Dr. Markus Ernst Gruner

Theoretical Physics
University of Duisburg-Essen

Contact

Hub
  • Influence of hydrogenation on the vibrational density of states of magnetocaloric LaFe11.4Si1.6 H1.6
    Terwey, A. and Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Eggert, B. and Ollefs, K. and Brabänder, V. and Radulov, I. and Skokov, K. and Faske, T. and Hu, M.Y. and Zhao, J. and Alp, E.E. and Giacobbe, C. and Gutfleisch, O. and Wende, H.
    Physical Review B 101 (2020)
    We report on the impact of magnetoelastic coupling on the magnetocaloric properties of LaFe11.4Si1.6H1.6 in terms of the vibrational (phonon) density of states (VDOS), which we determined with Fe57 nuclear resonant inelastic X-ray scattering (NRIXS) measurements and with density functional theory (DFT) based first-principles calculations in the ferromagnetic (FM) low-temperature and paramagnetic (PM) high-temperature phase. In experiments and calculations, we observe pronounced differences in the shape of the Fe-partial VDOS between nonhydrogenated and hydrogenated samples. This shows that hydrogen not only shifts the temperature of the first-order phase transition, but also affects the elastic response of the Fe subsystem significantly. In turn, the anomalous redshift of the Fe VDOS, observed by going to the low-volume PM phase, survives hydrogenation. As a consequence, the change in the Fe-specific vibrational entropy ΔSlat across the phase transition has the same sign as the magnetic and electronic contribution. DFT calculations show that the same mechanism, which is a consequence of the itinerant electron metamagnetism associated with the Fe subsystem, is effective in both the hydrogenated and the hydrogen-free compounds. Although reduced by 50% as compared to the hydrogen-free system, the measured change ΔSlat of (3.2±1.9)JkgK across the FM-to-PM transition contributes with ∼35% significantly and cooperatively to the total isothermal entropy change ΔSiso. Hydrogenation is observed to induce an overall blueshift of the Fe VDOS with respect to the H-free compound; this effect, together with the enhanced Debye temperature observed, is a fingerprint of the hardening of the Fe sublattice by hydrogen incorporation. In addition, the mean Debye velocity of sound of LaFe11.4Si1.6H1.6 was determined from the NRIXS and the DFT data. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.101.064415
  • Interface-related magnetic and vibrational properties in Fe/MgO heterostructures from nuclear resonant spectroscopy and first-principles calculations
    Eggert, B. and Gruner, M.E. and Ollefs, K. and Schuster, E. and Rothenbach, N. and Hu, M.Y. and Zhao, J. and Toellner, T.S. and Sturhahn, W. and Pentcheva, R. and Cuenya, B.R. and Alp, E.E. and Wende, H. and Keune, W.
    Physical Review Materials 4 (2020)
    We combine Fe57 Mössbauer spectroscopy and Fe57 nuclear resonant inelastic x-ray scattering (NRIXS) on nanoscale polycrystalline [bcc-Fe57/MgO] multilayers with various Fe-layer thicknesses and layer-resolved density-functional-theory (DFT)-based first-principles calculations of a (001)-oriented [Fe(8 ML)/MgO(8 ML)](001) heterostructure (where ML denotes monolayer) to unravel the interface-related atomic vibrational properties of a multilayer system. Being consistent in theory and experiment, we observe enhanced hyperfine magnetic fields Bhf in the multilayers as compared to Bhf in bulk bcc Fe; this effect is associated with the Fe/MgO interface layers. NRIXS and DFT both reveal a strong reduction of the longitudinal acoustic phonon peak in combination with an enhancement of the low-energy vibrational density of states (VDOS) suggesting that the presence of interfaces and the associated increase in the layer-resolved magnetic moments results in drastic changes in the Fe-partial VDOS. From the experimental and calculated VDOS, vibrational thermodynamic properties have been determined as a function of Fe thickness and were found to be in excellent agreement. © 2020 American Physical Society.
    view abstract10.1103/PhysRevMaterials.4.044402
  • Dynamics of optical excitations in a Fe/MgO(001) heterostructure from time-dependent density functional theory
    Gruner, M.E. and Pentcheva, R.
    Physical Review B 99 (2019)
    In the framework of real-time time-dependent density functional theory we unravel the layer-resolved dynamics of excited carriers in a (Fe)1/(MgO)3(001) multilayer after an optical excitation with a frequency below the band gap of bulk MgO. Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are mainly observed for in-plane polarized electric fields, corresponding to a laser pulse arriving perpendicular to the interface. While the strongest charge redistribution takes place in the Fe layer, a time-dependent change in the occupation numbers is visible in all layers, mediated by the presence of interface states. The time evolution of the layer-resolved time-dependent occupation numbers indicates a strong orbital dependence with the depletion from in-plane orbitals (e.g., dx2-y2 of Fe) and accumulation in out-of-plane orbitals (d3z2-r2 of Fe and pz of apical oxygen). We also observe a small net charge transfer of less than one percent of an electron away from oxygen towards the Mg sites, even for MgO layers which are not directly in contact with the metallic Fe. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.195104
  • Epitaxial strain adaptation in chemically disordered FeRh thin films
    Witte, R. and Kruk, R. and Wang, D. and Schlabach, S. and Brand, R.A. and Gruner, M.E. and Wende, H. and Hahn, H.
    Physical Review B 99 (2019)
    Strain and strain adaptation mechanisms in modern functional materials are of crucial importance for their performance. Understanding these mechanisms will advance innovative approaches for material properties engineering. Here we study the strain adaptation mechanism in a thin film model system as a function of epitaxial strain. Chemically disordered FeRh thin films are deposited on W-V buffer layers, which allow for large variation of the preset lattice constants, e.g., epitaxial boundary condition. It is shown by means of high-resolution x-ray reciprocal space maps and transmission electron microscopy that the system reacts with a tilting mechanism of the structural units in order to adapt to the lattice constants of the buffer layer. This response is explained by density functional theory calculations, which evidence an energetic minimum for structures with a distortion of c/a≈0.87. The experimentally observed tilting mechanism is induced by this energy gain and allows the system to remain in the most favorable structure. In general, it is shown that the use of epitaxial model heterostructures consisting of alloy buffer layers of fully miscible elements and the functional material of interest allows to study strain adaptation behaviors in great detail. This approach makes even small secondary effects observable, such as the directional tilting of the structural domains identified in the present case study. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.99.134109
  • Large thermopower anisotropy in PdCo O2 thin films
    Yordanov, P. and Sigle, W. and Kaya, P. and Gruner, M.E. and Pentcheva, R. and Keimer, B. and Habermeier, H.-U.
    Physical Review Materials 3 (2019)
    Motivated by recent theoretical studies predicting a large thermopower anisotropy in the layered delafossite PdCoO2, we have used pulsed laser deposition to synthesize thin films on (0001)-oriented and offcut Al2O3 substrates. By combining transport measurements on films with different offcut angles, tensor rotation relations, and an iterative fit procedure for the transport parameters, we have determined the resistivity and the thermopower along the main crystallographic axes in the temperature range 300-815 K. The data reveal a small positive Seebeck coefficient along the delafossite planes and a large negative Seebeck coefficient perpendicular to the planes, in excellent agreement with density functional calculations in the presence of moderate Coulomb correlations. The methodology introduced here is generally applicable for measurements of the thermoelectric properties of materials with highly anisotropic electronic structures. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.085403
  • Microscopic nonequilibrium energy transfer dynamics in a photoexcited metal/insulator heterostructure
    Rothenbach, N. and Gruner, M.E. and Ollefs, K. and Schmitz-Antoniak, C. and Salamon, S. and Zhou, P. and Li, R. and Mo, M. and Park, S. and Shen, X. and Weathersby, S. and Yang, J. and Wang, X.J. and Pentcheva, R. and Wende, H. and Bovensiepen, U. and Sokolowski-Tinten, K. and Eschenlohr, A.
    Physical Review B 100 (2019)
    The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]n as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield comprehensive insight into the microscopic processes leading to local relaxation within a single constituent or nonlocal relaxation between two constituents. Besides electronic excitations in Fe, which are monitored at the Fe L3 absorption edge and relax within 1 ps by electron-phonon coupling, soft X-ray analysis identifies a change at the oxygen K absorption edge of the MgO layers which occurs within 0.5 ps. This ultrafast energy transfer across the Fe-MgO interface is mediated by high-frequency, interface vibrational modes, which are excited by hot electrons in Fe and couple to vibrations in MgO in a mode-selective, nonthermal manner. A second, slower timescale is identified at the oxygen K pre-edge and the Fe L3 edge. The slower process represents energy transfer by acoustic phonons and contributes to thermalization of the entire heterostructure. We thus find that the interfacial energy transfer is associated with nonequilibrium behavior in the phonon system. Because our experiments lack signatures of charge transfer across the interface, we conclude that phonon-mediated processes dominate the competition of electronic and lattice excitations in these nonlocal, nonequilibrium dynamics. © 2019 American Physical Society.
    view abstract10.1103/PhysRevB.100.174301
  • Role of the exchange-correlation functional on the structural, electronic, and optical properties of cubic and tetragonal SrTiO3 including many-body effects
    Begum, V. and Gruner, M.E. and Pentcheva, R.
    Physical Review Materials 3 (2019)
    SrTiO3 is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the TiO6 octahedra. Here we study systematically the effect of different exchange-correlation functionals on the structural, electronic, and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard U term (SCAN+U, U=7.45 eV) allows us to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within GGA+U the gap is underestimated even for high U values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the GW approximation (single-shot G0W0) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to G0W0+BSE. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.065004
  • Segregation tendency of Heusler alloys
    Sokolovskiy, V.V. and Gruner, M.E. and Entel, P. and Acet, M. and Çaklr, A. and Baigutlin, D.R. and Buchelnikov, V.D.
    Physical Review Materials 3 (2019)
    Segregation in a series of Ni2Mn1+x(In,Sn,Ga,Al)1-x and Mn2Ni1+x(Ga, Al) as well as Ni2+xMn1-xGa Heusler alloys is studied by first-principles calculations. We show that Mn-rich Ni2Mn1+x(In,Sn,Al)1-x compounds are at low temperatures unstable in the whole concentration range against decomposition into a dual-phase system consisting of an L21-cubic Ni2MnX phase with ferromagnetic order and an L10-tetragonal NiMn phase ordered antiferromagnetically. In contrast, Ni2Mn1+xGa1-x and Mn2Ni1+x(Ga,Al)1-x are stable in the narrow concentration range near the 2-1-1 stoichiometry. This concentration range depends on the presence of a martensitic transformation and becomes wider with increasing energy difference between austenite and martensite phases in a parent system. We find that ferromagnetic Ni-rich Ni2+xMn1-xGa is stable in the concentration range 0<x≤0.6. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.084413
  • Tuning the magnetic and magnetocaloric properties of austenitic Ni-Mn-(In,Sn)Heuslers
    Cavazzini, G. and Cugini, F. and Gruner, M.E. and Bennati, C. and Righi, L. and Fabbrici, S. and Albertini, F. and Solzi, M.
    Scripta Materialia 170 (2019)
    In this work, we highlight the occurrence of different physical mechanisms that independently control the saturation magnetization and the ferro-paramagnetic transition temperature of Ni-Mn-based Heusler compounds, opening new possibilities in mastering the functional properties of this wide class of magnetic materials. We present the magnetic, structural and magnetocaloric features of a complete Ni48Mn36In16−xSnx (x = 0–16)series. The observed different trends of the critical temperature and of the saturation magnetization on varying the Sn to In ratio are discussed with the help of first-principles calculations of the electronic structure and magnetic interactions of the compound. © 2019 Elsevier Ltd
    view abstract10.1016/j.scriptamat.2019.05.027
  • Determining the vibrational entropy change in the giant magnetocaloric material LaFe11.6 Si1.4 by nuclear resonant inelastic x-ray scattering
    Landers, J. and Salamon, S. and Keune, W. and Gruner, M.E. and Krautz, M. and Zhao, J. and Hu, M.Y. and Toellner, T.S. and Alp, E.E. and Gutfleisch, O. and Wende, H.
    Physical Review B 98 (2018)
    Magnetocaloric LaFe13-xSix-based compounds belong to the outstanding materials with potential for efficient solid-state refrigeration. We have performed temperature-dependent Fe57 nuclear resonant inelastic x-ray scattering measurements (in a field μ0H of ∼0.7 T) of the vibrational (phonon) density of states, VDOS, in LaFe11.6Si1.4 across the metamagnetic isostructural first-order phase transition at TC∼192 K from the low-temperature ferromagnetic (FM) to the high-temperature paramagnetic (PM) phase, in order to determine the change in thermodynamic properties of the Fe lattice at TC. The experimental results are compared with density-functional-theory-based first-principles calculations using the fixed-spin moment approach. Our combined experimental and theoretical results reveal distinct and abrupt changes in the VDOS of the Fe sublattice across TC, occurring within a small temperature interval of ΔT≤12 K around TC. This indicates that strong magnetoelastic coupling (at the atomic scale) is present up to TC, leading to a pronounced lattice softening (phonon redshift) in the PM phase. These changes originate from the itinerant electron magnetism associated with Fe and are correlated with distinct modifications in the Fe-partial electronic density of states D(EF) at the Fermi energy EF. From the experimental VDOS we can infer an abrupt increase (jump) in the Fe-partial vibrational entropy ΔSvib of +6.9±2.6 J/(kg K) and in the vibrational specific heat ΔCvib of +2.7±1.6 J/(kg K) upon heating. The increase in magnitude of the vibrational entropy |ΔSvib|=6.9 J/(kg K) of the Fe sublattice at TC upon heating is substantial, if compared with the magnitude of the isothermal entropy change |ΔSiso| of 14.2 J/(kg K) in a field change ΔB from 0 to 1 T, as obtained from isothermal magnetization measurements on our sample and using the Maxwell relation. We demonstrate that ΔSvib obtained by nuclear resonant inelastic x-ray scattering is a sizable quantity and contributes directly and cooperatively to the total entropy change ΔSiso at the phase transition of LaFe13-xSix. © 2018 American Physical Society.
    view abstract10.1103/PhysRevB.98.024417
  • Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications
    Scheibel, F. and Gottschall, T. and Taubel, A. and Fries, M. and Skokov, K.P. and Terwey, A. and Keune, W. and Ollefs, K. and Wende, H. and Farle, M. and Acet, M. and Gutfleisch, O. and Gruner, M.E.
    Energy Technology 6 (2018)
    Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first-order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First-order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first-order magnetocaloric materials such as Ni–Mn-based Heusler alloys, FeRh, La(FeSi)13-based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size- and time-dependence of the MCE at magnetostructural transitions. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ente.201800264
  • Magnetocaloric materials for refrigeration near room temperature
    Waske, A. and Gruner, M.E. and Gottschall, T. and Gutfleisch, O.
    MRS Bulletin 43 (2018)
    This article overviews the current status of magnetocaloric materials for room-temperature refrigeration. We discuss the underlying mechanism of the magnetocaloric effect and illustrate differences between first- and second-order type materials starting with gadolinium as a reference system. Beyond the key functional properties of magnetocaloric materials, the adiabatic temperature, and entropy change, we briefly address the criticality of the most promising materials in terms of their supply risk. Looking at practical applications, suitable geometries and processing routes for magnetocaloric heat exchangers for device implementation are introduced. Copyright © Materials Research Society 2018.
    view abstract10.1557/mrs.2018.69
  • Modulations in martensitic Heusler alloys originate from nanotwin ordering
    Gruner, M.E. and Niemann, R. and Entel, P. and Pentcheva, R. and Rößler, U.K. and Nielsch, K. and Fähler, S.
    Scientific Reports 8 (2018)
    Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements. The origin of the so-called modulated structures is the subject of much controversy: They are either explained by phonon softening or adaptive nanotwinning. Here we used large-scale density functional theory calculations on the Ni2MnGa prototype system to demonstrate interaction energy between twin boundaries. Minimizing the interaction energy resulted in the experimentally observed ordered modulations at the atomic scale, it explained that a/b twin boundaries are stacking faults at the mesoscale, and contributed to the macroscopic hysteresis losses. Furthermore, we found that phonon softening paves the transformation path towards the nanotwinned martensite state. This unified both opposing concepts to explain modulated martensite. © 2018 The Author(s).
    view abstract10.1038/s41598-018-26652-6
  • Moment-Volume Coupling in La(Fe1−xSix)13
    Gruner, M.E. and Keune, W. and Landers, J. and Salamon, S. and Krautz, M. and Zhao, J. and Hu, M.Y. and Toellner, T. and Alp, E.E. and Gutfleisch, O. and Wende, H.
    Physica Status Solidi (B) Basic Research 255 (2018)
    We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe1−xSix)13 by means of first-principles calculations combined with the fixed-spin moment approach. We find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. In the same spirit, the dependence of the average local Fe moment on the Si content can account for the occurence of first- and second-order transitions in the system. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700465
  • Probing glassiness in Heuslers via density functional theory calculations
    Entel, P. and Gruner, M.E. and Acet, M. and Hucht, A. and Çakır, A. and Arróyave, R. and Karaman, I. and Duong, T.C. and Talapatra, A. and Bruno, N.M. and Salas, D. and Mankovsky, S. and Sandratskii, L. and Gottschall, T. and Gutfleisch, O. and Sahoo, S. and Fähler, S. and Lázpita, P. and Chernenko, V.A. and Barandiaran, J.M. and Buchelnikov, V.D. and Sokolovskiy, V.V. and Lookman, T. and Ren, X.
    Springer Series in Materials Science 275 (2018)
    Heusler compounds and alloys form a unique class of intermetallic systems with functional properties interfering with basic questions of fundamental aspects of materials science. Among the functional properties, the magnetic shape memory behavior (Planes et al., J Phys: Condens Matter 21:233201 (29 pp), 2009) and the ferrocaloric effects like the inverse magnetocaloric effect which is associated with the first order magnetostructural transformation with a jump-like change of the magnetization with lowering of temperature (Acet et al., Magnetic-field-induced effects in martensitic Heusler-based magnetic shape memory alloys. In: Bushow KHJ (ed) Handbook of magnetic materials, vol 19. North-Holland, Amsterdam, pp 231–289, 2011) have been intensively investigated in various reviews. Important references can be found in Acet et al. (Magnetic-field-induced effects in martensitc Heusler-based magnetic shape memory alloys. In: Bushow KHJ (ed) Handbook of magnetic materials, vol 19. North-Holland, Amsterdam, pp 231–289, 2011). Besides magnetocaloric effects, other ferroic cooling mechanisms of Heuslers (electrocaloric, barocaloric, and elastocaloric ones) have recently been discussed by Xavier Moya et al. (Nat Mater 13:439–450, 2014). A discussion of caloric effects in ferroic materials including a brief discussion of the importance of correlating time and length scales can be found in Fähler et al. (Adv Eng Mater 14:10–19, 2012). In the present article, we emphasize this item further by showing that, in particular, the physics at different time scales leads to markedly different properties of the Heusler materials. “Rapidly quenched” alloys behave differently from “less rapidly quenched” alloys. In the latter case, the so-called magnetostructural transformation may vanish altogether because of segregation of the alloys into the stoichiometric L2 1 Heusler phase and L1 0 Ni-Mn occurs. We argue that this tendency for segregation is at the origin of glassiness in Heuslers. © Springer Nature Switzerland AG 2018.
    view abstract10.1007/978-3-319-96914-5_6
  • Probing Structural and Magnetic Instabilities and Hysteresis in Heuslers by Density Functional Theory Calculations
    Entel, P. and Gruner, M.E. and Fähler, S. and Acet, M. and Çahır, A. and Arróyave, R. and Sahoo, S. and Duong, T.C. and Talapatra, A. and Sandratskii, L. and Mankowsky, S. and Gottschall, T. and Gutfleisch, O. and Lázpita, P. and Chernenko, V.A. and Barandiaran, J.M. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Physica Status Solidi (B) Basic Research 255 (2018)
    Martensitic transformations of rapidly quenched and less rapidly cooled Heusler alloys of type Ni–Mn–X with X = Ga, In, and Sn are investigated by ab initio calculatioms. For the rapidly cooled alloys, we obtain the magnetocaloric properties near the magnetocaloric transition. For the less rapidly quenched alloys these magnetocaloric properties start to change considerably, each alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic Ni–Mn–X and tetragonal NiMn. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700296
  • Properties and Decomposition of Heusler Alloys
    Entel, P. and Gruner, M.E. and Acet, M. and Çakır, A. and Arróyave, R. and Duong, T. and Sahoo, S. and Fähler, S. and Sokolovskiy, V.V.
    Energy Technology 6 (2018)
    Martensitic transformations of rapidly quenched and less-rapidly cooled Heusler alloys of type Ni–Mn–X with X=Ga, In, and Sn are investigated by ab initio calculations. For alloys rapidly cooled from 1173 K, we obtain the well-known magnetocaloric properties near the magnetocaloric phase transition. For the less-rapidly cooled alloys with secondary heat treatment these magnetocaloric properties start to change considerably, because each alloy transforms during temper-annealing into a dual-phase composite alloy. These two phases are identified to be cubic stoichiometric Ni2MnX and tetragonal disordered NiMn. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ente.201800256
  • Electronic structure and magnetism of epitaxial Ni-Mn-Ga(-Co) thin films with partial disorder: A view across the phase transition
    Schleicher, B. and Klar, D. and Ollefs, K. and Diestel, A. and Walecki, D. and Weschke, E. and Schultz, L. and Nielsch, K. and Fähler, S. and Wende, H. and Gruner, M.E.
    Journal of Physics D: Applied Physics 50 (2017)
    The influence of Co-doping in off-stoichiometric Ni-Mn-Ga and Ni-Mn-Ga-Co thin films on the magnetic coupling of the atoms is investigated with x-ray magnetic circular dichroism in both the martensitic as well as austenitic phase, respectively. Additionally, first principles calculations were performed to compare the experimentally obtained absorption spectra with theoretical predictions. Calculated exchange constants and density of states for the different atomic sites underline the large influence of chemical and magnetic order on the magnetocaloric properties of the material. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa8e7c
  • Interplay of phase sequence and electronic structure in the modulated martensites of Mn2NiGa from first-principles calculations
    Kundu, A. and Gruner, M.E. and Siewert, M. and Hucht, A. and Entel, P. and Ghosh, S.
    Physical Review B 96 (2017)
    We investigate the relative stability, structural properties, and electronic structure of various modulated martensites of the magnetic shape memory alloy Mn2NiGa by means of density functional theory. We observe that the instability in the high-temperature cubic structure first drives the system to a structure where modulation shuffles with a period of six atomic planes are taken into account. The driving mechanism for this instability is found to be the nesting of the minority band Fermi surface, in a similar way to that established for the prototype system Ni2MnGa. In agreement with experiments, we find 14M modulated structures with orthorhombic and monoclinic symmetries having energies lower than other modulated phases with the same symmetry. In addition, we also find energetically favorable 10M modulated structures which have not been observed experimentally for this system yet. The relative stability of various martensites is explained in terms of changes in the electronic structures near the Fermi level, affected mostly by the hybridization of Ni and Mn states. Our results indicate that the maximum achievable magnetic field-induced strain in Mn2NiGa would be larger than in Ni2MnGa. However, the energy costs for creating nanoscale adaptive twin boundaries are found to be one order of magnitude higher than that in Ni2MnGa. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.064107
  • Ordering tendencies and electronic properties in quaternary Heusler derivatives
    Neibecker, P. and Gruner, M.E. and Xu, X. and Kainuma, R. and Petry, W. and Pentcheva, R. and Leitner, M.
    Physical Review B 96 (2017)
    The phase stabilities and ordering tendencies in the quaternary full-Heusler alloys NiCoMnAl and NiCoMnGa have been investigated by in situ neutron diffraction, calorimetry, and magnetization measurements. NiCoMnGa was found to adopt the L21 structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the B2 phase with disorder also between Mn and Ga was observed at 1160K. In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient L21 ordering, otherwise the system displays only B2 order. Linked to L21 ordering, a drastic increase in the magnetic transition temperature was observed in NiCoMnAl, while annealing affected the magnetic behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder. First principles calculations were employed to study the thermodynamics as well as order-dependent electronic properties of both compounds. It was found that a near half-metallic pseudogap emerges in the minority spin channel only for the completely ordered Y structure. However, this structure is energetically unstable compared to a tetragonal structure with alternating layers of Ni and Co, which is predicted to be the low-temperature ground state. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies. © 2017 authors. Published by the American Physical Society.
    view abstract10.1103/PhysRevB.96.165131
  • Contradictory role of the magnetic contribution in inverse magnetocaloric Heusler materials
    Gottschall, T. and Skokov, K.P. and Benke, D. and Gruner, M.E. and Gutfleisch, O.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    In this paper, we illustrate the dilemma of inverse magnetocaloric materials using the example of Heusler alloys. For such materials, the magnetic and lattice contribution to the total entropy change are competing with each other. For the two paradigmatic Heusler systems of Ni-Mn-In and Ni-Mn-In-Co, we provide a systematic comparison of experimental data under different magnetic fields and hydrostatic pressures with magnetic and the magnetocaloric properties obtained from the Heisenberg model. This allows us to separate the lattice and the magnetic contribution to the total entropy of the martensitic transition. Our analysis reveals that a large magnetization change is parasitic, but at the same time it is necessary to drive the magnetocaloric effect. This contradicting role of the magnetic contribution - the dilemma - is a general characteristic of inverse magnetocaloric Heusler materials. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.184431
  • High Thermopower with Metallic Conductivity in p-Type Li-Substituted PbPdO2
    Lamontagne, L.K. and Laurita, G. and Gaultois, M.W. and Knight, M. and Ghadbeigi, L. and Sparks, T.D. and Gruner, M.E. and Pentcheva, R. and Brown, C.M. and Seshadri, R.
    Chemistry of Materials 28 (2016)
    PbPdO2 is a band semiconductor with a band gap arising from the filled d8 nature of square-planar Pd2+. We establish that hole doping through Li substitution for Pd in PbPdO2 results in a p-type metallic oxide with a positive temperature coefficient of resistance for substitution amounts as small as 2 mol % Li for Pd. Furthermore, PbPd1-xLixO2 demonstrates a high Seebeck coefficient and is therefore an oxide thermoelectric material with high thermopower despite the metallic conductivity. Up to 4 mol % Li is found to substitute for Pd as verified by Rietveld refinement of neutron diffraction data. At this maximal Li substitution, the resistivity is driven below the Mott metallic maximum to 3.5 × 10-3 ω cm with a Seebeck coefficient of 115 μV/K at room temperature, which increases to 175 μV/K at 600 K. These electrical properties are almost identical to those of the well-known p-type oxide thermoelectric NaxCoO2. Nonmagnetic Li-substituted PbPdO2 does not possess a correlated, magnetic state with high-spin degeneracy as found in some complex cobalt oxides. This suggests that there are other avenues to achieving high Seebeck coefficients with metallic conductivities in oxide thermoelectrics. The electrical properties coupled with the moderately low lattice thermal conductivities allow for a zT of 0.12 at 600 K, the maximal temperature measured here. The trend suggests yet higher values at elevated temperatures. First-principles calculations of the electronic structure and electrical transport provide insight into the observed properties. © 2016 American Chemical Society.
    view abstract10.1021/acs.chemmater.6b00447
  • Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films
    Wolloch, M. and Gruner, M.E. and Keune, W. and Mohn, P. and Redinger, J. and Hofer, F. and Suess, D. and Podloucky, R. and Landers, J. and Salamon, S. and Scheibel, F. and Spoddig, D. and Witte, R. and Roldan Cuenya, B. and Gutfleisch, O. and Hu, M.Y. and Zhao, J. and Toellner, T. and Alp, E.E. and Siewert, M. and Entel, P. and Pentcheva, R. and Wende, H.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. The FM VDOS exhibits a particular sensitivity to the slight tetragonal distortions present in epitaxial films, which is not encountered in the AF phase. This results in a notable change in lattice entropy, which is important for the comparison between thin film and bulk results. Our calculations confirm the recently reported lattice instability in the AF phase. The imaginary frequencies at the X point depend critically on the Fe magnetic moment and atomic volume. Analyzing these nonvibrational modes leads to the discovery of a stable monoclinic ground-state structure, which is robustly predicted from DFT but not verified in our thin film experiments. Specific heat, entropy, and free energy calculated within the quasiharmonic approximation suggest that the new phase is possibly suppressed because of its relatively smaller lattice entropy. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling. © 2016 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License.
    view abstract10.1103/PhysRevB.94.174435
  • Mastering hysteresis in magnetocaloric materials
    Gutfleisch, O. and Gottschall, T. and Fries, M. and Benke, D. and Radulov, I. and Skokov, K.P. and Wende, H. and Gruner, M. and Acet, M. and Entel, P. and Farle, M.
    Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 374 (2016)
    Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratoryscale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2 P-type compounds. This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'. © 2016 The Author(s) Published by the Royal Society. All rights reserved.
    view abstract10.1098/rsta.2015.0308
  • Scaling study and thermodynamic properties of the cubic helimagnet FeGe
    Wilhelm, H. and Leonov, A.O. and Rößler, U.K. and Burger, P. and Hardy, F. and Meingast, C. and Gruner, M.E. and Schnelle, W. and Schmidt, M. and Baenitz, M.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The critical behavior of the cubic helimagnet FeGe was obtained from isothermal magnetization data in very close vicinity of the ordering temperature. A thorough and consistent scaling analysis of these data revealed the critical exponents β=0.368,γ=1.382, and δ=4.787. The anomaly in the specific heat associated with the magnetic ordering can be well described by the critical exponent α=-0.133. The values of these exponents corroborate that the magnetic phase transition in FeGe belongs to the isotropic 3D-Heisenberg universality class. The specific heat data are well described by ab initio phonon calculations and confirm the localized character of the magnetic moments. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.144424
  • Tailoring magnetic frustration in strained epitaxial FeRh films
    Witte, R. and Kruk, R. and Gruner, M.E. and Brand, R.A. and Wang, D. and Schlabach, S. and Beck, A. and Provenzano, V. and Pentcheva, R. and Wende, H. and Hahn, H.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electron microscopy, and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.104416
  • Achieving large magnetocaloric effects in Co- and Cr-substituted Heusler alloys: Predictions from first-principles and Monte Carlo studies
    Sokolovskiy, V.V. and Entel, P. and Buchelnikov, V.D. and Gruner, M.E.
    Physical Review B - Condensed Matter and Materials Physics 91 (2015)
    The magnetocaloric properties of Ni-Co-Mn-Cr-In Heusler alloys have been studied by means of ab initio calculations and Monte Carlo simulations. We discuss the resulting complex spin configurations, the temperature behavior of entropy, as well as the critical temperatures of the phase transitions. The substitution of 5% Co for Ni and 5% Cr for Mn results in a first-order magnetostructural transition from ferromagnetic austenite to antiferromagnetic martensite, which is accompanied by a spin-flip transition upon cooling. As a result, a large magnetization drop and giant inverse magnetocaloric effect can be achieved of ΔTad≈10 K in a 2 T field across the magnetostructural phase transition. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.91.220409
  • Effect of substitution on elastic stability, electronic structure and magnetic property of Ni-Mn based Heusler alloys: An ab initio comparison
    Roy, T. and Gruner, M.E. and Entel, P. and Chakrabarti, A.
    Journal of Alloys and Compounds 632 (2015)
    First-principles density functional theory based calculations have been used to predict the bulk mechanical properties of magnetic shape memory Heusler alloy Ni2MnGa substituted by copper (Cu), platinum (Pt), palladium (Pd) and manganese (Mn) at the Ni site. The elastic constants of Ni2MnGa alloy with and without substitution are calculated. We analyze and compare in detail the bulk mechanical properties for these alloys, in particular, the ratio between the calculated bulk and shear modulii, as well as the Poisson's ratio and Young's modulii. This analysis further based on an empirical relation, indicates that Pt2MnGa may inherently be the least brittle material, among the above-mentioned alloys. Interesting difference has been observed between the shear modulii calculated from Voigt's and Reuss's method. This has been explained in terms of the values of the tetragonal shear constant C′ of the materials. Study of Heisenberg exchange coupling parameters and Curie temperature as well as density of states of the materials shows the effect of substitution at the Ni site on the magnetic and electronic properties, respectively. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2015.01.255
  • Element-resolved thermodynamics of magnetocaloric lafe13-xsix
    Gruner, M.E. and Keune, W. and Roldan Cuenya, B. and Weis, C. and Landers, J. and Makarov, S.I. and Klar, D. and Hu, M.Y. and Alp, E.E. and Zhao, J. and Krautz, M. and Gutfleisch, O. and Wende, H.
    Physical Review Letters 114 (2015)
    By combination of two independent approaches, nuclear resonant inelastic x-ray scattering and first-principles calculations in the framework of density functional theory, we demonstrate significant changes in the element-resolved vibrational density of states across the first-order transition from the ferromagnetic low temperature to the paramagnetic high temperature phase of LaFe13-xSix. These changes originate from the itinerant electron metamagnetism associated with Fe and lead to a pronounced magneto-elastic softening despite the large volume decrease at the transition. The increase in lattice entropy associated with the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties. © 2015 American Physical Society.
    view abstract10.1103/PhysRevLett.114.057202
  • First-Principles Calculations of Magnetic Properties of Cr-Doped Ni45Co5Mn37In13 Heusler Alloys
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Gruner, M.E. and Entel, P.
    IEEE Transactions on Magnetics 51 (2015)
    The magnetic and electronic properties of Co- and Cr-doped Ni50Mn37In13 Heusler alloys with a substitution of 5 at.% Co for Ni and 5 at.% Cr for Ni, Mn, or In are investigated in the framework of the density functional theory method. The chemical disorder in the off-stoichiometric Ni-Co-Mn-Cr-In systems was treated in the coherent potential approximation. Three different ferrimagnetic and one ferromagnetic (FM) spin states for austenite and martensite were considered in ab initio calculations. It is found that for both structures, the intersublattice interactions (MnY(Z)-Co, MnY(Z)-Ni, MnY(Z)-MnZ(Y), MnY(Z)-Cr, and Cr-Co) provide the largest contribution to the exchange due to the shorter distance compared with the intrasublattice interactions (MnY(Z)-MnY(Z), Co-Co, Ni-Ni, and Cr-Cr). Besides, the MnY-MnZ and MnY(Z)-Cr exchanges in the first shell become five times larger in martensite compared with austenite. The largest anti-FM interaction is observed between MnY(Z)-Cr atoms in martensite. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2015.2439391
  • First-principles calculations of magnetic properties of Cr-doped Ni45Co5Mn37In13 Heusler alloys
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Gruner, M. and Entel, P.
    2015 IEEE International Magnetics Conference, INTERMAG 2015 (2015)
    For the past 20 years, ferromagnetic compounds with first-order magnetic phase transition and simultaneous structural changes have attracted interest due to emerging magnetocaloric effects (MCE) and their possible application in magnetic refrigeration technology working at room temperatures. Nowadays, the search for new magnetocaloric materials with better MCE as refrigerants is an important objective for the development of this technology. Among ferromagnetic materials having acceptable magnetocaloric properties, Ni-Mn-Z (Z = Ga, In, Sn, Sb) Heusler alloys have been widely investigated because of their potential applications as intelligent functional materials showing also the thermally and field-induced shape memory effect, superelasticity, magnetic-field-induced strains, magnetoresistance and exchange bias effects [1-4]. © 2015 IEEE.
    view abstract10.1109/INTMAG.2015.7156782
  • Impact of strain-induced electronic topological transition on the thermoelectric properties of PtCoO2 and PdCoO2
    Gruner, M.E. and Eckern, U. and Pentcheva, R.
    Physical Review B - Condensed Matter and Materials Physics 92 (2015)
    By a combination of first-principles calculations and semiclassical Boltzmann transport theory, we investigate the effect of epitaxial strain on the electronic structure and transport properties of PtCoO2 and PdCoO2. In contrast to the rather uniform elastic response of both systems, we predict for PtCoO2 a high sensitivity of the out-of-plane transport properties to strain, which is not present in PdCoO2. At ambient temperature we identify a considerable absolute change in the thermopower from -107μV/K at -5% compressive strain to -303μV/K at +5% tensile strain. This remarkable response is related to distinct changes of the Fermi surface, which involve the crossing of two additional bands at a moderate compressive in-plane strain. Combining our transport results with available experimental data on electrical and lattice thermal conductivity we predict a thermoelectric figure of merit of up to ZT=0.25 at T=600 K for strained PtCoO2. © 2015 American Physical Society.
    view abstract10.1103/PhysRevB.92.235140
  • Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstract10.1051/matecconf/20153302001
  • 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 abstract10.1109/TMAG.2015.2438953
  • The magnetic states of the Ni1.75Co0.25Mn1.25Cr0.25In0.5 Heusler alloy
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Gruner, M. and Entel, P.
    2015 IEEE International Magnetics Conference, INTERMAG 2015 (2015)
    The functional properties of Mn excess Ni-Mn-Z (Z = Ga, In, Sn, Sb) Heusler alloys with addition of fourth and fifth elements like the magnetic shape memory effect, exchange bias effect and the magnetocaloric effect (MCE) are promising for future technologies [1] These effects are associated with the complex magnetic ordering which changes from ferromagnetism in austenite to composite magnetic structures in martensite below the magnetostructural transformation (MST) [2] The competing magnetic interactions lead to the characteristic drops of magnetization curves across the MST for the Ni-(Co)-Mn-In alloys [3] The strong antiferromagnetic correlations start to appear with the Mn excess atoms which mainly occupy sites of the Z sublattice, causing the nearest neighbor Mn-Mn distances to shrink. © 2015 IEEE.
    view abstract10.1109/INTMAG.2015.7156752
  • 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 (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 abstract10.1016/j.jmmm.2015.03.003
  • Chemically ordered decahedral FePt nanocrystals observed by electron microscopy
    Li, Z.-A. and Spasova, M. and Ramasse, Q.M. and Gruner, M.E. and Kisielowski, C. and Farle, M.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The crystal structure of FePt nanoparticles of mean size of 6 nm produced by gas-phase condensation is characterized using a combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field (HAADF) imaging in scanning transmission electron microscopy (STEM). These FePt nanoparticles are found to be chemically ordered, decahedral shaped, and Pt enriched at the surfaces. The experimentally determined crystallographic lattice constants and distribution of Fe and Pt atoms are compared with first-principles calculations of ordered decahedral FePt nanoparticles to confirm the discovery of a unique decahedral structure with Fe/Pt ordering and Pt surface segregation. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.161406
  • First-principles calculation of the instability leading to giant inverse magnetocaloric effects
    Comtesse, D. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Singh, N. and Gottschall, T. and Gutfleisch, O. and Chernenko, V.A. and Albertini, F. and Fähler, S. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The structural and magnetic properties of functional Ni-Mn-Z (Z=Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.184403
  • First-principles studies on graphene-supported transition metal clusters
    Sahoo, S. and Gruner, M.E. and Khanna, S.N. and Entel, P.
    Journal of Chemical Physics 141 (2014)
    Theoretical studies on the structure, stability, and magnetic properties of icosahedral TM13 (TM = Fe, Co, Ni) clusters, deposited on pristine (defect free) and defective graphene sheet as well as graphene flakes, have been carried out within a gradient corrected density functional framework. The defects considered in our study include a carbon vacancy for the graphene sheet and a five-membered and a seven-membered ring structures for graphene flakes (finite graphene chunks). It is observed that the presence of defect in the substrate has a profound influence on the electronic structure and magnetic properties of graphene-transition metal complexes, thereby increasing the binding strength of the TM cluster on to the graphene substrate. Among TM 13 clusters, Co13 is absorbed relatively more strongly on pristine and defective graphene as compared to Fe13 and Ni 13 clusters. The adsorbed clusters show reduced magnetic moment compared to the free clusters. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4893328
  • Interacting magnetic cluster-spin glasses and strain glasses in Ni-Mn based Heusler structured intermetallics
    Entel, P. and Gruner, M.E. and Comtesse, D. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Physica Status Solidi (B) Basic Research 251 (2014)
    Magnetic Ni-Mn based Heusler intermetallics show complex magnetic behavior in connection with martensitic transformations (see, for instance, the phase diagram of Ni-Co-Mn-Sn on the right-hand side). The cubic austenitic phase at high temperature shows long-range ferromagnetic order which can considerably be weakened by the appearance of competing antiferromagnetic interactions which are induced by Mn excess and chemical disorder. With decreasing temperature a martensitic/magnetostructural transformation takes place from cubic to non-modulated/modulated tetragonal/monoclinic or orthorhombic structure, where long-range ferromagnetic order can no longer be maintained, leading to superparamagnetic behavior. At still lower temperatures superparamagnetism changes to superspin glass because of strong competition of ferromagnetic and antiferromagnetic interactions and chemical disorder. In addition, disorder and local structural distortions can lead to strain glass in austenite, as observed for some non-magnetic martensitic systems. The magnetic intermetallics are of technological importance in view of their functional properties involving magnetic shape-memory and exchange-bias effects as well as magnetocaloric effects. The 'ferroic cooling' is of particular relevance since it avoids the use of ozone-depleting and greenhouse chemicals compared with conventional fluid-compression technology. Experimental phase diagram of Ni50-x Cox Mn39 Sn11 for 0≤x≤10. Here, TC is the Curie temperature of austenite; at TM the system transforms to paramagnetic martensite and at TS to superparamagnetic martensite (SPM) and then to superspin-glass martensite (SSG) at TP. The possible strain-glass phases (labeled by question marks) are predicted because of kinetic arrest phenomena and local distortions associated with the magnetostructural transition and ergodicity breaking by field-cooling/zero-field-cooling experiments. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201451059
  • Magnetoelastic coupling and the formation of adaptive martensite in magnetic shape memory alloys
    Gruner, M.E. and Fähler, S. and Entel, P.
    Physica Status Solidi (B) Basic Research 251 (2014)
    Reviewing the results of recent first-principles calculations, we work out a close analogy between the two paradigmatic classes of magnetic shape memory materials, the ordered Ni2 MnGa Heusler compound and the disordered Fe70 Pd30 alloy. Despite fundamental differences between both systems, we can demonstrate that in both cases the very low formation energy for tetragonal twins on the smallest length scale opens an alternative transformation path into an adaptive hierarchical microstructure which is important for the functional behavior. The low energy of the (101) twin boundary corresponds to a shear instability which is associated to the soft transversal acoustic phonon in both systems. In turn, changes of the energy landscape upon magnetic disorder are responsible for the stability of austenite. This points out the strong influence of magnetoelastic coupling on the transformation process. Nanotwinned adaptive microstructures in Ni2 MnGa and Fe68 Pd32 magnetic shape memory alloys obtained from first-principles calculations. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201350397
  • Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstract10.1109/TMAG.2014.2330845
  • Ab initio studies of effect of copper substitution on the electronic and magnetic properties of Ni2MnGa and Mn2NiGa
    Chakrabarti, A. and Siewert, M. and Roy, T. and Mondal, K. and Banerjee, A. and Gruner, M.E. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    Using first-principles density functional theory based calculations, we study systematically the effect of medium to large Cu substitution at the Mn, Ga as well as Ni sites on the geometric, bulk mechanical, electronic, and magnetic properties of Ni2MnGa and Mn2NiGa. The calculations have been carried out for possible austenite and martensite phases using a supercell approach. Partial Cu substitutions at Mn and Ga sites show promises in terms of the electronic and magnetic properties for both Ni2MnGa and Mn 2NiGa alloys from an application point of view. Our calculations predict that for certain partial substitutions, the austenite to martensite transition temperature is likely to increase and the system remains magnetic in nature. On the other hand, a significantly large amount of Cu substitution at the Ni site seems to stabilize the austenite phase in both of these alloy systems rendering a martensite transition unlikely. Interestingly, the overall trend in the changes in the structural, bulk mechanical, electronic, and magnetic properties of these two different types of alloy systems, Ni 2MnGa and Mn2NiGa, as a result of substantial Cu substitution in all the three different sites, is found to be the same. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.174116
  • Complex magnetic ordering as a driving mechanism of multifunctional properties of Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Arróyave, R. and Singh, N. and Talapatra, A. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Albertini, F. and Righi, L. and Chernenko, V.A.
    European Physical Journal B 86 (2013)
    First-principles calculations are used to study the structural, electronic and magnetic properties of (Pd, Pt)-Mn-Ni-(Ga, In, Sn, Sb) alloys, which display multifunctional properties like the magnetic shape-memory, magnetocaloric and exchange bias effect. The ab initio calculations give a basic understanding of the underlying physics which is associated with the complex magnetic behavior arising from competing ferro- and antiferromagnetic interactions with increasing number of Mn excess atoms in the unit cell. This information allows to optimize, for example, the magnetocaloric effect by using the strong influence of compositional changes on the magnetic interactions. Thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. We present guidelines of how to improve the functional properties. For Pt-Ni-Mn-Ga alloys, a shape memory effect with 14% strain can be achieved in an external magnetic field. © 2013 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstract10.1140/epjb/e2012-30936-9
  • Compositional trends and magnetic excitations in binary and ternary Fe-Pd-X magnetic shape memory alloys
    Gruner, M.E. and Hamann, S. and Brunken, H. and Ludwig, Al. and Entel, P.
    Journal of Alloys and Compounds 577 (2013)
    High throughput thin film experiments and first-principles calculations are combined in order to get insight into the relation between finite temperature transformation behavior and structural ground state properties of ternary Fe-Pd-X alloys. In particular, we consider the binding surface, i.e., the energy of the disordered alloy calculated along the Bain path between bcc and fcc which we model by a 108 atom supercell. We compare stoichiometric Fe 75Pd25 with ternary systems, where 4.6% of the Fe atoms were substituted by Cu and Mn, respectively. The computational trends are related to combinatorial experiments on thin film libraries for the systems Fe-Pd-Mn and Fe-Pd-Cu which reveal a systematic evolution of the martensitic start temperature with composition within the relevant concentration range for magnetic shape memory (MSM) applications. Our calculations include atomic relaxations, which were shown to be relevant for a correct description of the structural properties. Furthermore, we find that magnetic excitations can substantially alter the binding surface. The comparison of experimental and theoretical trends indicates that, both, compositional changes and magnetic excitations contribute significantly to the structural stability which may thus be tailored by specifically adding antiferromagnetic components. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2012.02.033
  • Interaction of phase transformation and magnetic properties of heusler alloys: A density functional theory study
    Entel, P. and Gruner, M.E. and Comtesse, D. and Wuttig, M.
    JOM 65 (2013)
    The structural, electronic, and magnetic properties of functional Ni-Mn-Z (Z = Ga, In, Sn, and Sb) Heusler alloys are studied by first-principles and Monte Carlo tools. The ab initio calculations give a basic understanding of the underlying physics that are associated with the complex magnetic behavior arising from the competition of ferromagnetic and antiferromagnetic interactions with increasing chemical disorder in the super cell. This complex magnetic ordering is the driving mechanism of structural transformations. It also essentially determines the multifunctional properties of the Heusler alloys such as magnetic shape-memory and magnetocaloric effects. The thermodynamic properties can be calculated by using the ab initio magnetic exchange parameters in finite-temperature Monte Carlo simulations. The experimental entropy and specific heat changes across the magnetostructural transition are accurately reproduced by the Monte Carlo simulations. The predictive power of the first-principles calculations allows one to optimize the functional features by choosing optimal compositions. © 2013 The Minerals, Metals & Materials Society.
    view abstract10.1007/s11837-013-0757-2
  • Optimization of smart Heusler alloys from first principles
    Entel, P. and Siewert, M. and Gruner, M.E. and Chakrabarti, A. and Barman, S.R. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Journal of Alloys and Compounds 577 (2013)
    The strong magnetoelastic interaction in ternary X2YZ Heusler alloys is reponsible for the appearance of magnetostructural phase transitions and related functional properties such as the magnetocaloric and magnetic shape-memory effects. Here, X and Y are transition metal elements and Z is usually an element from the III-V group. In order to discuss possibilities to optimize the multifunctional effects, we use density functional theory calculations from which the martensitic driving forces of the magnetic materials can be derived. We find that the electronic contribution arising from the band Jahn-Teller effect is one of the major driving forces. The ab initio calculations also give a hint of how to design new intermetallics with higher martensitic transformation temperatures compared to the prototype alloy system Ni-Mn-Ga. As an example, we discuss quarternary PtxNi 2-xMnGa alloys which have properties very similar to Ni-Mn-Ga but exhibit a higher maximal eigenstrain of 14%. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jallcom.2012.03.005
  • Trends in spin and orbital magnetism of free and encapsulated FePt nanoparticles
    Gruner, M.E.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    Owed to the large magneto-crystalline anisotropy (MCA) of the bulk FePt alloys, nanostructures with a few nm in diameter are considered for ultra-high density recording applications. First principles calculations in the framework of density functional theory (DFT) permit insight into the close interrelation between particle composition, morphology, and magnetism with access to the electronic level. The present survey will systematically highlight the impact of an additional encapsulation with Cu, Au, Al, and further main group elements on spin- and orbital magnetism and MCA with special emphasis on the role of the interface. Site resolved orbital moment anisotropy (OMA) of an uncovered 147 atom FePt nanoparticle. Large-scale first principles calculations in the framework of density functional theory offer detailed insight into the close interrelation between particle composition, morphology and magnetism with electronic resolution. Exploiting the power of contemporary supercomputers, one can identify systematic trends in spin and orbital magnetism of nanometer-sized hard magnetic particles related to their structure or chemical environment. This Feature Article concentrates on Fe-Pt nanoparticles, which are considered as promising candidates for ultra-high density recording media. Special emphasis is made on the role of the surfaces and the impact of a protective encapsulation with Cu, Au, Al or further main group elements on the hard magnetic properties. The anisotropy of the orbital moments turns out to be a valuable quantity characterizing the particular contribution of surfaces and interfaces on the atomic scale. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201329048
  • A first-principles investigation of the compositional dependent properties of magnetic shape memory heusler alloys
    Siewert, M. and Gruner, M.E. and Hucht, A. and Herper, H.C. and Dannenberg, A. and Chakrabarti, A. and Singh, N. and Arróyave, R. and Entel, P.
    Advanced Engineering Materials 14 (2012)
    The interplay of structural and magnetic properties of magnetic shape memory alloys is closely related to their composition. In this study the influence of the valence electron concentration on the tetragonal transformation in Ni 2Mn 1 + xZ 1 - x (Z = Ga, In, Sn, Sb) and Co 2Ni 1 + xGa 1 - x is investigated by means of ab initio calculations. While the type of magnetic interaction is different for the two series, the trends of the total energy changes under a tetragonal transformation are very similar. We find that tetragonal structures become energetically preferred with respect to the cubic one as the valence electron concentration e/a is increased regardless of the system under consideration. In particular, the energy difference between the austenite and martensite structures increases linearly with e/a, which is in part responsible for the linear increase of the matensite transformation temperature. The substitution of nickel by platinum increases even further the transformation temperature. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200063
  • Anisotropic ferromagnetism in carbon-doped zinc oxide from first-principles studies
    Nayak, S.K. and Gruner, M.E. and Sakong, S. and Sil, S. and Kratzer, P. and Behera, S.N. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    A density functional theory study of substitutional carbon impurities in ZnO has been performed, using both the generalized gradient approximation (GGA) and a hybrid functional (HSE06) as exchange-correlation functional. It is found that the nonspinpolarized C Zn impurity is under almost all conditions thermodynamically more stable than the C O impurity which has a magnetic moment of 2μ B, with the exception of very O-poor and C-rich conditions. This explains the experimental difficulties in sample preparation in order to realize d0 ferromagnetism in C-doped ZnO. From GGA calculations with large 96-atom supercells, we conclude that two C O-C O impurities in ZnO interact ferromagnetically, but the interaction is found to be short-ranged and anisotropic, much stronger within the hexagonal ab plane of wurtzite ZnO than along the c axis. This layered ferromagnetism is attributed to the anisotropy of the dispersion of carbon impurity bands near the Fermi level for C O impurities in ZnO. From the calculated results, we derive that a C O concentration between 2% and 6% should be optimal to achieve d0-ferromagnetism in C-doped ZnO. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.054441
  • 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 (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 abstract10.1007/s11661-011-0832-7
  • Competition between ordering, twinning, and segregation in binary magnetic 3d-5d nanoparticles: A supercomputing perspective
    Gruner, M.E. and Entel, P.
    International Journal of Quantum Chemistry 112 (2012)
    The benefit of massively parallel supercomputers for technologically relevant applications in the field of materials science is demonstrated at the example of first-principles total energy calculations of magnetic binary transition metal nanoparticles containing up to 1415 3d and 5d transition metal atoms. The simulations, which take into account structural optimizations without symmetry constraints, reveal the size-dependent evolution of the energetic order of single crystalline and multiply twinned Fe-Pt nanoparticles up to 4 nm in diameter, which are discussed as promising building blocks for future ultra-high density data recording media. Although at small diameters, multiply twinned morphologies are preferred, we can show that an energetic crossover to a single crystalline, ordered arrangement can be expected at diameters around four nanometers. The comparison with Co-Pt indicates that the contributions of the interfaces in multiply twinned structures are of similar importance as the surface and cannot be neglected especially for small particle sizes. The results imply that for Co-Pt particles segregation of Pt to the surface and the formation of a Pt-depleted subsurface layer is also dominant for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, whereas for Fe-Pt multiple twinning is the most important equilibrium mechanism for small particle sizes. Hybrid combinations of the most favorable ordering motifs, that is, L10-type ordering in the particle core in combination with segregation in the outer shells, may thus lead to highly stable morphologies, which could dominate the growth process. © 2011 Wiley Periodicals, Inc.
    view abstract10.1002/qua.23254
  • Domain structure in the tetragonal phase of BaTiO 3-From bulk to nanoparticles
    Grnebohm, A. and Gruner, M.E. and Entel, P.
    Ferroelectrics 426 (2012)
    We present a first-principles density functional theory study of domain wall structures in tetragonal BaTiO 3 and its nanoparticles. For the bulk material the domain wall profiles, their width and their formation energy are computed and preliminary investigations on thin BaTiO 3 films up to 4 monolayers and small nanoparticles of 15.8 have been performed. While the 180 wall is atomically sharp, we find a lower bond for the 90 wall width of 16.5 . Although, no ferroelectric state can be stabilized neither in films nor in the nanoparticles of this small size, a large local polarization exits in both cases. © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2012.671090
  • Effect of temperature and compositional changes on the phonon properties of Ni-Mn-Ga shape memory alloys
    Ener, S. and Neuhaus, J. and Petry, W. and Mole, R. and Hradil, K. and Siewert, M. and Gruner, M.E. and Entel, P. and Titov, I. and Acet, M.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    We report on the vibrational properties of the ferromagnetic shape memory alloy system Ni-Mn-Ga in its stoichiometric Ni 2MnGa and off-stoichiometric Ni 49Mn 32Ga 19 compositions. Elastic and inelastic neutron scattering measurements at different temperatures are presented with a focus on the austenite phase and compared to first-principles calculations. The overall behavior of the full phonon dispersion is similar for both compositions with remarkable exceptions for the TA 2[ξξ0] acoustic branch and optical phonon branches. Less dispersion is found in the optical phonons for Ni 49Mn 32Ga 19 in the whole reciprocal space when compared to Ni 2MnGa and is explained by the occupation of regular Ga sites by excess Mn atoms. A pronounced softening in the TA 2[ξξ0] phonon branch within the austenite phase is observed in both samples when approaching the martensitic transition. Its location in reciprocal space reveals the martensitic transition mechanism. The austenite L2 1 structure transforms to the tetragonal modulated martensite structure by shuffling (110) planes in the [11̄0] direction, similarly to what has been observed at the martensitic transitions of the d1 and d2 transition metals. Whereas the temperature dependence of the softening of the TA 2[ξξ0] phonons in the stoichiometric sample coincides perfectly with the magnetic and structural transitions, this is not the case for the off-stoichiometric sample. Here the relation between the magnetic ordering and the vibrational properties is still an open question. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.144305
  • 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 (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 abstract10.1016/j.jmmm.2012.02.081
  • Enhancing magnetocrystalline anisotropy of the Fe 70Pd 30 magnetic shape memory alloy by adding Cu
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Schultz, L. and Ludwig, Al. and Fähler, S.
    Acta Materialia 60 (2012)
    Fe-Pd-Cu thin films are of great interest for applications in magnetic shape memory microsystems due to their increased martensitic transformation temperature. Here we analyse the consequences of Cu addition to Fe-Pd on the binding energy and magnetic properties by a combination of thin film experiments and first-principles calculations. Strained epitaxial growth of Fe 70Pd 30-xCu x with x = 0, 3, 7 is used to freeze intermediate stages during the martensitic transformation. This makes a large range of tetragonal distortion susceptible for analysis, ranging from body-centred cubic to beyond face-centred cubic (1.07 < c/a bct < 1.57). We find that Cu enhances the quality of epitaxial growth, while spontaneous polarization and Curie temperature are reduced only moderately, in agreement with our calculations. Beyond c/a bct &gt; 1.41 the samples undergo structural relaxations through adaptive nanotwinning. Cu enhances the magnetocrystalline anisotropy constant K 1 at room temperature, which reaches a maximum of -2.4 × 10 5 J m -3 around c/a bct = 1.33. This value exceeds those of binary Fe 70Pd 30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K 1 represents the maximum driving energy for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuator applications with significantly improved work output. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2012.08.001
  • Nucleation, structure and magnetism of transition metal clusters from first principles
    Sahoo, S. and Gruner, M.E. and Hucht, A. and Rollmann, G. and Entel, P.
    NanoScience and Technology 79 (2012)
    Properties of transition metal (TM) clusters such as structural stability, growth and magnetic properties are studied using the density functional theory (DFT). We find that for both elemental and binary clusters, different morphologies are stable for different ranges of cluster sizes. We discuss possible structural transformations namely Jahn-Teller (JT) and Mackay transformation (MT) occurring in TM clusters. While the JT-distorted cluster is stable for a Fe 13 icosahedron, the MT-distorted structure is stable for Co 13. For Ni 13, however, both distortions lead to similar energies. In larger clusters, both JT and MT compete with each other, and as a result we find a higher stability for large Fe clusters with a shellwiseMackay transformation. Studies on binary Fe-Pt clusters show a segregation tendency of Pt atoms to the surfaces of the clusters. The ordered Fe-Pt icosahedral structures show enhanced stability compared to the L1 0 cuboctahedron. From the studies on magnetocrystalline anisotropy (MAE) for clusters, we find that relaxed Fe 13 and Ni 13 have several orders of magnitude larger MAE as compared to the corresponding bulk values. However, Co 13 does not follow this trend. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_3
  • The role of adaptive martensite in magnetic shape memory alloys
    Niemann, R. and Röáßler, U.K. and Gruner, M.E. and Heczko, O. and Schultz, L. and Fähler, S.
    Advanced Engineering Materials 14 (2012)
    Magnetic shape memory materials require a high twin boundary mobility and low hysteresis for applications mainly as actuators, sensors, and magnetocaloric cooling elements. Usually, outstanding properties are found only in samples with a modulated martensitic structure. Here, we analyze the question why a modulated structure is beneficial and show evidence that the modulated martensite is not an equilibrium phase but a nanoscale microstructure of non-modulated (NM) martensite. In this review, we combine results from continuum and atomistic theory, as well as local and integral measurements on the model system Ni-Mn-Ga. Following the concept of adaptive martensite the modulated phase forms to minimize elastic energy near the phase boundary by introducing low-energy twin boundaries between lamellae of the NM martensite that have widths of a few unit cells. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200058
  • Understanding the magnetic shape memory system Fe-Pd-X by thin film experiments and first principle calculations
    Kauffmann-Weiss, S. and Hamann, S. and Gruner, M.E. and Buschbeck, J. and Ludwig, Al. and Schultz, L. and Fähler, S.
    Advanced Engineering Materials 14 (2012)
    The magnetic shape memory (MSM) alloy Fe 70Pd 30 is of particular interest for novel microactuator and sensor applications. This review summarizes the underlying physical and material science concepts for this MSM alloy system. First-principles calculations of the electronic and crystallographic structure together with combinatorial and epitaxial film studies are presented. By these complementary methods we can address the open key questions of MSM alloys and microsystems: Which are the driving forces for a martensitic transformation and how does this transformation proceed? How is it possible to improve the MSM properties by adding third elements? What is the role of external interfaces and which routes allow the preparation of freestanding epitaxial films? Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200052
  • A guideline for atomistic design and understanding of ultrahard nanomagnets
    Antoniak, C. and Gruner, M.E. and Spasova, M. and Trunova, A.V. and Römer, F.M. and Warland, A. and Krumme, B. and Fauth, K. and Sun, S. and Entel, P. and Farle, M. and Wende, H.
    Nature Communications 2 (2011)
    Magnetic nanoparticles are of immense current interest because of their possible use in biomedical and technological applications. Here we demonstrate that the large magnetic anisotropy of FePt nanoparticles can be significantly modified by surface design. We employ X-ray absorption spectroscopy offering an element-specific approach to magnetocrystalline anisotropy and the orbital magnetism. Experimental results on oxide-free FePt nanoparticles embedded in Al are compared with large-scale density functional theory calculations of the geometric- and spin-resolved electronic structure, which only recently have become possible on world-leading supercomputer architectures. The combination of both approaches yields a more detailed understanding that may open new ways for a microscopic design of magnetic nanoparticles and allows us to present three rules to achieve desired magnetic properties. In addition, concrete suggestions of capping materials for FePt nanoparticles are given for tailoring both magnetocrystalline anisotropy and magnetic moments. © 2011 Macmillan Publishers Limited. All rights reserved.
    view abstract10.1038/ncomms1538
  • Ab-initio modeling of Fe-Mn based alloys and nanoclusters
    Entel, P. and Comtesse, D. and Herper, H.C. and Gruner, M.E. and Siewert, M. and Sahoo, S. and Hucht, A.
    Materials Research Society Symposium Proceedings 1296 (2011)
    New methods in steel design and basic understanding of the novel materials require large scale ab initio calculations of ground state and finite temperature properties of transition metal alloys. In this contribution we present ab initio modeling of the structural and magnetic properties of XYZ compounds and alloys where X, Y = Mn, Fe, Co Ni and Z = C, Si with emphasis on the Fe-Mn steels. The optimization of structural and magnetic properties is performed by using different simulation tools. In particular, the finite-temperature magnetic properties are simulated using a Heisenberg model with magnetic exchange interactions from first-principles calculations. Part of the calculations are extended to the nanoparticle range showing how ferromagnetic and antiferromagnetic trends influence the nucleation, morphologies and growth of Fe-Mn-based nanoparticles. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.1449
  • Absence of ferromagnetic interaction in Co-Co nearest neighbor impurity pairs in ZnO: An analysis from GGA+U studies
    Nayak, S.K. and Ney, A. and Gruner, M.E. and Tripathi, G.S. and Behera, S.N. and Entel, P.
    AIP Conference Proceedings 1461 (2011)
    We study the magnetic interactions of Co doped in ZnO, with the Co atoms occupying the nearest neighbor cation sites. We perform electronic structure calculations using the local density approximation (LDA), generalized gradient approximation (GGA), and GGA+U. The Hubbard U is treated separately on d-orbitals of Zn and Co, and simultaneously on the d-orbitals of both Zn and Co. Results of GGA+U studies confirm that the nearest neighbor Co-Co pair favor antiferromagnetic interaction, where the Co spins align oppositely. This is different from the LDA and GGA predictions. A general comparison of our results with experiments shows fairly good agreement. © 2012 American Institute of Physics.
    view abstract10.1063/1.4736900
  • Composition-dependent basics of smart heusler materials from first-principles calculations
    Entel, P. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Buchelnikov, V.D. and Chernenko, V.A.
    Materials Science Forum 684 (2011)
    The structural and magnetic order are the decisive elements which vastly determine the properties of smart ternary intermetallics such as X2YZ Heusler alloys. Here, X and Y are transition metal elements and Z is an element from the III-V group. In order to give a precise prescription of the possibilities to optimize the magnetic shape memory and magnetocaloric effects of these alloys, we use density functional theory calculations. In particular, we outline how one may find new intermetallics which show higher Curie and martensite transformation temperatures when compared with the prototypical magnetic shape-memory alloy Ni2MnGa. Higher operation temperatures are needed for technological applications at elevated temperatures. © (2011) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.684.1
  • Designing shape-memory Heusler alloys from first-principles
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Chakrabarti, A. and Herper, H.C. and Wuttig, M. and Barman, S.R. and Singh, S. and Al-Zubi, A. and Hickel, T. and Neugebauer, J. and Gillessen, M. and Dronskowski, R. and Entel, P.
    Applied Physics Letters 99 (2011)
    The phase diagrams of magnetic shape-memory Heusler alloys, in particular, ternary Ni-Mn-Z and quarternary (Pt, Ni)-Mn-Z alloys with Z = Ga, Sn, have been addressed by density functional theory and Monte Carlo simulations. Finite temperature free energy calculations show that the phonon contribution stabilizes the high-temperature austenite structure while at low temperatures magnetism and the band Jahn-Teller effect favor the modulated monoclinic 14M or the nonmodulated tetragonal structure. The substitution of Ni by Pt leads to a series of magnetic shape-memory alloys with very similar properties to Ni-Mn-Ga but with a maximal eigenstrain of 14. © 2011 American Institute of Physics.
    view abstract10.1063/1.3655905
  • Erratum: Competing structural ordering tendencies in Heusler-type alloys with high Curie temperatures: Fe2CoGa1-xZnx studied by first-principles calculations (Physical Review B - Condensed Matter and Materials Physics (2010) 82 (214421))
    Dannenberg, A. and Siewert, M. and Gruner, M.E. and Wuttig, M. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    view abstract10.1103/PhysRevB.83.059903
  • First-principles investigations of caloric effects in ferroic materials
    Entel, P. and Sahoo, S. and Siewert, M. and Gruner, M.E. and Herper, H.C. and Comtesse, D. and Acet, M. and Buchelnikov, V.D. and Sokolovskiy, V.V.
    AIP Conference Proceedings 1461 (2011)
    We study the magnetic interactions in Ni-Mn-based Heusler alloys which are suitable candidates for refrigeration based on magnetocaloric, barocaloric, and elastocaloric effects, where the adiabatic temperature change of the Heusler material is induced by applying a magnetic field, hydrostatic pressure, or compressive strain. The predominantly ferromagnetic interactions of the Heusler alloys with austenite cubic structure at high temperatures are modified by the appearance of antiferromagnetic interactions in the alloys with Mn-excess because of the much shorter distances between the Mn-excess atoms and those on the original Mn-sublattice. This leads to a larger entropy change across the magnetostructural transformation in Ni 50Mn 25+x(Ga, In, Sn, Sb) 25-x alloys and is also responsible for the appearance of the inverse magnetocaloric effect in the martensitic phase. In Ni-excess Ni-Mn-Ga alloys the influence of antiferromagnetic correlations is weaker and the large entropy change across the magnetostructural transition is mainly caused by the disorder introduced in the Mn sublattice by the excess Ni atoms. We find that the magnetocaloric effect and other functional properties like the magnetic shape-memory effect and the exchange bias effect, are goverened by these complex mixed magnetic features. © 2012 American Institute of Physics.
    view abstract10.1063/1.4736867
  • From tiny magnetic clusters to functional magnetic materials
    Entel, P. and Sahoo, S. and Hucht, A. and Gruner, M.E.
    AIP Conference Proceedings 1393 (2011)
    Magnetic nanoparticles have been of continuous interest, in part, because of the surface effects on the magnetic properties and, in particular, because of their possible use in future magnetic storage devices. In this contribution we discuss the results of ab initio calculations for tiny magnetic transition metal clusters up to ∼103atoms and show the variation of the magnetic moments and magnetic anisotropy as a function of the cluster size, morphology and kind of transition metal atoms. Since the ab initio calculations show that the magnetic moments of the clusters have rather localized character, we have also considered quantum Heisenberg models of spin-1/2 and spin-1 clusters. The exact diagonalization of these models provide some supplementary informations to the first-principles calculations regarding correlation functions and thermodynamic properties. © 2011 American Institute of Physics.
    view abstract10.1063/1.3653606
  • Impact of local lattice distortions on the structural stability of Fe-Pd magnetic shape-memory alloys
    Gruner, M.E. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    The binding surface of Fe-rich Fe-Pd alloys is explored by means of first-principles calculations in the framework of density functional theory involving unconstrained optimization of the atomic positions within a 108-atom supercell. We find that static displacements arising from geometric optimization provide an important contribution to the total energy, effectively compensating favorable contributions gained from introducing L12 order in stoichiometric Fe3Pd. In the concentration range for magnetic shape-memory applications, the energy profile with respect to tetragonal distortion is altered qualitatively, shifting the ground state of the intermixed disordered system from face-centered cubic (fcc) to body-centered tetragonal (bct). From the radial pair distribution function and electronic density of states obtained from a 500-atom supercell calculation we identify the origin of the displacements. These arise from the size-dependent relaxations of the larger Pd atoms, on the fcc side in combination with a Jahn-Teller-like rearrangement of Fe d states at the Fermi level. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.83.214415
  • Magnetic interactions governing the inverse magnetocaloric effect in martensitic Ni-Mn-based shape-memory alloys
    Aksoy, S. and Acet, M. and Krenke, T. and Wassermann, E.F. and Gruner, M. and Entel, P. and Mañosa, L. and Planes, A. and Deen, P.P.
    Springer Series in Materials Science 148 (2011)
    Ni-Mn-X Heusler-type alloys (X: group IIIB-VB elements) undergo martensitic transformations, and many of them exhibit magnetic shape-memory and field-induced effects, one of the most predominant being the inverse magnetocaloric effect. To understand the cause of the inverse magnetocaloric effect, which involves a magnetic entropy increase with applied field, it is necessary to understand the nature of the magnetic coupling in the temperature vicinity of the martensitic transition. We present results on neutron polarization analysis experiments on Ni-Mn-based martensitic Heusler systems, with which we show that around Ms, the magnetic short-range correlations at temperatures T < Ms are antiferromagnetic. We discuss the relationship of the magnetic coupling and the inverse magnetocaloric effect. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-20943-7_4
  • Magnetic nanostructures by adaptive twinning in strained epitaxial films
    Kauffmann-Weiss, S. and Gruner, M.E. and Backen, A. and Schultz, L. and Entel, P. and Fähler, S.
    Physical Review Letters 107 (2011)
    We exploit the intrinsic structural instability of the Fe 70Pd30 magnetic shape memory alloy to obtain functional epitaxial films exhibiting a self-organized nanostructure. We demonstrate that coherent epitaxial straining by 54% is possible. The combination of thin film experiments and large-scale first-principles calculations enables us to establish a lattice relaxation mechanism, which is not expected for stable materials. We identify a low twin boundary energy compared to a high elastic energy as key prerequisite for the adaptive nanotwinning. Our approach is versatile as it allows to control both, nanostructure and intrinsic properties for ferromagnetic, ferroelastic, and ferroelectric materials. © 2011 American Physical Society.
    view abstract10.1103/PhysRevLett.107.206105
  • Sphase diagrams of conventional and inverse functional magnetic heusler alloys: New theoretical and experimental investigations
    Entel, P. and Gruner, M.E. and Hucht, A. and Dannenberg, A. and Siewert, M. and Herper, H.C. and Kakeshita, T. and Fukuda, T. and Sokolovskiy, V.V. and Buchelnikov, V.D.
    Springer Series in Materials Science 148 (2011)
    First-principles calculations allow to characterize the electronic and magnetic ground-state properties of the full-Heusler alloys of type X2YZ. Functionality of the materials strongly depends on the type of elements and composition. A halfmetallic state with 100% spin polarization at the Fermi level, which is an ideal spintronics material for tunneling devices, is, for instance, achieved for (X D Co, Y D Mn, and Z D Ge and Si). Replacing Co by Ni and Ge or Si by Ga yields the prototypical magnetic shape-memory compound Ni2MnGa, which undergoes a (martensitic) tetragonal distortion at ca. 200 K, where the magnetic shape-memory features can be exploited by an external magnetic field and external stress in the martensitic state. Quite another functionality, the conventional or inverse magnetocaloric effect, is observed in the off-stoichiometric samples of (X D Ni, Y D Mn, and Z D Ga, In, Sn, and Sb), where the efficiency of the magnetocaloric effect depends on the size of the isothermal entropy change across the magnetostructural phase transition in an applied magnetic field. Here, we discuss how some of these material properties can be improved in order to obtain room temperature or higher operation temperatures needed for a technological breakthrough. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-20943-7_2
  • Structural and magnetic properties of ternary Fe1-xmnxpt nanoalloys from first principles
    Gruner, M.E. and Entel, P.
    Beilstein Journal of Nanotechnology 2 (2011)
    Background: Structural and magnetic properties of binary Mn-Pt and ternary Fe1-xMnxPt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared. Results: The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices. Conclusion: Addition of Mn effectively increases the stability of single crystalline L10 particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications. © 2011 Gruner and Entel.
    view abstract10.3762/bjnano.2.20
  • 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 abstract10.1088/1742-6596/200/7/072038
  • Chemical trends in structure and magnetism of bimetallic nanoparticles from atomistic calculations
    Gruner, M.E.
    Journal of Physics D: Applied Physics 43 (2010)
    By means of large scale first-principles calculations in the framework of density functional theory, structure and magnetism of 561 atom nanoparticles are compared in order to obtain a systematic picture of the evolution with respect to a change in the constitutional elements. The investigation comprises ordered and disordered, cuboctahedral, icosahedral and decahedral morphologies of composition A265B296, where A is one of Mn, Fe and Co and B is Pt and, additionally, with A = Fe and B = Ni, Pd, Pt, Ir and Au. Fe-Ir and Fe-Pd and Co-Pt exhibit in comparison with Fe-Pt an increased tendency to form multiply-twinned structures and prefer segregation of the heavier element to the surface. The latter trend also applies to Fe-Au, where, on the other hand, icosahedral and crystalline motifs are very close in energy. Only in Mn-Pt the formation of multiply-twinned structures is effectively suppressed. The combinations with reduced valence electron concentration, Mn-Pt and Fe-Ir, exhibit a strong preference for antiferromagnetic spin order. The structural and magnetic trends are tentatively related to the change in features in the element and site-resolved electronic density of states. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/43/47/474008
  • Competing structural ordering tendencies in Heusler-type alloys with high Curie temperatures: Fe2 CoGa1-x Znx studied by first-principles calculations
    Dannenberg, A. and Siewert, M. and Gruner, M.E. and Wuttig, M. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The influence of Zn substitution on the structural, magnetic, and electronic properties and lattice vibrations of ferromagnetic Fe2 CoGa1-x Znx alloys in the conventional X2 YZ and inverse (XY) XZ Heusler structures is investigated, by means of ab initio and Monte Carlo calculations, which predict strong ferromagnetic coupling and high Curie temperatures between 770 and 925 K. In the Ga-rich systems the inverse Heusler structure is energetically favored but no indication for a structural instability is found in contrast to Fe-Co-Ga-Zn alloys in the conventional Heusler structure. The origin of the remarkably strong preference of the cubic (c/a=1) inverse phase is believed to originate from the bcc-like environment of the two inequivalent Fe atoms and their stronger hybridization with the Co states compared to the conventional structure. In the quaternary compounds, Fe2 CoGa1-x Znx, substitution of Ga by Zn reduces the energetic preference of the inverse structure caused by weakening of the Co-Fe hybridization. Simultaneously, Zn leads to higher magnetic moments and Curie temperatures because of localization effects. In addition, since Zn weakens the miscibility of the alloys, we propose that the composition of Fe2 CoGa1-x Znx alloys has to be carefully chosen in order to yield an interesting future ferromagnetic shape-memory alloy system. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.214421
  • Core-shell morphologies of FePt and CoPt nanoparticles: An ab initio comparison
    Gruner, M.E.
    Journal of Physics: Conference Series 200 (2010)
    Large-scale first principles calculations for near-stoichiometric FePt and CoPt clusters with up to 923 atoms are presented including unconstrained structural relaxations. The energetic order and magnetic properties of 561 atom FePt and CoPt nanoparticles are evaluated with emphasis on segregated morphologies with Pt covered surfaces. The results imply that for CoPt particles segregation of Pt to the surface and the formation of a Pt depleted subsurface layer is dominant also for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, while for FePt multiple twinning is the most important mechanism at small particle sizes. The mixed (100) surfaces of the fully L10 ordered FePt and CoPt isomers exhibit a characteristic reconstruction. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/7/072039
  • Electronic structure and lattice dynamics of the magnetic shape-memory alloy Co2 NiGa
    Siewert, M. and Gruner, M.E. and Dannenberg, A. and Hucht, A. and Shapiro, S.M. and Xu, G. and Schlagel, D.L. and Lograsso, T.A. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    In addition to the prototypical Ni-Mn-based Heusler alloys, the Co-Ni-Ga systems have recently been suggested as another prospective materials class for magnetic shape-memory applications. We provide a characterization of the dynamical properties of this material and their relation to the electronic structure within a combined experimental and theoretical approach. This relies on inelastic neutron scattering to obtain the phonon dispersion while first-principles calculations provide the link between dynamical properties and electronic structure. In contrast to Ni2 MnGa, where the softening of the TA2 phonon branch is related to Fermi-surface nesting, our results reveal that the respective anomalies are absent in Co-Ni-Ga, in the phonon dispersions as well as in the electronic structure. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.064420
  • First-principles and Monte Carlo study of magnetostructural transition and magnetocaloric properties of Ni2+x Mn1-x Ga
    Buchelnikov, V.D. and Sokolovskiy, V.V. and Herper, H.C. and Ebert, H. and Gruner, M.E. and Taskaev, S.V. and Khovaylo, V.V. and Hucht, A. and Dannenberg, A. and Ogura, M. and Akai, H. and Acet, M. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    First-principles calculations of magnetic exchange parameters of the austenitic and martensitic phases of Ni-Mn-Ga allow one to characterize this Heusler system across the phase diagram in agreement with experimental trends. The ab initio investigations have been combined with Monte Carlo simulations for a detailed description of the magnetic, martensitic, and magnetocaloric properties of Ni2+x Mn1-x Ga (0.18≤x≤0.27) Heusler alloys, which undergo a first-order magnetostructural phase transition. For these alloys, the calculated temperature dependence of the magnetic and lattice contributions to the total specific heat as well as the evaluation of the isothermal magnetic entropy Δ Smag (T, Hext) and adiabatic temperature ΔT (T, Hext) changes around the magnetostructural transition in an external magnetic field agree fairly well with the experimental data. In particular, results for Δ Smag (T, Hext) and ΔT (T, Hext) may be used to speculate about designing new magnetic Heusler alloys with better magnetocaloric properties, i.e., larger ΔT (T, Hext) values. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.094411
  • First-principles study of the structural stability of L11 order in Pt-based alloys
    Dannenberg, A. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    We investigate in the framework of density functional theory the structural and electronic properties of stoichiometric L11 ordered transition metal alloys of Pt and the 3d transition metals Mn, Fe, Co, Ni and Cu. A marked dependence of the energy difference between L11 and L10 structure on the valence electron concentration is encountered, with the L1 1 order being the preferred structure for CuPt, whereas the other alloys favor the L10 arrangement. The changes of the electronic density of states on composition are well represented within a rigid-band-picture, while the transition from L10 to L11 order is accompanied by a characteristic redistribution of the minority spin states around the Fermi level. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/7/072021
  • Fundamental aspects of magnetic shape memory alloys: Insights from ab initio and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Dannenberg, A. and Siewert, M. and Nayak, S.K. and Herper, H.C. and Buchelnikov, V.D.
    Materials Science Forum 635 (2010)
    Ferromagnetic Heusler alloys like Ni-Mn-Z (Z = Al, Ga, In, Sn, Sb), which undergo a martensitic phase transformation, are on the edge of being used in technological applications involving actuator and magnetocaloric devices. The other class of ferromagnetic full Heusler alloys like Co-Mn-Z (Z = Al, Si, Ga, Ge, Sn) not undergoing a structural phase transition, are half-metals (in contrast to the Ni-based systems) with high spin polarization at the Fermi level and are of potential importance for future spintronics devices. On the basis of recent ab initio calculations, we highlight the main differences between the two classes of Heusler based materials. © (2010) Trans Tech Publications.
    view abstract10.4028/www.scientific.net/MSF.635.3
  • Lattice dynamics and structural stability of ordered Fe3 Ni, Fe3 Pd and Fe3 Pt alloys using density functional theory
    Gruner, M.E. and Adeagbo, W.A. and Zayak, A.T. and Hucht, A. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 81 (2010)
    We investigate the binding surface along the Bain path and phonon dispersion relations for the cubic phase of the ferromagnetic binary alloys Fe3 X (X=Ni,Pd,Pt) for L 12 and D 022 ordered phases from first principles by means of density functional theory. The phonon dispersion relations exhibit a softening of the transverse acoustic mode at the M point in the L 12 phase in accordance with experiments for ordered Fe3 Pt. This instability can be associated with a rotational movement of the Fe atoms around the Ni-group element in the neighboring layers and is accompanied by an extensive reconstruction of the Fermi surface. In addition, we find an incomplete softening in [111] direction which is strongest for Fe3 Ni. We conclude that besides the valence electron density also the specific Fe-content and the masses of the alloying partners should be considered as parameters for the design of Fe-based functional magnetic materials. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.81.064109
  • Magnetic properties of small Pt-capped Fe, Co, and Ni clusters: A density functional theory study
    Sahoo, S. and Hucht, A. and Gruner, M.E. and Rollmann, G. and Entel, P. and Postnikov, A. and Ferrer, J. and Fernández-Seivane, L. and Richter, M. and Fritsch, D. and Sil, S.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    We have undertaken theoretical studies of spin and orbital magnetic moments as well as magnetic anisotropy energies for M13(M=Fe,Co,Ni) and M13 Ptn (n=3,4,5,20) clusters including the spin-orbit coupling in the framework of density functional theory. For all M13 clusters considered we find tendencies for small structural distortions which can be characterized by either Jahn-Teller (JT) or Mackay transformations (MT). The magnetic anisotropy energy (MAE) along with the spin and orbital moments are calculated for M13 icosahedral clusters and the angle-dependent energy differences are modeled using a Néel model with local anisotropies. From our studies, the MAE for JT-distorted M13 clusters are found to be larger relative to the MT clusters and more than two orders of magnitude larger compared to the corresponding bcc or fcc bulk values. In addition, we demonstrate for one example that Pt capping may further enhance the MAE compared to the uncapped JT- and the Mackay-distorted Fe13 cluster. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.054418
  • Structural ordering tendencies in the new ferromagnetic Ni-Co-Fe-Ga-Zn Heusler alloys
    Dannenberg, A. and Siewert, M. and Gruner, M.E. and Wuttig, M. and Entela, P.
    Physics Procedia 10 (2010)
    In search for new ferromagnetic shape memory alloys (FSMA) we have calculated structural energy differences, magnetic exchange interaction constants and mixing energies of quaternary (X 1X 2)YZ Heusler alloys with X 1, X 2, Y = Ni, Co, Fe and Z = Ga, Zn using density functional theory. The comparison of the energy profiles of (NiCo)FeZ, (FeNi)CoZ, and (FeCo)NiZ with Z = Ga and Zn as a function of the tetragonal distortion c/a reveals that the energetically preferred ordering type is (NiCo)FeGa and (NiCo)FeZn which shows that Fe prefers to occupy the same cubic sublattice as Ga or Zn what implies that Fe favors Co and Ni as nearest neighbors, respectively. The Curie temperatures of (NiCo)FeGa and (NiCo)FeZn are high of the order of 600 K. NiCo)FeGa, which has the same valence electron concentration (e/a = 7.5) as Ni 2MnGa and also possesses a high martensitic ransformation temperature (&gt; 500 K), is of interest for future magnetic shape memory devices.
    view abstract10.1016/j.phpro.2010.11.090
  • 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 (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 abstract10.1016/j.phpro.2010.11.089
  • The ferromagnetic shape memory system Fe-Pd-Cu
    Hamann, S. and Gruner, M.E. and Irsen, S. and Buschbeck, J. and Bechtold, C. and Kock, I. and Mayr, S.G. and Savan, A. and Thienhaus, S. and Quandt, E. and Fähler, S. and Entel, P. and Ludwig, Al.
    Acta Materialia 58 (2010)
    A new ferromagnetic shape memory thin film system, Fe-Pd-Cu, was developed using ab initio calculations, combinatorial fabrication and high-throughput experimentation methods. Reversible martensitic transformations are found in extended compositional regions, which have increased fcc-fct transformation temperatures in comparison to previously published results. High resolution transmission electron microscopy verified the existence of a homogeneous ternary phase without precipitates. Curie temperature, saturation polarization and orbital magnetism are only moderately decreased by alloying with nonmagnetic Cu. Compared to the binary system; enhanced Invar-type thermal expansion anomalies in terms of an increased volume magnetostriction are predicted. Complementary experiments on splat-fabricated bulk Fe-Pd-Cu samples showed an enhanced stability of the disordered transforming Fe70Pd30 phase against decomposition. From the comparison of bulk and thin film results, it can be inferred that, for ternary systems, the Fe content, rather than the valence electron concentration, should be regarded as the decisive factor determining the fcc-fct transformation temperature. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.actamat.2010.07.011
  • ab initio calculations

  • density functional theory

  • energy conversion

  • first-principles calculation

  • Lattice dynamics

  • magnetism

  • magnetocaloric effects

  • martensite

  • metallic nanoparticles

  • microstructure

  • shape-memory alloys

« back