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  • 1.
    Jönsson, Johan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Bykov, Maxim
    Univ Bayreuth, Germany.
    Dubrovinsky, Leonid
    Univ Bayreuth, Germany.
    van Smaalen, Sander
    Univ Bayreuth, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Natl Univ Sci and Technol MISIS, Russia.
    Inverse pressure-induced Mott transition in TiPO42019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 24, article id 245132Article in journal (Refereed)
    Abstract [en]

    TiPO4 shows interesting structural and magnetic properties as temperature and pressure are varied, such as a spin-Peierls phase transition and the development of incommensurate modulations of the lattice. Recently, high-pressure experiments for TiPO4 reported two structural phases appearing at high pressures, the so-called phases IV and V [M. Bykov et al., Angew. Chem. Int. Ed. 55, 15053 (2016).]. The latter was shown to include the first example of fivefold O-coordinated P atoms in an inorganic phosphate compound. In this work, we characterize the electronic structure and other physical properties of these phases by means of ab initio calculations and investigate the structural transition. We find that the appearance of phases IV and V coincides with a collapse of the Mott insulating gap and quenching of magnetism in phase III as pressure is applied. Remarkably, our calculations show that in the high-pressure phase V, these features reappear, leading to an antiferromagnetic Mott insulating phase, with robust local moments.

  • 2.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Univ Bayreuth, Germany.
    Schoenleber, A.
    Univ Bayreuth, Germany.
    van Smaalen, S.
    Univ Bayreuth, Germany.
    The role of magnetic order in VOCl2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 32, article id 325502Article in journal (Refereed)
    Abstract [en]

    VOCl and other transition metal oxychlorides are candidate materials for next-generation rechargeable batteries. We have investigated the influence of the underlying magnetic order on the crystallographic and electronic structure by means of density functional theory. Our study shows that antiferromagnetic ordering explains the observed low-temperature monoclinic distortion of the lattice, which leads to a decreased distance between antiferromagnetically coupled V-V nearest neighbors. We also show that the existence of a local magnetic moment removes the previously suggested degeneracy of the occupied levels, in agreement with experiments. To describe the electronic structure, it turns out crucial to take the correct magnetic ordering into account, especially at elevated temperature.

  • 3.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Gambino, Davide
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Jönsson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Max Planck Inst Eisenforsch GmbH, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. Natl Univ Sci and Technol MISIS, Russia.
    Assessing the SCAN functional for itinerant electron ferromagnets2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 9, article id 094413Article in journal (Refereed)
    Abstract [en]

    Density functional theory is a standard model for condensed-matter theory and computational material science. The accuracy of density functional theory is limited by the accuracy of the employed approximation to the exchange-correlation functional. Recently, the so-called strongly constrained appropriately normed (SCAN) [Sun, Ruzsinszky, and Perdew, Phys. Rev. Lett. 115, 036402 (2015)] functional has received a lot of attention due to promising results for covalent, metallic, ionic, as well as hydrogen- and van der Waals-bonded systems alike. In this work, we focus on assessing the performance of the SCAN functional for itinerant magnets by calculating basic structural and magnetic properties of the transition metals Fe, Co, and Ni. We find that although structural properties of bcc-Fe seem to be in good agreement with experiment, SCAN performs worse than standard local and semilocal functionals for fcc-Ni and hcp-Co. In all three cases, the magnetic moment is significantly overestimated by SCAN, and the 3d states are shifted to lower energies, as compared to experiments.

  • 4.
    Feng, Qingguo
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Tasnadi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Jönsson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. NUST MISIS, Russia.
    Topological transitions of the Fermi surface of osmium under pressure: an LDA plus DMFT study2017In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 19, article id 033020Article in journal (Refereed)
    Abstract [en]

    The influence of pressure on the electronic structure of Os has attracted substantial attention recently due to reports on isostructural electronic transitions in this metal. Here, we theoretically investigate the Fermi surface of Os from ambient to high pressure, using density functional theory combined with dynamical mean field theory. Weprovide a detailed discussion of the calculated Fermi surface and its dependence on the level of theory used for the treatment of the electron-electron interactions. Although we confirm that Os can be classified as weakly correlated metal, the inclusion of local quantum fluctuations between 5d electrons beyond the local density approximation explains the most recent experimental reports regarding the occurrence of electronic topological transitions in Os.

  • 5.
    Potapkin, V.
    et al.
    Forschungszentrum Julich, Germany.
    Dubrovinsky, L.
    University of Bayreuth, Germany.
    Sergueev, I.
    DESY, Germany.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Kantor, I.
    European Synchrotron Radiat Facil, France.
    Bessas, D.
    European Synchrotron Radiat Facil, France.
    Bykova, E.
    University of Bayreuth, Germany.
    Prakapenka, V.
    University of Chicago, IL 60437 USA.
    Hermann, R. P.
    Forschungszentrum Julich, Germany; Oak Ridge National Lab, TN 37831 USA.
    Rueffer, R.
    European Synchrotron Radiat Facil, France.
    Cerantola, V.
    University of Bayreuth, Germany.
    Jönsson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Olovsson, W.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, Faculty of Science & Engineering.
    Mankovsky, S.
    University of Munich, Germany.
    Ebert, H.
    University of Munich, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia; Tomsk State University, Russia.
    Magnetic interactions in NiO at ultrahigh pressure2016In: PHYSICAL REVIEW B, ISSN 2469-9950, Vol. 93, no 20, p. 201110-Article in journal (Refereed)
    Abstract [en]

    Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mossbauer transition of Ni-61. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to similar to 24 T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa.

  • 6.
    Tal, Alexey
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia.
    Katsnelson, Mikhail I.
    Radboud University of Nijmegen, Netherlands; Ural Federal University, Russia.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Jönsson, Johan
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Dubrovinsky, Leonid
    University of Bayreuth, Germany.
    Dubrovinskaia, Natalia
    University of Bayreuth, Germany.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia.
    Pressure-induced crossing of the core levels in 5d metals2016In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 20, p. 205150-Article in journal (Refereed)
    Abstract [en]

    A pressure-induced interaction between core electrons, the core-level crossing (CLC) transition, has been observed in hcp Os at P approximate to 400 GPa [L. Dubrovinsky et al., Nature (London) 525, 226 (2015)]. By carrying out a systematic theoretical study for all metals of the 5d series (Hf, Ta, W, Re, Os, Ir, Pt, Au) we have found that the CLC transition is a general effect for this series of metals. While in Pt it occurs at approximate to 1500 GPa, at a pressure substantially higher than in Os, in Ir it occurs already at 80 GPa. Moreover, we predict that in Re the CLC transition may take place already at ambient pressure. We explain the effect of the CLC and analyze the shift of the transition pressure across the series within the Thomas-Fermi model. In particular, we show that the effect has many common features with the atomic collapse in rare-earth elements.

  • 7.
    Dubrovinsky, L.
    et al.
    University of Bayreuth, Germany.
    Dubrovinskaia, N.
    University of Bayreuth, Germany.
    Bykova, E.
    University of Bayreuth, Germany; University of Bayreuth, Germany.
    Bykov, M.
    University of Bayreuth, Germany.
    Prakapenka, V.
    University of Chicago, IL 60437 USA.
    Prescher, C.
    University of Chicago, IL 60437 USA.
    Glazyrin, K.
    Deutsch Elektronen Synchrotron DESY, Germany.
    Liermann, H. -P.
    Deutsch Elektronen Synchrotron DESY, Germany.
    Hanfland, M.
    European Synchrotron Radiat Facil, France.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering.
    Feng, Qingguo
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Pourovskii, L. V.
    Linköping University, Faculty of Science & Engineering. Ecole Polytech, France.
    Katsnelson, M. I.
    Radboud University of Nijmegen, Netherlands; Ural Federal University, Russia.
    Wills, J. M.
    Los Alamos National Lab, NM 87545 USA.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, Faculty of Science & Engineering. National University of Science and Technology MISIS, Russia.
    The most incompressible metal osmium at static pressures above 750 gigapascals2015In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 525, no 7568, p. 226-+Article in journal (Refereed)
    Abstract [en]

    Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures(1). It is also very incompressible(2-4), but its high-pressure behaviour is not well understood because it has been studied(2-6) so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells(7), with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.

  • 8.
    Reeh, S
    et al.
    Rhein Westfal TH Aachen, Germany .
    Kasprzak, M
    Rhein Westfal TH Aachen, Germany .
    Klusmann, C D.
    Rhein Westfal TH Aachen, Germany .
    Stalf, F
    Rhein Westfal TH Aachen, Germany .
    Music, D
    Rhein Westfal TH Aachen, Germany .
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Schneider, J M.
    Rhein Westfal TH Aachen, Germany .
    Elastic properties of fcc Fe-Mn-X (X = Cr, Co, Ni, Cu) alloys studied by the combinatorial thin film approach and ab initio calculations2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 24Article in journal (Refereed)
    Abstract [en]

    The elastic properties of fcc Fe-Mn-X (X = Cr, Co, Ni, Cu) alloys with additions of up to 8 at.% X were studied by combinatorial thin film growth and characterization and by ab initio calculations using the disordered local moments (DLM) approach. The lattice parameter and Youngs modulus values change only marginally with X. The calculations and experiments are in good agreement. We demonstrate that the elastic properties of transition metal alloyed Fe-Mn can be predicted by the DLM model.

  • 9.
    Reeh, S.
    et al.
    Materials Chemistry, RWTH Aachen University, Aachen, Germany.
    Music, D.
    Materials Chemistry, RWTH Aachen University, Aachen, Germany.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Schneider, J. M.
    Materials Chemistry, RWTH Aachen University, Aachen, Germany.
    Elastic properties of fcc Fe-Mn-X (X=Cr, Co, Ni, Cu) alloys from first-principles calculations2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 22Article in journal (Refereed)
    Abstract [en]

    The influence of the valence electron concentration of X in fcc Fe-Mn-X (X=Cr, Co, Ni, Cu) alloys on the elastic and magnetic properties has been studied by means of ab initio calculations for alloy element concentrations of up to 8 at. % X. We observe that Cu increases the bulk-to-shear modulus (B/G) ratio by 19.2%. Simultaneously magnetic moments of Fe and Mn increase strongly. The other alloying elements induce less significant changes in B/G. The trends in B/G may be understood by considering the changes in G induced by the variation in valence electron concentration (VEC). As the VEC is increased, more pronounced metallic bonds are formed, giving rise to smaller shear modulus values. The changes in magnetic moments may be explained by the magnetovolume effect. Alloys with smaller VEC as Fe-Mn exhibit a decrease in local magnetic moments and equilibrium lattice parameters, while alloys with larger VEC as Fe-Mn demonstrate an increase in local magnetic moments and equilibrium lattice parameters. These data provide the basis for the design of Mn-rich steels with enhanced elastic properties.

  • 10.
    Sergueev, I.
    et al.
    Deutsches Elektronen-Synchrotron, D-22607 Hamburg, Germany.
    Dubrovinsky, Leonid
    Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany.
    Ekholm, Marcus
    Linköping University, Department of Electrical Engineering. Linköping University, The Institute of Technology.
    Vekilova, Olga
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Chumakov, A. I.
    European Synchrotron Radiation Facility (ESRF), Grenoble, France.
    Zając, M.
    European Synchrotron Radiation Facility (ESRF), Grenoble, France.
    Potapkin, V.
    European Synchrotron Radiation Facility (ESRF), Grenoble, France.
    Kantor, I.
    European Synchrotron Radiation Facility (ESRF), Grenoble, France.
    Bornemann, S.
    Department of Chemistry, Ludwig-Maximilians-Universität München, München, Germany.
    Ebert, H.
    Department of Chemistry, Ludwig-Maximilians-Universität München, München, Germany.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ruffer, R.
    European Synchrotron Radiation Facility (ESRF), P.O. Box 220, F-38043 Grenoble, France.
    Hyperfine Splitting and Room-Temperature Ferromagnetism of Ni at Multimegabar Pressure2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 111, no 15Article in journal (Refereed)
    Abstract [en]

    Magnetic and elastic properties of Ni metal have been studied up to 260 GPa by nuclear forward scattering of synchrotron radiation with the 67.4 keV Mossbauer transition of Ni-61. The observed magnetic hyperfine splitting confirms the ferromagnetic state of Ni up to 260 GPa, the highest pressure where magnetism in any material has been observed so far. Ab initio calculations reveal that the pressure evolution of the hyperfine field, which features a maximum in the range of 100 to 225 GPa, is a relativistic effect. The Debye energy obtained from the Lamb-Mossbauer factor increases from 33 me V at ambient pressure to 60 me V at 100 GPa. The change of this energy over volume compression is well described by a Gruneisen parameter of 2.09.

  • 11.
    Glazyrin, K.
    et al.
    Bayerisches Geoinstitut, Universität Bayreuth, Germany.
    Pourovskii, L.
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Dubrovinsky, L.
    Bayerisches Geoinstitut, Universität Bayreuth, Germany.
    Narygina, O.
    School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK.
    McCammon, C.
    Bayerisches Geoinstitut, Universität Bayreuth, Germany.
    Hewener, B.
    Technische Universität Kaiserslautern, Kaiserslautern, Germany.
    Schünemann, V.
    Technische Universität Kaiserslautern, Kaiserslautern, Germany.
    Wolny, J.
    Technische Universität Kaiserslautern, Kaiserslautern, Germany.
    Muffler, K.
    Technische Universität Kaiserslautern, Kaiserslautern, Germany.
    Chumakov, A. I.
    ESRF, Grenoble Cedex, France.
    Crichton, W.
    ESRF, Grenoble Cedex, France.
    Hanfland, M.
    ESRF, Grenoble Cedex, France.
    Prakapenka, V.
    GeoSoilEnviroCARS, University of Chicago, Argonne National Laboratory, USA.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Aichhorn, M.
    Centro Atómico Constituyentes, GIyANN, CNEA, San Martin, Buenos Aires, Comisión Nacional de Investigaciones Científicas y Técnicas, Ciudad de Buenos Aires, Argentina.
    Vildosola, V.
    Centro Atómico Constituyentes, GIyANN, CNEA, San Martin, Buenos Aires, Comisión Nacional de Investigaciones Científicas y Técnicas, Ciudad de Buenos Aires, Argentina.
    Ruban, A. V.
    12Department of Materials Science and Engineering, Royal Institute of Technology, Stockholm, Sweden.
    Katsnelson, M. I.
    Radboud University Nijmegen, Institute for Molecules and Materials, Netherlands.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition2013In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 11Article in journal (Refereed)
    Abstract [en]

    We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. DOI: 10.1103/PhysRevLett.110.117206

  • 12.
    Abrikosov, Igor
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ponomareva, Alena V.
    Theoretical Physics Department, Moscow Steel and Alloys Institute, Russua.
    Barannikova, Svetlana A.
    Institute of Strength Physics and Materials Science, Siberian Branch of Russian Academy of Science, Tomsk, Russia.
    Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys2012In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 190, p. 291-294Article in journal (Refereed)
    Abstract [en]

    We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12 face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.

  • 13.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Theoretical Descriptions of Complex Magnetism in Transition Metals and Their Alloys2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis, various methods for studying solids by simulations of quantummechanical equations, have been applied to transition metals and their alloys. Transition metals such as Fe, Ni, and Mn, are not only cornerstones in modern technology, but also key components in the very fabric of the Earth interior. Such systems show highly complex magnetic properties. As shown within this thesis, to understand and predict their properties from a microscopic level, is still a highly demanding task for the the quantum theory of solids. This is especially crucial at elevated temperature and pressure.

    It is found that the magnetic degrees of freedom are inseparable from the structural, elastic and chemical properties of such alloy systems. This requires theoretical descriptions capable of handling this interplay. Such schemes are discussed and demonstrated.

    Furthermore, the importance of the description of Coulomb correlation effects is demonstrated by DFT calculations and also by going beyond the one-electron description by the LDA+DMFT method.

    It is also shown how magnetic interactions in the half-metallic compound NiMnSb can be manipulated by alloying. The stability of these alloys is  also evaluated in calculations, and verified by experimental synthesis at ambient conditions.

    List of papers
    1. Structural and magnetic ground-state properties of gamma-FeMn alloys from ab initio calculations
    Open this publication in new window or tab >>Structural and magnetic ground-state properties of gamma-FeMn alloys from ab initio calculations
    2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 10, p. 104423-Article in journal (Refereed) Published
    Abstract [en]

    The magnetic properties of fcc-FeMn alloys, especially at the Fe(0.5)Mn(0.5) composition, have been the subject of intense experimental and theoretical investigations for several decades. We carry out an ab initio theoretical study of this system, including simultaneous optimization of structural and magnetic properties, and find that the ground state is the locally relaxed noncollinear 3Q antiferromagnetic structure. We also show that the two most frequently used parameterizations of the generalized gradient approximation not only fail to reproduce the equilibrium lattice constant of FeMn alloys, and consequently the magnetic properties, but also internally yield qualitatively different results. For practical studies of these alloys, which currently attract great attention, we propose a set of approximations, which is internally consistent, and brings the equilibrium lattice constant and magnetic properties in good agreement with the experiment in the whole range of alloy compositions.

    Place, publisher, year, edition, pages
    American Physical Society, 2011
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-71074 (URN)10.1103/PhysRevB.84.104423 (DOI)000294922100002 ()
    Note
    Funding Agencies|Goran Gustafsson Foundation for Research in Natural Sciences and Medicine||Swedish e-Science Research Centre (SeRC)||Available from: 2011-09-30 Created: 2011-09-30 Last updated: 2017-12-08
    2. Influence of chemical composition and magnetic effects on the elastic properties of fcc Fe-Mn alloys
    Open this publication in new window or tab >>Influence of chemical composition and magnetic effects on the elastic properties of fcc Fe-Mn alloys
    Show others...
    2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 4, p. 1493-1501Article in journal (Refereed) Published
    Abstract [en]

    The influence of the Mn content on the elastic properties of face centered cubic Fe-Mn alloys was studied using the combinatorial approach. Fe-Mn thin films with a graded chemical composition were synthesized. Nanoindentation experiments were carried out to investigate the elastic properties as a function of the Mn content. As the Mn content increases from similar to 23 to similar to 39 at.%, the average bulk modulus varies from 143 to 105 GPa. Ab initio calculations served to probe the impact of magnetic effects on the elastic properties. The magnetic state description with disordered local moments yields the best agreement with the experimental results, whereas with non-magnetic and antiferromagnetic configurations the bulk modulus is overestimated. The strong impact of the magnetic configuration may be understood based on the differences in the chemical bonding and the magnetovolume effect. It is suggested that, owing to minute energy differences of competing antiferromagnetic configurations, a mixture of these with a "notional magnetic disorder" is present, which is in fact well described by the disordered local moments model. These results show that the combinatorial thin film synthesis with subsequent nanoindentation is an appropriate tool for investigating the elastic properties of Fe-Mn alloys systematically as a function of the chemical composition, to validate theoretical models.

    Place, publisher, year, edition, pages
    Elsevier Science B.V., Amsterdam., 2011
    Keywords
    Iron alloys, Sputtering, Elastic properties, Ab initio calculations, Nanoindentation
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-66331 (URN)10.1016/j.actamat.2010.11.013 (DOI)000287265100018 ()
    Available from: 2011-03-11 Created: 2011-03-11 Last updated: 2017-12-11Bibliographically approved
    3. Ab initio lattice stability of fcc and hcp Fe-Mn random alloys
    Open this publication in new window or tab >>Ab initio lattice stability of fcc and hcp Fe-Mn random alloys
    Show others...
    2010 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 29, p. 295402-Article in journal (Refereed) Published
    Abstract [en]

    We have studied the lattice stability of face centred cubic (fcc) versus hexagonal close packed (hcp) Fe-Mn random alloys using ab initio calculations. In the calculations we considered the antiferromagnetic order of local moments, which for fcc alloys models the magnetic configuration of this phase at room temperature (below its Neel temperature) as well as their complete disorder, corresponding to paramagnetic fcc and hcp alloys. For both cases, the results are consistent with our thermodynamic calculations, obtained within the Calphad approach. For the room temperature magnetic configuration, the cross-over of the total energies of the hcp phase and the fcc phase of Fe-Mn alloys is at the expected Mn content, whereas for the magnetic configuration above the fcc Neel temperature, the hcp lattice is more stable within the whole composition range studied. The increase of the total energy difference between hcp and antiferromagnetic fcc due to additions of Mn as well as the stabilizing effect of antiferromagnetic ordering on the fcc phase are well displayed. These results are of relevance for understanding the deformation mechanisms of these random alloys.

    Place, publisher, year, edition, pages
    Iop Publishing Ltd, 2010
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-58224 (URN)10.1088/0953-8984/22/29/295402 (DOI)000279616000012 ()
    Available from: 2010-08-10 Created: 2010-08-09 Last updated: 2017-12-12Bibliographically approved
    4. The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Open this publication in new window or tab >>The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys
    Show others...
    2011 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 24, p. 246003-Article in journal (Refereed) Published
    Abstract [en]

    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Neel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase.

    Place, publisher, year, edition, pages
    Iop Publishing Ltd, 2011
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-69173 (URN)10.1088/0953-8984/23/24/246003 (DOI)000291152500019 ()
    Available from: 2011-06-17 Created: 2011-06-17 Last updated: 2017-12-11Bibliographically approved
    5. Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment
    Open this publication in new window or tab >>Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment
    Show others...
    2011 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 8, p. 3145-3155Article in journal (Refereed) Published
    Abstract [en]

    We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Youngs modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Youngs modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Youngs modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Youngs modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.

    Place, publisher, year, edition, pages
    Elsevier Science B.V., Amsterdam., 2011
    Keywords
    Iron alloys, Sputtering, Elastic properties, Ab initio calculations, Nanoindentation
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-68186 (URN)10.1016/j.actamat.2011.01.054 (DOI)000290053100024 ()
    Available from: 2011-05-13 Created: 2011-05-13 Last updated: 2017-12-11Bibliographically approved
    6. Supercell Calculations of Hyperfine Interactions in Transition-Metal Alloys
    Open this publication in new window or tab >>Supercell Calculations of Hyperfine Interactions in Transition-Metal Alloys
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    We have performed calculations for the hyperfine field in the disordered Fe0.5Ni0.5 alloy using supercells with up to 864 atoms. The computational scheme is based on exact muffin-tin orbitals and the locally self-consistent Green’s function formalism, which scales linearly with the number of atoms in the supercell. This scheme allows local environment effects, such as chemical and magnetic environment, and short-range order, to be explicitly included. Supercell calculations for Fe-Ni show that while the average fields coincides with that obtained using the coherent potential approximation, there is a significant distribution of the hyperfine fields depending on the local environment. The fields of Fe and Ni show qualitatively different behaviour as a function of the on-site magnetic moment, but scale linearly with the average magnetic moment in the first coordination shell.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-78774 (URN)
    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2015-08-19Bibliographically approved
    7. Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys
    Open this publication in new window or tab >>Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys
    2012 (English)In: Solid State Phenomena, ISSN 1012-0394, E-ISSN 1662-9779, Vol. 190, p. 291-294Article in journal (Refereed) Published
    Abstract [en]

    We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12 face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.

    Keywords
    Magnetic transition metal alloys, magnetic ground state, disordered local moments, elastic properties, first-principles simulations
    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:liu:diva-78775 (URN)10.4028/www.scientific.net/SSP.190.291 (DOI)000308061600070 ()
    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2017-12-07Bibliographically approved
    8. Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy
    Open this publication in new window or tab >>Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy
    Show others...
    2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 16, p. 167208-Article in journal (Refereed) Published
    Abstract [en]

    In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.

    Place, publisher, year, edition, pages
    American Physical Society, 2010
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-61178 (URN)10.1103/PhysRevLett.105.167208 (DOI)000283056100008 ()
    Note
    Original Publication: Marcus Ekholm, H Zapolsky, A V Ruban, I Vernyhora, D Ledue and Igor Abrikosov, Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy, 2010, PHYSICAL REVIEW LETTERS, (105), 16, 167208. http://dx.doi.org/10.1103/PhysRevLett.105.167208 Copyright: American Physical Society http://www.aps.org/Available from: 2010-11-08 Created: 2010-11-05 Last updated: 2017-12-12Bibliographically approved
    9. Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition
    Open this publication in new window or tab >>Importance of correlation effects in hcp iron revealed by a pressure-induced electronic topological transition
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    2013 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 110, no 11Article in journal (Refereed) Published
    Abstract [en]

    We discover that hcp phases of Fe and Fe0.9Ni0.1 undergo an electronic topological transition at pressures of about 40 GPa. This topological change of the Fermi surface manifests itself through anomalous behavior of the Debye sound velocity, c/a lattice parameter ratio, and Mossbauer center shift observed in our experiments. First-principles simulations within the dynamic mean field approach demonstrate that the transition is induced by many-electron effects. It is absent in one-electron calculations and represents a clear signature of correlation effects in hcp Fe. DOI: 10.1103/PhysRevLett.110.117206

    Place, publisher, year, edition, pages
    American Physical Society, 2013
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-78777 (URN)10.1103/PhysRevLett.110.117206 (DOI)000316172500027 ()
    Note

    Funding Agencies|Swedish e-Science Research Centre (SeRC)||Swedish Research Council|621-2011-4426|Swedish Foundation for Strategic Research (SSF) programs SRL Grant|10-0026|German Science Foundation (DFG)||German Ministry for Education and Research (BMBF)||National Science Foundation-Earth Sciences|EAR-1128799|Department of Energy-Geosciences|DE-FG02-94ER14466|U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences|DE-AC02-06CH11357|

    Available from: 2012-06-20 Created: 2012-06-20 Last updated: 2017-12-07Bibliographically approved
    10. Configurational thermodynamics of Fe-Ni alloys at Earths core conditions
    Open this publication in new window or tab >>Configurational thermodynamics of Fe-Ni alloys at Earths core conditions
    Show others...
    2011 (English)In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 308, no 1-2, p. 90-96Article in journal (Refereed) Published
    Abstract [en]

    By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.

    Place, publisher, year, edition, pages
    Elsevier, 2011
    Keywords
    Earths inner core, Fe-Ni alloy, ab-initio calculations, crystal structure
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-70218 (URN)10.1016/j.epsl.2011.05.035 (DOI)000293486100009 ()
    Note
    Original Publication: Marcus Ekholm, Arkady Mikhaylushkin, Sergey Simak, B Johansson and Igor Abrikosov, Configurational thermodynamics of Fe-Ni alloys at Earths core conditions, 2011, Earth and Planetary Science Letters, (308), 1-2, 90-96. http://dx.doi.org/10.1016/j.epsl.2011.05.035 Copyright: Elsevier http://www.elsevier.com/Available from: 2011-08-26 Created: 2011-08-26 Last updated: 2017-12-08Bibliographically approved
    11. Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb
    Open this publication in new window or tab >>Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb
    2008 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 77, no 14, p. 144414-Article in journal (Refereed) Published
    Abstract [en]

    We have performed a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d metals Cr, Mn, Fe, Co, and Ni, with respect to both energetics and magnetic properties. Starting from the formation energies, we discuss the possibility of placing the dopant on different crystallographic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions, and density of states and also outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn have the largest impact on magnetic interactions in the system while preserving its half-metallic property. Therefore, we suggest the possibility that these dopants increase the thermal stability of half-metallicity in NiMnSb, with implications for its possible usage in spintronics applications.

    Keywords
    NiMnSb, spintronics, half-metallic, doping, defects
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-60433 (URN)10.1103/PhysRevB.77.144414 (DOI)
    Available from: 2010-10-13 Created: 2010-10-13 Last updated: 2017-12-12
    12. Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni1-xMn1+xSb
    Open this publication in new window or tab >>Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni1-xMn1+xSb
    Show others...
    2010 (English)In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979, Vol. 108, no 9Article in journal (Refereed) Published
    Abstract [en]

    We perform a combined theoretical and experimental study of the phase stability and magnetism of the off-stoichiometric Ni1-xMn1+xSb in the half-Heusler crystal phase. Our work is motivated by the need for strategies to engineer the magnetism of potentially half-metallic materials, such as NiMnSb, for improved performance at elevated temperatures. By means of ab initio calculations we investigate Ni1-xMn1+xSb over the whole composition range 0 andlt;= x andlt;= 1 of Ni replacing Mn and show that at relevant temperatures, the half-Heusler phase should be thermodynamically stable up to at least x=0.20 with respect to the competing C38 structure of Mn2Sb. Furthermore we find that half-Heusler Ni1-xMn1+xSb retains half-metallic band structure over the whole concentration range and that the magnetic moments of substitutional Mn-Ni atoms display magnetic exchange interactions an order of magnitude larger than the Ni-Mn interaction in NiMnSb. We also demonstrate experimentally that the alloys indeed can be created by synthesizing off-stoichiometric Ni1-xMn1+xSb films on MgO substrates by means of magnetron sputtering.

    Place, publisher, year, edition, pages
    American Institute of Physics, 2010
    National Category
    Engineering and Technology
    Identifiers
    urn:nbn:se:liu:diva-63149 (URN)10.1063/1.3476282 (DOI)000284270900068 ()
    Note
    Original Publication: Marcus Ekholm, Petter Larsson, Björn Alling, Ulf Helmersson and Igor Abrikosov, Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni1-xMn1+xSb, 2010, JOURNAL OF APPLIED PHYSICS, (108), 9, 093712. http://dx.doi.org/10.1063/1.3476282 Copyright: American Institute of Physics http://www.aip.org/Available from: 2010-12-13 Created: 2010-12-13 Last updated: 2015-08-19Bibliographically approved
  • 14.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Mikhaylushkin, Arkady
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Simak, Sergey
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Johansson, B
    Royal Institute Technology KTH.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Configurational thermodynamics of Fe-Ni alloys at Earths core conditions2011In: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 308, no 1-2, p. 90-96Article in journal (Refereed)
    Abstract [en]

    By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.

  • 15.
    Gebhardt, Thomas
    et al.
    Rhein Westfal TH Aachen.
    Music, Denis
    Rhein Westfal TH Aachen.
    Kossmann, Daniel
    Rhein Westfal TH Aachen.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Vitos, Levente
    Royal Institute Technology, SE-75121 Uppsala, Sweden .
    Schneider, Jochen M
    Rhein Westfal TH Aachen.
    Elastic properties of fcc Fe-Mn-X (X = Al, Si) alloys studied by theory and experiment2011In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 59, no 8, p. 3145-3155Article in journal (Refereed)
    Abstract [en]

    We have studied the influence of Al and Si additions on the elastic properties of face-centered cubic (fcc) Fe-Mn random alloys with Fe/Mn ratios of 4.00 and 2.33 using ab initio calculations. When Al is added up to 8 at.% the shearing elastic constants (C-11-C-12)/2 and C-44 decrease, resulting in a drop of similar to 20% in shear and similar to 19% in Youngs modulus. In fcc Fe-Mn-Si alloys, the trends in the elastic constants are similar, but less drastic, with a similar to 7% shear and similar to 6% Youngs modulus decrease when Si is added up to 8 at.%. The Fe/Mn ratio exhibits a minor influence on the shear and Youngs modulus values at constant Al and Si contents. To assess the quality of the ab initio data Fe-Mn-Al and Fe-Mn-Si thin films with an fcc structure were combinatorially synthesized and the elastic properties measured using nanoindentation. For both systems the measured and calculated lattice parameters are in good agreement. Although the measured Youngs modulus data showed significant scatter due to the high surface roughness, they are in good agreement with the predicted values. For the Fe-Mn-Al system the calculations generally underestimate the experimental data by similar to 15%. For the Fe-Mn-Si system the calculated data are in general lower by similar to 10% than the experimentally determined values. The presented results are of relevance for multicomponent alloy design, since the effect of Si and Al addition on the elastic properties of Fe-Mn alloys can be predicted based on ab initio data.

  • 16.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Structural and magnetic ground-state properties of gamma-FeMn alloys from ab initio calculations2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 10, p. 104423-Article in journal (Refereed)
    Abstract [en]

    The magnetic properties of fcc-FeMn alloys, especially at the Fe(0.5)Mn(0.5) composition, have been the subject of intense experimental and theoretical investigations for several decades. We carry out an ab initio theoretical study of this system, including simultaneous optimization of structural and magnetic properties, and find that the ground state is the locally relaxed noncollinear 3Q antiferromagnetic structure. We also show that the two most frequently used parameterizations of the generalized gradient approximation not only fail to reproduce the equilibrium lattice constant of FeMn alloys, and consequently the magnetic properties, but also internally yield qualitatively different results. For practical studies of these alloys, which currently attract great attention, we propose a set of approximations, which is internally consistent, and brings the equilibrium lattice constant and magnetic properties in good agreement with the experiment in the whole range of alloy compositions.

  • 17.
    Gebhardt, T
    et al.
    Rhein Westfal TH Aachen.
    Music, D
    Rhein Westfal TH Aachen.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Vitos, L
    Royal Institute Technology KTH.
    Dick, A
    Max Planck Institute Eisenforsch GmbH.
    Hickel, T
    Max Planck Institute Eisenforsch GmbH.
    Neugebauer, J
    Max Planck Institute Eisenforsch GmbH.
    Schneider, J M
    Rhein Westfal TH Aachen.
    The influence of additions of Al and Si on the lattice stability of fcc and hcp Fe-Mn random alloys2011In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 24, p. 246003-Article in journal (Refereed)
    Abstract [en]

    We have studied the influence of additions of Al and Si on the lattice stability of face-centred-cubic (fcc) versus hexagonal-closed-packed (hcp) Fe-Mn random alloys, considering the influence of magnetism below and above the fcc Neel temperature. Employing two different ab initio approaches with respect to basis sets and treatment of magnetic and chemical disorder, we are able to quantify the predictive power of the ab initio methods. We find that the addition of Al strongly stabilizes the fcc lattice independent of the regarded magnetic states. For Si a much stronger dependence on magnetism is observed. Compared to Al, almost no volume change is observed as Si is added to Fe-Mn, indicating that the electronic contributions are responsible for stabilization/destabilization of the fcc phase.

  • 18.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Larsson, Petter
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics.
    Alling, Björn
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Helmersson, Ulf
    Linköping University, Department of Physics, Chemistry and Biology, Plasma and Coating Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni1-xMn1+xSb2010In: JOURNAL OF APPLIED PHYSICS, ISSN 0021-8979, Vol. 108, no 9Article in journal (Refereed)
    Abstract [en]

    We perform a combined theoretical and experimental study of the phase stability and magnetism of the off-stoichiometric Ni1-xMn1+xSb in the half-Heusler crystal phase. Our work is motivated by the need for strategies to engineer the magnetism of potentially half-metallic materials, such as NiMnSb, for improved performance at elevated temperatures. By means of ab initio calculations we investigate Ni1-xMn1+xSb over the whole composition range 0 andlt;= x andlt;= 1 of Ni replacing Mn and show that at relevant temperatures, the half-Heusler phase should be thermodynamically stable up to at least x=0.20 with respect to the competing C38 structure of Mn2Sb. Furthermore we find that half-Heusler Ni1-xMn1+xSb retains half-metallic band structure over the whole concentration range and that the magnetic moments of substitutional Mn-Ni atoms display magnetic exchange interactions an order of magnitude larger than the Ni-Mn interaction in NiMnSb. We also demonstrate experimentally that the alloys indeed can be created by synthesizing off-stoichiometric Ni1-xMn1+xSb films on MgO substrates by means of magnetron sputtering.

  • 19.
    Gebhardt, T.
    et al.
    Rhein Westfal TH Aachen.
    Music, D.
    Rhein Westfal TH Aachen.
    Hallstedt, B.
    Rhein Westfal TH Aachen.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Vitos, L.
    Royal Institute of Technology.
    M. Schneider, J.
    Rhein Westfal TH Aachen.
    Ab initio lattice stability of fcc and hcp Fe-Mn random alloys2010In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 29, p. 295402-Article in journal (Refereed)
    Abstract [en]

    We have studied the lattice stability of face centred cubic (fcc) versus hexagonal close packed (hcp) Fe-Mn random alloys using ab initio calculations. In the calculations we considered the antiferromagnetic order of local moments, which for fcc alloys models the magnetic configuration of this phase at room temperature (below its Neel temperature) as well as their complete disorder, corresponding to paramagnetic fcc and hcp alloys. For both cases, the results are consistent with our thermodynamic calculations, obtained within the Calphad approach. For the room temperature magnetic configuration, the cross-over of the total energies of the hcp phase and the fcc phase of Fe-Mn alloys is at the expected Mn content, whereas for the magnetic configuration above the fcc Neel temperature, the hcp lattice is more stable within the whole composition range studied. The increase of the total energy difference between hcp and antiferromagnetic fcc due to additions of Mn as well as the stabilizing effect of antiferromagnetic ordering on the fcc phase are well displayed. These results are of relevance for understanding the deformation mechanisms of these random alloys.

  • 20.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Zapolsky, H
    University of Rouen.
    Ruban, A V
    Royal Institute of Technology.
    Vernyhora, I
    University of Rouen.
    Ledue, D
    University of Rouen.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 16, p. 167208-Article in journal (Refereed)
    Abstract [en]

    In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.

  • 21.
    Alling, Björn
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Ekholm, Marcus
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Abrikosov, Igor
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics .
    Doping the half-Heusler alloy NiMnSb with magnetic 3d-metals2008In: Workshop New Challenges in the Electronic Structure of Complex Materials,2008, Hungary: Hungary , 2008Conference paper (Refereed)
  • 22.
    Alling, Björn
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Ekholm, Marcus
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics . Linköping University, The Institute of Technology.
    Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb2008In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 77, no 14, p. 144414-Article in journal (Refereed)
    Abstract [en]

    We have performed a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d metals Cr, Mn, Fe, Co, and Ni, with respect to both energetics and magnetic properties. Starting from the formation energies, we discuss the possibility of placing the dopant on different crystallographic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions, and density of states and also outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn have the largest impact on magnetic interactions in the system while preserving its half-metallic property. Therefore, we suggest the possibility that these dopants increase the thermal stability of half-metallicity in NiMnSb, with implications for its possible usage in spintronics applications.

  • 23.
    Ekholm, Marcus
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Tasnádi, Ferenc
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Abrikosov, Igor
    Linköping University, Department of Physics, Chemistry and Biology, Theoretical Physics. Linköping University, The Institute of Technology.
    Supercell Calculations of Hyperfine Interactions in Transition-Metal AlloysManuscript (preprint) (Other academic)
    Abstract [en]

    We have performed calculations for the hyperfine field in the disordered Fe0.5Ni0.5 alloy using supercells with up to 864 atoms. The computational scheme is based on exact muffin-tin orbitals and the locally self-consistent Green’s function formalism, which scales linearly with the number of atoms in the supercell. This scheme allows local environment effects, such as chemical and magnetic environment, and short-range order, to be explicitly included. Supercell calculations for Fe-Ni show that while the average fields coincides with that obtained using the coherent potential approximation, there is a significant distribution of the hyperfine fields depending on the local environment. The fields of Fe and Ni show qualitatively different behaviour as a function of the on-site magnetic moment, but scale linearly with the average magnetic moment in the first coordination shell.

1 - 23 of 23
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