, including a detailed interpretation of the relevant parameters, physical limiting cases and potential extensions. In addition, Bragg-Williams-based approaches as well as cluster-variation methods of chemical order-disorder transitions are discussed. On the other hand, it is shown how magnetic contributions can be introduced based on various microscopic model Hamiltonians (Hubbard model, Heisenberg model and beyond) in combination with DFT-computed parameters. As a result of the investigation we were able to indicate similarities between the treatment of chemical and magnetic degrees of freedom as well as the treatment within the calphad and DFT approaches. Potential synergy effects resulting from this overlap have been derived and alternative approaches have been suggested, in order to improve future thermodynamic modelling of lambda transitions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssb.201350136
  • 2014 • 46 Magnetic field templated patterning of the soft magnetic alloy CoFe
    Karnbach, F. and Uhlemann, M. and Gebert, A. and Eckert, J. and Tschulik, K.
    Electrochimica Acta 123 477-484 (2014)
    The ongoing miniaturization of multifunctional electronic consumer products demands for cost-efficient production of functional metal and alloy structures. Recently, an electrochemical method of structuring by means of magnetic gradient fields has been introduced for metal deposition. Here, not only the structured deposition of the metals Co and Fe is presented, but it is further demonstrated that this method can be applied to structure alloys. This is shown for the industrially highly relevant magnetic CoFe alloy. Deposits with maximum layer thickness in regions of high magnetic gradients are formed and the chemical composition of the alloy is constant across the structure. Electrochemical quartz crystal microbalance studies revealed that in applied magnetic gradient fields the current efficiency for alloy deposition is significantly increased with respect to that for hydrogen reduction in the additive free sulphate based electrolyte used in this study. © 2014 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2014.01.055
  • 2014 • 45 Magnetic vortices induced by a monopole tip
    Magiera, M.P. and Schulz, S.
    IEEE Transactions on Magnetics 50 (2014)
    A ferromagnetic monolayer with an easy-plane anisotropy scanned by a magnetic tip that is moved with constant velocity \({v}\) is studied using atomistic computer simulations. The spin dynamics are treated using the Landau-Lifshitz-Gilbert equation. To study the influence of the tip's field, it is modeled by a monopole field instead of a dipole field, which is a common near-field approximation of a scanning probe microscopy cantilever. The magnetic structures induced by the moving tip are analyzed with respect to the strength of the coupling as well as the scanning velocity, and the energy dissipation is calculated. The results agree with calculations in a continuum model using Thiele's equation, as well as with earlier results obtained from simulations using a dipolar tip. The quantitative influence of the field is illustrated using energetic arguments. © 2014 IEEE.
    view abstractdoi: 10.1109/TMAG.2014.2317457
  • 2014 • 44 Magnetically driven suppression of nematic order in an iron-based superconductor
    Avci, S. and Chmaissem, O. and Allred, J.M. and Rosenkranz, S. and Eremin, I. and Chubukov, A.V. and Bugaris, D.E. and Chung, D.Y. and Kanatzidis, M.G. and Castellan, J.-P. and Schlueter, J.A. and Claus, H. and Khalyavin, D.D. and...
    Nature Communications 5 (2014)
    A theory of superconductivity in the iron-based materials requires an understanding of the phase diagram of the normal state. In these compounds, superconductivity emerges when stripe spin density wave (SDW) order is suppressed by doping, pressure or atomic disorder. This magnetic order is often pre-empted by nematic order, whose origin is yet to be resolved. One scenario is that nematic order is driven by orbital ordering of the iron 3d electrons that triggers stripe SDW order. Another is that magnetic interactions produce a spin-nematic phase, which then induces orbital order. Here we report the observation by neutron powder diffraction of an additional fourfold-symmetric phase in Ba 1 ̂'x Na x Fe 2 As 2 close to the suppression of SDW order, which is consistent with the predictions of magnetically driven models of nematic order.
    view abstractdoi: 10.1038/ncomms4845
  • 2014 • 43 Molecular spintronics: Topology communicates
    Cinchetti, M.
    9 965-966 (2014)
    doi: 10.1038/nnano.2014.284
  • 2014 • 42 Optical tweezers as manufacturing and characterization tool in microfluidics
    Köhler, J. and Ghadiri, R. and Ksouri, S.I. and Gurevich, E.L. and Ostendorf, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9164 (2014)
    Pumping and mixing of small volumes of liquid samples are basic processes in microfluidic applications. Among the number of different principles for active transportation of the fluids microrotors have been investigated from the beginning. The main challenge in microrotors, however, has been the driving principle. In this work a new approach for a very simple magnetic driving principle has been realized. More precisely, we take advantage of optical grippers to fabricate various microrotors and introduce an optical force method to characterize the fluid flow generated by rotating the structures through magnetic actuation. The microrotors are built of silica and magnetic microspheres which are initially coated with Streptavidin or Biotin molecules. Holographic optical tweezers (HOT) are used to trap, to position, and to assemble the microspheres with the chemical interaction of the biomolecules leading to a stable binding. Using this technique, complex designs of microrotors can be realized. The magnetic response of the magnetic microspheres enables the rotation and control of the structures through an external magnetic field. The generated fluid flow around the microrotor is measured optically by inserting a probe particle next to the rotor. While the probe particle is trapped by optical forces the flow force leads to a displacement of the particle from the trapping position. This displacement is directly related to the flow velocity and can be measured and calibrated. Variations of the microrotor design and rotating speed lead to characteristic flow fields. © 2014 SPIE.
    view abstractdoi: 10.1117/12.2063080
  • 2014 • 41 Single core-shell nanoparticle probes for non-invasive magnetic force microscopy
    Uhlig, T. and Wiedwald, U. and Seidenstücker, A. and Ziemann, P. and Eng, L.M.
    Nanotechnology 25 (2014)
    We present an easy, fast and reliable method for the preparation of magnetic force microscopy (MFM) probes based on single Co nanoparticles (NPs). Due to their dipolar character, these magnetic probes open up a new approach for quantitative and non-invasive MFM measurements on the nanometer length scale. To guarantee long-term stability of these tips under ambient conditions, an ultrathin protecting Au shell was grown around the Co NPs through photochemical deposition. Single magnetic particles were firmly attached to standard silicon AFM tips using bifunctional self-assembling molecules. Such probes were tested on longitudinal magnetic recording media and compared to the results as recorded with conventional thin-film MFM tips. Easy data interpretation of the magnetic nanoparticle probes in a point dipole model is shown. Our nanoparticle tips provide excellent endurance for MFM recording, enable non-invasive probing while maintaining a high sensitivity, resolution, and reproducibility. © 2014 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/25/25/255501
  • 2014 • 40 Structural complexity in metal-organic frameworks: Simultaneous modification of open metal sites and hierarchical porosity by systematic doping with defective linkers
    Fang, Z. and Dürholt, J.P. and Kauer, M. and Zhang, W. and Lochenie, C. and Jee, B. and Albada, B. and Metzler-Nolte, N. and Pöppl, A. and Weber, B. and Muhler, M. and Wang, Y. and Schmid, R. and Fischer, R.A.
    Journal of the American Chemical Society 136 9627-9636 (2014)
    A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/ja503218j
  • 2014 • 39 Transmission electron microscopy and ferromagnetic resonance investigations of tunnel magnetic junctions using Co2MnGe Heusler alloys as magnetic electrodes
    Belmeguenai, M. and Genevois, C. and Zighem, F. and Roussigné, Y. and Chérif, S.M. and Westerholt, K. and El Bahoui, A. and Fnidiki, A. and Moch, P.
    Thin Solid Films 551 163-170 (2014)
    High resolution transmission electron microscopy, nano-beam electronic diffraction, energy dispersive X-rays scanning spectroscopy, vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) techniques are used in view of comparing (static and dynamic) magnetic and structural properties of Co 2MnGe(13 nm)/Al2O3(3 nm)/Co(13 nm) tunnel magnetic junctions (TMJs), deposited on various single crystalline substrates (a-plane sapphire, MgO(100) and Si(111)). They allow for providing a correlation between these magnetic properties and the fine structure investigated at atomic scale. The Al2O3 tunnel barrier is always amorphous and contains a large concentration of Co atoms, which, however, is significantly reduced when using a sapphire substrate. The Co layer is polycrystalline and shows larger grains for films grown on a sapphire substrate. The VSM investigation reveals in-plane anisotropy only for samples grown on a sapphire substrate. The FMR spectra of the TMJs are compared to the obtained ones with a single Co and Co2MnGe films of identical thickness deposited on a sapphire substrate. As expected, two distinct modes are detected in the TMJs while only one mode is observed in each single film. For the TMJ grown on a sapphire substrate, the FMR behavior does not significantly differ from the superposition of the individual spectra of the single films, allowing for a conclusion that the exchange coupling between the two magnetic layers is too small to give rise to observable shifts. For TMJs grown on a Si or on a MgO substrate, the resonance spectra reveal one mode which is nearly identical to the obtained one in the single Co film, while the other observed resonance shows a considerably smaller intensity and cannot be described using the magnetic parameters appropriate to the single Co2MnGe film. The large Co concentration in the Al2O3 interlayer prevents for a simple interpretation of the observed spectra when using Si or MgO substrates. © 2013 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2013.11.090
  • 2013 • 38 Accessing 4f-states in single-molecule spintronics
    Fahrendorf, S. and Atodiresei, N. and Besson, C. and Caciuc, V. and Matthes, F. and Blügel, S. and Kögerler, P. and Bürgler, D.E. and Schneider, C.M.
    Nature Communications 4 (2013)
    Magnetic molecules are potential functional units for molecular and supramolecular spintronic devices. However, their magnetic and electronic properties depend critically on their interaction with metallic electrodes. Charge transfer and hybridization modify the electronic structure and thereby influence or even quench the molecular magnetic moment. Yet, detection and manipulation of the molecular spin state by means of charge transport, that is, spintronic functionality, mandates a certain level of hybridization of the magnetic orbitals with electrode states. Here we show how a judicious choice of the molecular spin centres determines these critical molecule-electrode contact characteristics. In contrast to late lanthanide analogues, the 4f-orbitals of single bis(phthalocyaninato)-neodymium(III) molecules adsorbed on Cu(100) can be directly accessed by scanning tunnelling microscopy. Hence, they contribute to charge transport, whereas their magnetic moment is sustained as evident from comparing spectroscopic data with ab initio calculations. Our results showcase how tailoring molecular orbitals can yield all-electrically controlled spintronic device concepts. © 2013 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms3425
  • 2013 • 37 Medical applications of surface-enhanced Raman scattering
    Xie, W. and Schlücker, S.
    Physical Chemistry Chemical Physics 15 5329-5344 (2013)
    This perspective article provides an overview of selected medical applications of surface-enhanced Raman scattering (SERS), highlighting recent developments and trends. The use of SERS for detection, analysis and imaging has attracted great interest in the past decade owing to its high sensitivity and molecular fingerprint specificity. SERS can deliver chemical and structural information from analytes rapidly and nondestructively in a label-free manner. Alternatively, SERS labels or nanotags, when conjugated to target-specific ligands, can be employed for the selective detection and localization of the corresponding target molecule. Biomedical applications based on both approaches are highlighted. © 2013 the Owner Societies.
    view abstractdoi: 10.1039/c3cp43858a
  • 2013 • 36 Ultrafast element-specific magnetization dynamics of complex magnetic materials on a table-top
    Mathias, S. and La-O-Vorakiat, C. and Shaw, J.M. and Turgut, E. and Grychtol, P. and Adam, R. and Rudolf, D. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Schneider, C.M. and Kapteyn, H.C. and Murnane, M.M.
    Journal of Electron Spectroscopy and Related Phenomena 189 164-170 (2013)
    We review recent progress in femtosecond magnetization dynamics probed by extreme ultraviolet pulses from high-harmonic generation. In a transverse magneto-optical Kerr geometry, we established an ultrafast, element-specific experimental capability - on a table-top - for the measurement of magnetization dynamics in complex multi-sublattice magnets and multilayer magnetic structures. We show that this newly introduced technique is an artifact-free magnetic sensor, with only negligible non-magnetic (optical) contributions from the transient variation of the refractive index due to the presence of a non equilibrium hot-electron distribution. We then use these new experimental capabilities of ultrahigh time-resolution, combined with element-specific simultaneous probing, to disentangle important microscopic processes that drive magnetization dynamics on femtosecond timescales. We elucidate the role of exchange interaction on magnetization dynamics in strongly exchange-coupled alloys, and the role of photo-induced superdiffusive spin currents in magnetic multilayer stacks. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.elspec.2012.11.013
  • 2012 • 35 Ab-initio investigation of spin states of sodium cobaltate Na 2/3CoO2
    Lysogorskiy, Y.V. and Nedopekin, O.V. and Krivenko, S.A. and Minisini, B. and Tayurskii, D.A.
    Journal of Physics: Conference Series 394 (2012)
    Resent experiments in the lamellar system NaxCoO2 detected a transition of Co planes into a puzzling metallic state at x ≥ 2/3, which co-exists with a robust arrangement of the 3d cobalt electrons: The triangular Co lattices are disproportionated in the spinless Co3+ sites (Co1), and Co3.44+ sites (Co2) with enhanced magnetism forming conducting sublattices. This textures concur with a tightening of the ferromagnetic (FM) interaction in planes, and emerge when the sodium ions become arranged in layers in between the CoO2 slabs. In the present research we have investigated ab-initio the appearance of such state in Na2/3CoO2. Towards this end in view we studied an interplay between the electronic coupling to the superstructure of the Na+ ions and local correlations of the itinerant d electrons treated within the GGA+U approximation. Employing the exact crystallographic supercell, the electronic organization has been analyzed upon increasing the energy U of the Coulomb repulsion within the 3d shells at T = 0. The metallic ground state, being a spin density wave with the inplane FM and antiferromagnetic interplane correlations, has been obtained and established to posses two regimes. When U &gt; 2 eV, a crossover develops from a uniform state of the d-lattice to the regular phase with the spin/charge disproportionation between the sites. In particular at the representative value U = 5 eV, the Co13+ sites with suppressed magnetism appears, while the spin-active Co4+ holes are accumulated by the Co2 sites. A related formation of an isolated, narrow conduction band at the Fermi level implies a considerable enhancement of the electron correlations in the crystal field imposed by the Na+ patterns.
    view abstractdoi: 10.1088/1742-6596/394/1/012019
  • 2012 • 34 Advancing density functional theory to finite temperatures: Methods and applications in steel design
    Hickel, T. and Grabowski, B. and Körmann, F. and Neugebauer, J.
    Journal of Physics Condensed Matter 24 (2012)
    The performance of materials such as steels, their high strength and formability, is based on an impressive variety of competing mechanisms on the microscopic/atomic scale (e.g. dislocation gliding, solid solution hardening, mechanical twinning or structural phase transformations). Whereas many of the currently available concepts to describe these mechanisms are based on empirical and experimental data, it becomes more and more apparent that further improvement of materials needs to be based on a more fundamental level. Recent progress for methods based on density functional theory (DFT) now makes the exploration of chemical trends, the determination of parameters for phenomenological models and the identification of new routes for the optimization of steel properties feasible. A major challenge in applying these methods to a true materials design is, however, the inclusion of temperature-driven effects on the desired properties. Therefore, a large range of computational tools has been developed in order to improve the capability and accuracy of first-principles methods in determining free energies. These combine electronic, vibrational and magnetic effects as well as structural defects in an integrated approach. Based on these simulation tools, one is now able to successfully predict mechanical and thermodynamic properties of metals with a hitherto not achievable accuracy. © 2012 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/24/5/053202
  • 2012 • 33 Basic properties of magnetic shape-memory materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Comtesse, D. and Elmers, H.-J. and Kallmayer, M.
    Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science 43 2891-2900 (2012)
    The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the lowtemperature martensite phase with body-centered-tetragonal (bct) or rather face-centeredtetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds. © The Minerals, Metals & Materials Society and ASM International 2011.
    view abstractdoi: 10.1007/s11661-011-0832-7
  • 2012 • 32 Carbon nanotube bolometer: Transport properties and noise characteristics
    Rod, I. and Wirtz, C. and Kazakova, O. and Panchal, V. and Cox, D. and Zähres, H. and Posth, O. and Lindner, J. and Meckenstock, R. and Farle, M.
    Solid State Phenomena 190 510-513 (2012)
    The temperature dependent resistance and the noise characteristics of an individual multiwall carbon nanotube (CNT) decorated with a finite number of magnetic nanocubes are investigated. We show that CNT is a highly sensitive bolometer and can enable measurements of magnetic resonance in a single nanoparticle. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 31 Electron transport in partially filled iron carbon nanotubes
    Migunov, V. and Li, Z.-A. and Spasova, M. and Farle, M.
    Solid State Phenomena 190 498-501 (2012)
    We report electron transport properties of iron filled multiwalled carbon nanotubes (MWCNT) with outer diameters of 30 to 80 nm and lengths of 1 to 10 μm. Our study is combined with a structural investigation of the iron core using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). It was found that high current densities of 1.8×107A/cm2 increase the conductivity of the MWCNT by a factor of two at 300 K, while the Fe core disappears probably forming defect states in the carbon shells. The enhanced diffusion of iron is most probably the result of local heating of the iron followed by implantation of iron atoms in the nanotube layers. © (2012) Trans Tech Publications.
    view abstractdoi: 10.4028/
  • 2012 • 30 Electronic and magnetic trends in martensitically transforming Fe-Pd alloys
    Gruner, M.E. and Entel, P. and Minár, J. and Polesya, S. and Mankovsky, S. and Ebert, H.
    Journal of Magnetism and Magnetic Materials 324 3524-3529 (2012)
    We discuss the compositional trends in the electronic structure of Fe-Pd alloys in the composition range relevant for martensitic transformations and magnetic shape-memory applications. The results are obtained within the framework of density functional theory based on the Korringa-Kohn-Rostoker Greens function approach in combination with the coherent potential approximation for the description of disorder. For the body centered cubic structure, we observe with increasing Pd content the gradual disappearance of the pseudogap like structure in the center of the minority spin d-band, while on the face centered cubic side a small peak passes the Fermi level around 80 at. % Fe, which has been related previously to a band-Jahn-Teller instability. In addition, we investigate the structural variation of magnetic properties of the magnetic shape-memory composition Fe 70Pd 30 based on magnetic exchange parameters obtained from our ab initio calculations being mapped onto a classical Heisenberg model. This allows us to estimate the magnetic transition temperature within a mean field approach and Monte Carlo simulations taking into account the induced nature of the Pd atoms, leading to a close, semi-quantitative agreement with experiment in the latter case. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.jmmm.2012.02.081
  • 2012 • 29 Magnetic switchable alginate beads
    Degen, P. and Leick, S. and Siedenbiedel, F. and Rehage, H.
    Colloid and Polymer Science 290 97-106 (2012)
    Calcium alginate beads are enclosed in a wide range of products including food, pharmaceuticals, and cosmetic formulations. The biopolymer matrix is often used to stabilize active ingredients and to provide a controlled release under well-defined conditions. In this context, it is of high interest to study the magnetic-induced attraction, elongation, and rupture of capsules or beads. In this work, we synthesized new types of magnetic switchable alginate beads. The magnetic sensitivity was achieved by incorporation of magnetic nanoparticles (MNPs) within the alginate gel. We measured the mechanical properties of single alginate beads in squeezing experiments, the evaporation of water and the magnetic sensitivity by stimulation of these beads in external fields. In all these measurements, the alginate and the nanoparticle concentration were systematically varied. We could show that the incorporation of MNPs generates a magnetic response of the beads and reduces the evaporation of water but has no influence on the mechanical stability of the beads during compression. Calculations of the shear modulus by means of the squeezing data result in good agreement in comparison to the shear moduli measured by rheological frequency sweep tests. With scanning electron microscopy, we could analyze the molecular structure of such composite systems, and we observed a homogeneous distribution of the MNPs within the gel matrix. © Springer-Verlag 2011.
    view abstractdoi: 10.1007/s00396-011-2524-7
  • 2012 • 28 Probing the timescale of the exchange interaction in a ferromagnetic alloy
    Mathias, S. and La-O-Vorakiat, C. and Grychtol, P. and Granitzka, P. and Turgut, E. and Shaw, J.M. and Adam, R. and Nembach, H.T. and Siemens, M.E. and Eich, S. and Schneider, C.M. and Silva, T.J. and Aeschlimann, M. and Murnane, ...
    Proceedings of the National Academy of Sciences of the United States of America 109 4792-4797 (2012)
    The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies.
    view abstractdoi: 10.1073/pnas.1201371109
  • 2012 • 27 Synthesis of hard magnetic ordered mesoporous Co3O 4/CoFe2O4 nanocomposites
    Tüysüz, H. and Salabaş, E.L. and Bill, E. and Bongard, H. and Spliethoff, B. and Lehmann, C.W. and Schüth, F.
    Chemistry of Materials 24 2493-2500 (2012)
    The nanocomposite Co3O4/CoFe2O4 heterostructured mesoporous material was produced via a simple solid-solid reaction of an iron precursor with ordered mesoporous Co3O 4 that had been prepared via nanocasting from mesoporous silica as hard template. The magnetic behavior of the exchange-coupled antiferromagnetic/ferrimagnetic (AFM/FM) system was investigated via superconducting quantum interference device (SQUID) magnetometry and 57Fe Mössbauer spectroscopy. The low-temperature magnetization loops of the Co3O4/CoFe2O4 heterostructure present exchange bias under cooling in an applied magnetic field. The antiferromagnetic ordering temperature of Co3O4 is increased due to the proximity of the hard magnetic CoFe2O 4 phase. The nanocomposite Co3O4/CoFe 2O4 behaves as an exchange coupled system with a cooperative magnetic switching. © 2012 American Chemical Society.
    view abstractdoi: 10.1021/cm3005166
  • 2012 • 26 Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
    Rudolf, D. and La-O-Vorakiat, C. and Battiato, M. and Adam, R. and Shaw, J.M. and Turgut, E. and Maldonado, P. and Mathias, S. and Grychtol, P. and Nembach, H.T. and Silva, T.J. and Aeschlimann, M. and Kapteyn, H.C. and Murnane, M...
    Nature Communications 3 (2012)
    Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferro-or antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers. © 2012 Macmillan Publishers Limited. All rights reserved.
    view abstractdoi: 10.1038/ncomms2029
  • 2011 • 25 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 abstractdoi: 10.1038/ncomms1538
  • 2011 • 24 An incremental variational formulation of dissipative magnetostriction at the macroscopic continuum level
    Miehe, C. and Kiefer, B. and Rosato, D.
    International Journal of Solids and Structures 48 1846-1866 (2011)
    This paper outlines a new variational-based modeling and computational implementation of macroscopic continuum magneto-mechanics involving non-linear, inelastic material behavior, with a special focus on dissipative magnetostriction. It is based on a constitutive variational principle that optimizes a generalized incremental work function with respect to the internal state variables. In an incremental setting at finite time steps, this variational problem defines a quasi-hyper-magnetoelastic potential for the stresses and the magnetic induction, and incorporates energy storage as well as dissipative mechanisms. The existence of this potential further allows the incremental boundary-value problem of quasi-static inelastic magneto-mechanics to be recast into a principle of stationary incremental energy. The second focus of this paper is on the careful construction of the energy storage and dissipation functions for the model problem of hysteretic magnetostriction at the macroscopic level. It is then demonstrated that the proposed model is capable of predicting the ferromagnetic and field-induced strain hysteresis curves characteristic of magnetostrictive material response in good agreement with experiments. The numerical solution of the coupled non-linear boundary-value problem is based on a monolithic multi-field finite element implementation. As a consequence of the proposed incremental variational principle, the discretization of the multi-field problem appears in a compact symmetric format. In this sense, the proposed formulation provides a canonical framework for the simulation of boundary-value-problems in dissipative magnetostriction at the macro-level. The performance of the proposed algorithm is tested by application to relevant numerical examples. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.ijsolstr.2011.02.011
  • 2011 • 23 Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes
    Kronast, F. and Friedenberger, N. and Ollefs, K. and Gliga, S. and Tati-Bismaths, L. and Thies, R. and Ney, A. and Weber, R. and Hassel, C. and Römer, F.M. and Trunova, A.V. and Wirtz, C. and Hertel, R. and Dürr, H.A. and Farle, M.
    Nano Letters 11 1710-1715 (2011)
    Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/nl200242c
  • 2011 • 22 In situ analysis of three-dimensional electrolyte convection evolving during the electrodeposition of copper in magnetic gradient fields
    Tschulik, K. and Cierpka, C. and Gebert, A. and Schultz, L. and Kähler, C.J. and Uhlemann, M.
    Analytical Chemistry 83 3275-3281 (2011)
    A novel three-dimensional particle tracking velocimetry technique was used to examine the flow during electrodeposition of Cu. For the first time electrode-normal, circumferential, and radial velocities were spatially resolved during deposition in superimposed low and high magnetic gradient fields. In this way the complex interaction of magnetic field gradient force and Lorentz force induced convective effects could be measured and analyzed. Magnetic field gradient force induced electrolyte flow was detected only in high gradient magnetic fields, and it was found to be directed toward regions of gradient maxima. Since this electrode-normal flow causes enhanced transport of Cu 2+ ions from the bulk electrolyte to those regions of the working electrode where maxima of magnetic gradients are present, a structured deposit is formed during diffusion-limited electrodeposition. Lorentz force driven convection was observed during deposition in the low and the high magnetic gradient experiments. The overall fluid motion and the convection near the working electrode were determined experimentally and discussed with regard to the acting magnetic forces and numerical simulations. © 2011 American Chemical Society.
    view abstractdoi: 10.1021/ac102763m
  • 2011 • 21 Magnetic bond-order potential for iron
    Mrovec, M. and Nguyen-Manh, D. and Elsässer, C. and Gumbsch, P.
    Physical Review Letters 106 (2011)
    We present a magnetic bond-order potential (BOP) that is able to provide a correct description of both directional covalent bonds and magnetic interactions in iron. This potential, based on the tight binding approximation and the Stoner model of itinerant magnetism, forms a direct bridge between the electronic-structure and the atomistic modeling hierarchies. Even though BOP calculations are computationally more demanding than those using common empirical potentials, the formalism can be used for studies of complex defect configurations in large atomic ensembles exceeding 105 atoms. Our studies of dislocations in α-Fe demonstrate that correct descriptions of directional covalent bonds and magnetism are crucial for a reliable modeling of these defects. © 2011 American Physical Society.
    view abstractdoi: 10.1103/PhysRevLett.106.246402
  • 2011 • 20 Magnetic field effects on the active dissolution of iron
    Sueptitz, R. and Tschulik, K. and Uhlemann, M. and Schultz, L. and Gebert, A.
    Electrochimica Acta 56 5866-5871 (2011)
    The impact of superimposed magnetic fields on the behavior of iron in 0.05 M H2SO4 at low anodic polarization was investigated by means of potentiostatic polarization measurements. Significant magnetic field effects were observed even though the active dissolution reaction in the investigated potential region is formally charge transfer controlled. The current density can be enhanced or reduced dependent on the magnetic field to electrode configuration. The results are discussed in terms of the magnetic field impact on the surface pH value during the anodic dissolution. Our findings are likely to have important consequences for the life-time prediction of ferromagnetic components in electromagnetic devices and for future studies on magneto-electrodeposition processes. © 2011 Elsevier Ltd. All Rights Reserved.
    view abstractdoi: 10.1016/j.electacta.2011.04.126
  • 2011 • 19 Magnetic properties of EuS spin filter tunnel contacts to silicon
    Müller, M. and Schreiber, R. and Schneider, C.M.
    Journal of Applied Physics 109 (2011)
    We investigate the magnetic properties of the ferromagnetic insulator EuS in view of its potential in spin-filter tunnel contacts to silicon. We prepared thin polycrystalline EuS films directly on (001) oriented Si substrates that show well-defined magnetic properties down to the monolayer regime. Addressing the question of magnetic coupling between a EuS magnetic tunnel barrier and a CoOCo magnetic electrode, we succeeded in realizing an independent magnetic switching behavior in this spin-valve-type system. These results are important prerequisites for future spin-dependent transport experiments. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3549609
  • 2011 • 18 Photomodulation of the magnetisation of Co nanocrystals decorated with Rhodamine B
    Comesaña-Hermo, M. and Estivill, R. and Ciuculescu, D. and Amiens, C. and Farle, M. and Batat, P. and Jonusauskas, G. and McClenaghan, N.D. and Lecante, P. and Tardin, C. and Mazeres, S.
    ChemPhysChem 12 2915-2919 (2011)
    How exciting! Upon excitation of Rhodamine B with visible light in magnetic Co nanocrystal-Rhodamine B nanocomposites, electron transfer from the nanocrystal to the dye is evidenced as well as an increase in magnetisation (see picture), affording a new access to photomodulation of the magnetic properties of nanocrystal assemblies. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201100616
  • 2011 • 17 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 57-60 (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 abstractdoi: 10.1063/1.3601785
  • 2011 • 16 Structural and magnetic characterization of self-assembled iron oxide nanoparticle arrays
    Benitez, M.J. and Mishra, D. and Szary, P. and Badini Confalonieri, G.A. and Feyen, M. and Lu, A.H. and Agudo, L. and Eggeler, G. and Petracic, O. and Zabel, H.
    Journal of Physics Condensed Matter 23 (2011)
    We report about a combined structural and magnetometric characterization of self-assembled magnetic nanoparticle arrays. Monodisperse iron oxide nanoparticles with a diameter of 20nm were synthesized by thermal decomposition. The nanoparticle suspension was spin-coated on Si substrates to achieve self-organized arrays of particles and subsequently annealed at various conditions. The samples were characterized by x-ray diffraction, and bright and dark field high resolution transmission electron microscopy. The structural analysis is compared to magnetization measurements obtained by superconducting quantum interference device magnetometry. We can identify either multi-phase FexO/γ-Fe2O3 or multi-phase Fe xO/Fe3O4 nanoparticles. The Fe xO/γ-Fe2O3 system shows a pronounced exchange bias effect which explains the peculiar magnetization data found for this system. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/12/126003
  • 2011 • 15 Structural, ferroelectric and magnetic properties of Bi 0.85Sm0.15FeO3 perovskite
    Khomchenko, V.A. and Paixão, J.A. and Costa, B.F.O. and Karpinsky, D.V. and Kholkin, A.L. and Troyanchuk, I.O. and Shvartsman, V.V. and Borisov, P. and Kleemann, W.
    Crystal Research and Technology 46 238-242 (2011)
    Room temperature crystal structure, ferroelectric and magnetic properties of polycrystalline Bi0.85Sm0.15FeO3 samples were investigated. X-ray diffraction study shows that the compound possesses a dominant PbZrO3-like orthorhombic structure with √2a× 2√2a × √2a superlattice (a is the parameter of the cubic perovskite subcell). In contrast to piezoresponse force microscopy data demonstrating some features characteristic of ferroelectrics, polarization vs. electric field measurements reveal the behavior expected for nonpolar materials. Investigation of magnetic properties confirms that 15% samarium substitution suppresses the spin modulation typical of BiFeO3 and induces the appearance of spontaneous magnetization. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/crat.201100040
  • 2011 • 14 The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Gebhardt, T. and Music, D. and Ekholm, M. and Abrikosov, I.A. and Vitos, L. and Dick, A. and Hickel, T. and Neugebauer, J. and Schneider, J.M.
    Journal of Physics Condensed Matter 23 (2011)
    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Néel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/23/24/246003
  • 2011 • 13 Time-resolved measurements of Ni80Fe20/MgO/Co trilayers in the extreme ultraviolet range
    Adam, R. and Grychtol, P. and Cramm, S. and Schneider, C.M.
    Journal of Electron Spectroscopy and Related Phenomena 184 291-295 (2011)
    We performed an element-selective magneto-optic characterization of Ni 80Fe20/MgO/Co magnetic trilayers employing the resonant magnetic reflectivity of extreme ultraviolet (XUV) radiation tuned to the M absorption edges of cobalt (60.2 eV) and nickel (67.5 eV). Static reflectivity shows a large magnetic contrast of up to 80% for the top Co and 20% for the buried Ni80Fe20 layers. The magneto-dynamic response of the trilayers to the ultrashort field pulse exhibits oscillations in a frequency range of up to 6.5 GHz associated exclusively with magnetization dynamics of the top Co layer. The presented results demonstrate the feasibility of element-specific magneto-dynamic studies of magnetic multilayers employing resonant XUV reflectivity at the M absorption edges. © 2010 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elspec.2010.12.017
  • 2011 • 12 Visualization of spin dynamics in single nanosized magnetic elements
    Banholzer, A. and Narkowicz, R. and Hassel, C. and Meckenstock, R. and Stienen, S. and Posth, O. and Suter, D. and Farle, M. and Lindner, J.
    Nanotechnology 22 (2011)
    The design of future spintronic devices requires a quantitative understanding of the microscopic linear and nonlinear spin relaxation processes governing the magnetization reversal in nanometer-scale ferromagnetic systems. Ferromagnetic resonance is the method of choice for a quantitative analysis of relaxation rates, magnetic anisotropy and susceptibility in a single experiment. The approach offers the possibility of coherent control and manipulation of nanoscaled structures by microwave irradiation. Here, we analyze the different excitation modes in a single nanometer-sized ferromagnetic stripe. Measurements are performed using a microresonator set-up which offers a sensitivity to quantitatively analyze the dynamic and static magnetic properties of single nanomagnets with volumes of (100nm)3. Uniform as well as non-uniform volume modes of the spin wave excitation spectrum are identified and found to be in excellent agreement with the results of micromagnetic simulations which allow the visualization of the spatial distribution of these modes in the nanostructures. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/29/295713
  • 2010 • 11 A comparative study of (Fe, Fe3Si)/GaAs and Heusler/MgO for spintronics applications
    Grünebohm, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    Many Heusler-like compounds as well as elemental Fe show large spin polarization in their bulk phase, which makes them possible candidates for spintronic applications. However, it has turned out that the magnetic properties may change significantly if the ferromagnet is grown on a semiconductor. Here, we investigate from first principles the magnetic properties of thin Fe, Fe 3Si, and Heusler films on GaAs(110) and MgO(001) substrates, which show rather flat interfaces due to the absence of reconstruction. We observe high spin polarization for the Fe containing systems and both Heusler alloys considered here, even for ultra thin layers. These systems are mainly unaffected by the substrate. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1742-6596/200/7/072038
  • 2010 • 10 Ab initio study of the anomalous volume-composition dependence in Fe-Al alloys
    Friák, M. and Neugebauer, J.
    Intermetallics 18 1316-1321 (2010)
    The experimentally observed anomalous compositional dependence of the lattice constant of Fe-Al crystals has been theoretically investigated employing density functional theory (DFT) within the generalized gradient approximation (GGA). The formation energies, equilibrium volumes and magnetic states have been determined for a dense set of different aluminium concentrations and a large variety of atomic configurations. The spin-polarized calculations for Fe-rich compounds reproduce very well the anomalous lattice-constant behavior in contrast to both the nonmagnetic and fixed-spin-moment calculations that result in nearly linear trends without any anomaly. We thus identify the change in magnetism of iron atoms as caused by an increasing number of Al atoms in the first coordination spheres to be the decisive driving force of the anomalous behavior. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.intermet.2010.03.014
  • 2010 • 9 Charge ordering of magnetic dipoles in artificial honeycomb patterns
    Schumann, A. and Sothmann, B. and Szary, P. and Zabel, H.
    Applied Physics Letters 97 (2010)
    Artificial spin ice offers the possibility to investigate a variety of dipolar orderings, spin frustrations and ground states. We have investigated magnetic dipoles arranged on a honeycomb lattice as a function of applied field, using magnetic force microscopy. The patterns were prepared by electron beam lithography where the basic units are polycrystalline Fe islands with dimensions length, width, and thickness of 3 μm, 0.3 μm, and 20 nm, respectively. These islands are in a single domain dipolar state at remanence. We have measured the magnetization reversal of the honeycomb patterns with different field directions. For the easy direction with the field parallel to one of the three dipole sublattices we observe at coercivity a maximum of magnetic charge order of alternating charges ±3, where the magnetic charge refers to the number and sign of magnetic poles pointing into any of the vertices. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3463482
  • 2010 • 8 Electrocrystallisation of metallic films under the influence of an external homogeneous magnetic field - Early stages of the layer growth
    Koza, J.A. and Mogi, I. and Tschulik, K. and Uhlemann, M. and Mickel, C. and Gebert, A. and Schultz, L.
    Electrochimica Acta 55 6533-6541 (2010)
    The effects of a uniform magnetic field on the early stages of Ag, Fe and CoFe alloys electrocrystallisation have been investigated. It was found for Fe and CoFe alloys, irrespective of the applied parameters, that early stages of the layer growth can be characterised by a nucleation and 3D diffusion controlled growth. The influence of the deposition parameters on the nucleation behaviour was studied on the basis of the Sharifker-Mostany (SM) model. A modification to the existing model has been proposed in order to model alloy systems. It is reported that a magnetic field superposed parallel to the electrode surface has a significant influence on the early stages of Fe and CoFe alloys growth. The growth of the nuclei is enhanced by the Lorentz-force-driven convection, while the nucleation processes remain unaffected. The hydrodynamic origin of these phenomena is confirmed by independent rotating disk electrode (RDE) investigations. Moreover, the proposed mechanism of a magnetic field influence on the 3D diffusion controlled growth is supported by a microscopic investigation of Ag deposits. It was found that Ag deposits obtained without a magnetic field superposition are characterised by a relatively large number of small 3D growth centres, whereas the deposits obtained in a field show fewer 3D centres but their size is greatly increased. © 2010 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.06.026
  • 2010 • 7 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 96-99 (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 abstractdoi: 10.1016/j.chemphys.2010.08.020
  • 2010 • 6 Magnetic alloy nanoparticles from laser ablation in cyclopentanone and their embedding into a photoresist
    Jakobi, J. and Petersen, S. and Menéndez-Manjón, A. and Wagener, P. and Barcikowski, S.
    Langmuir 26 6892-6897 (2010)
    The generation of nonoxidized magnetic alloy nanoparticles is still a challenge using conventional chemical reduction methods. However, because these nanoparticles are currently attracting much attention, alternative methods are required. In this context, the applicability of femtosecond laser ablation, which has evolved as a powerful tool for the generation of colloidal metal nanoparticles, has been investigated using the example of Ni48Fe 52 and Sm2Co17 ablation in cyclopentanone. Besides stability and size measurements, the focus has been placed on the analysis of the elemental composition of nanoparticles, which proved the preservation of the stoichiometry of the target in Ni-Fe nanoparticles but not in Sm-Co. It is assumed that this is due to a greater difference in the heat of evaporation of the bulk alloy components in Sm-Co than in Ni-Fe. Hence, the successful generation of magnetic alloy nanoparticles is possible for alloys composed of elements with similar heats of evaporation. This one-step approach allows the fabrication of nanomagnetic polymer composites (e.g., with application prospects in microtechnology such as microactuators). © 2010 American Chemical Society.
    view abstractdoi: 10.1021/la101014g
  • 2010 • 5 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 abstractdoi: 10.1088/1742-6596/200/7/072105
  • 2010 • 4 On the action of magnetic gradient forces in micro-structured copper deposition
    Mutschke, G. and Tschulik, K. and Weier, T. and Uhlemann, M. and Bund, A. and Fröhlich, J.
    Electrochimica Acta 55 9060-9066 (2010)
    In order to shed more light on the role of magnetic gradient forces and Lorentz forces on the deposition pattern found recently at copper electrodes, experiments and numerical simulations have been performed in a simple geometry that consists of a single small cylindrical permanent magnet which is placed behind the cathode. The cylinder axis coincides with the magnetization direction and points normal to the electrode surface. The electrode is oriented vertically which allows a separate discussion of the influence of both forces. Experiments and numerical simulations are found to give very good qualitative agreement with respect to the deposition pattern. Our analysis clearly shows that the major influence is due to the action of the magnetic gradient force. Numerical simulations prove that the separate action of the Lorentz force does not reproduce the deposition structure. A detailed analytical discussion of the motion forced by the different magnetic forces in superposition with natural convection is given. © 2010 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.08.046
  • 2010 • 3 Structure, lattice dynamics and Fermi surface of the magnetic shape memory system Co-Ni-Ga from first principles calculations
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Entel, P.
    Physics Procedia 10 138-143 (2010)
    Advanced magnetic shape memory materials like the prototypical Ni-Mn-Ga alloy system are limited to operating temperatures that are too low for many practical applications. To overcome this problem, an intensive search for new magnetic shape memory compounds has been started. One interesting system, showing magnetic as well as conventional shape memory behavior, is Co- Ni-Ga. In this work we report systematic studies of stoichiometric Co-Ni-Ga based alloys in the full and inverse Heusler structure by means of density functional theory. A prediction of the martensitic transition temperatures can be obtained by the structural energy differences calculated for different crystal structures. In prototype, near-stoichiometric Ni-Mn-Ga, the (pre-)martensitic transformation is accompanied by an anomalous softening of one transversal acoustic phonon branch along the [110] direction which has been frequently linked to nesting features of the Fermi surface in the past. In order to clarify this aspect for the Co-Ni- Ga system, we will discuss the influence of structure on the phonon dispersions determined from first principles and investigate whether the Fermi surface of the Co-Ni-Ga compound reveals nesting features as well.
    view abstractdoi: 10.1016/j.phpro.2010.11.089
  • 2010 • 2 Studies on the patterning effect of copper deposits in magnetic gradient fields
    Tschulik, K. and Sueptitz, R. and Koza, J. and Uhlemann, M. and Mutschke, G. and Weier, T. and Gebert, A. and Schultz, L.
    Electrochimica Acta 56 297-304 (2010)
    Electrochemical Cu deposition was performed from electrolytes containing 0.1-1 M CuSO4 in a well-defined magnetic gradient field. Patterned deposits resulted in all cases showing a strong correlation to this gradient field. It was observed that with increasing Cu2+ concentration the structuring effect decreases in terms of differences in topography and morphology. An explanation of this effect is presented based on local convection induced by the magnetic field gradient force which is dependent on the concentration gradient established during the deposition. Superimposed effects of Lorentz force driven convection were observed for high Cu2+ concentrations, and their influence on the deposition process was discussed. © 2010 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2010.08.080
  • 2009 • 1 Structural, magnetic and superconducting phase transitions in CaFe2As2 under ambient and applied pressure
    Canfield, P.C. and Bud'ko, S.L. and Ni, N. and Kreyssig, A. and Goldman, A.I. and McQueeney, R.J. and Torikachvili, M.S. and Argyriou, D.N. and Luke, G. and Yu, W.
    Physica C: Superconductivity and its Applications 469 404-412 (2009)
    At ambient pressure CaFe2As2 has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T ∼ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ∼0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ∼1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe2As2 transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures. © 2009 Elsevier B.V.
    view abstractdoi: 10.1016/j.physc.2009.03.033