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  • 1.
    Arapan, S.
    et al.
    VSB Tech Univ Ostrava, IT4Innovat, 17 Listopadu 15, CZ-70833 Ostrava, Czech Republic;Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Nieves, P.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Cuesta-Lopez, S.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain;Int Ctr Adv Mat & Raw Mat Castilla & Leon, ICAMCyL, Leon 24492, Spain.
    Gusenbauer, M.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria.
    Oezelt, H.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria.
    Schrefl, T.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria.
    Delczeg-Czirjak, Erna Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Orebro Univ, Sch Sci & Engn, SE-70182 Orebro, Sweden.
    Influence of antiphase boundary of the MnAl tau-phase on the energy product2019In: Physical Review Materials, E-ISSN 2475-9953, Vol. 3, no 6, article id 064412Article in journal (Refereed)
    Abstract [en]

    In this paper, we use a multiscale approach to describe a realistic model of a permanent magnet based on MnAl tau-phase and elucidate how the antiphase boundary defects present in this material affect the energy product. We show how the extrinsic properties of a microstructure depend on the intrinsic properties of a structure with defects by performing micromagnetic simulations. For an accurate estimation of the energy product of a realistic permanent magnet based on the MnAl tau-phase with antiphase boundaries, we quantify exchange interaction strength across the antiphase boundary defect with a simple approach derived from first-principles calculations. These two types of calculations, performed at different scales, are linked via atomistic spin-dynamics simulations.

  • 2.
    Arapan, S.
    et al.
    VSB Tech Univ Ostrava, IT4Innovat, 17 Listopadu 2172-15, Ostrava 70800, Czech Republic.
    Nieves, P.
    VSB Tech Univ Ostrava, IT4Innovat, 17 Listopadu 2172-15, Ostrava 70800, Czech Republic;Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Legut, D.
    VSB Tech Univ Ostrava, IT4Innovat, 17 Listopadu 2172-15, Ostrava 70800, Czech Republic;VSB Tech Univ Ostrava, Nanotechnol Ctr, 17 Listopadu 2172-15, Ostrava 70800, Czech Republic.
    Computational screening of Fe-Ta hard magnetic phases2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 1, article id 014426Article in journal (Refereed)
    Abstract [en]

    In this paper, we perform a systematic calculation of the Fe-Ta phase diagram to discover hard magnetic phases. By using structure prediction methods based on evolutionary algorithms, we identify two energetically stable magnetic structures: a tetragonal Fe3Ta (space group 122) and a cubic Fe5Ta (space group 216) binary phase. The tetragonal structure is estimated to have both high saturation magnetization (mu M-0(s) = 1.14 T) and magnetocrystalline anisotropy (K-1 = 2.17 MJ/m(3)) suitable for permanent magnet applications. The high-throughput screening of magnetocrystalline anisotropy also reveals two low-energy metastable hard magnetic phases: Fe5Ta2 (space group 156) and Fe6Ta (space group 194), that may exhibit intrinsic magnetic properties comparable to SmCo5 and Nd2Fe14B, respectively.

  • 3.
    Balatsky, Alexander V.
    et al.
    NORDITA, Roslagstullsbacken 11, S-10691 Stockholm, Sweden.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Functional Dirac Materials: Status and Perspectives2018In: Physica Status Solidi. Rapid Research Letters, ISSN 1862-6254, E-ISSN 1862-6270, Vol. 12, no 11, article id 1870334Article in journal (Other academic)
  • 4.
    Bidermane, Ieva
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Brumboiu, Iulia
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Totani, Roberta
    University of L'Aquila.
    Grazioli, Cesare
    University of Trieste.
    Shariati Nilsson, Masumeh Nina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Herper, Heike
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ressel, B
    Univ Nova Gorica, Ajdovscina 5270, Slovenia.
    de Simone, Monica
    Lozzi, Luca
    University of L'Aquila.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Theoretical Physics.
    Puglia, Carla
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Atomic Contributions to the Valence Band Photoelectron Spectra of Metal-free, Iron and Manganese Phthalocyanines2015In: Journal of Electron Spectroscopy and Related Phenomena, ISSN 0368-2048, E-ISSN 1873-2526, Vol. 205, p. 92-97Article, review/survey (Other academic)
    Abstract [en]

    The present work reports a photoelectron spectroscopy study of the low-energy region of the valence band of metal-free phthalocyanine (H2Pc) compared with those of iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc). We have analysed in detail the atomic orbital composition of the valence band both experimentally, by making use of the variation in photoionization cross-sections with photon energy, and theoretically, by means of density functional theory. The atomic character of the Highest Occupied Molecular Orbital (HOMO), reflected on the outermost valence band binding energy region, is different for MnPc as compared to the other two molecules. The peaks related to the C 2p contributions, result in the HOMO for H2Pc and FePc and in the HOMO-1 for MnPc as described by the theoretical predictions, in very good agreement with the experimental results. The DFT simulations, discerning the atomic contribution to the density of states, indicate how the central metal atom interacts with the C and N atoms of the molecule, giving rise to different partial and total density of states for these three Pc molecules.

  • 5.
    Brena, Barbara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of ligands on the electronic and magnetic properties of Fe porphyrin in gas phase and on Cu(001)2015In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 117, no 17, article id 17B318Article in journal (Refereed)
    Abstract [en]

    A study of the adsorption of different axial ligands on Fe porphyrin (FeP), both isolated and adsorbed on Cu(001), was performed by means of Density Functional Theory. The electronic and magnetic properties of the adsorbed FeP resulted to be strongly influenced by the axial ligands considered, Cl and O. Cl induces an enhancement of the overall molecular magnetic moment of 3.0 mu(B) while O or O-2 leave the spin state of the molecule unchanged. The influence of the Cl in the electronic states was moreover studied by means of theoretical NEXAFS N K-edge, where the spectra of isolated FeP and FeP with Cl ligand were calculated. The adsorption of the FeP molecules on Cu(001) leads in case of Cl to a further increase of the magnetic moment due to strong deformation of the Fe-N bond.

  • 6.
    Brena, Barbara
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Chemical Switching of the Magnetic Coupling in a MnPc Dimer by Means of Chemisorption and Axial Ligands2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 49, p. 27185-27193Article in journal (Refereed)
    Abstract [en]

    We present an ab initio density functional theory study of the magnetic properties of manganese phthalocyanine dimers, where we focus on the magnetic coupling between the Mn centers and on how it is affected by external factors like chemisorption or atomic axial ligands. We have studied several different configurations for the gas phase dimers, which resulted in ferromagnetic couplings of different magnitudes. For the bare dimer we find a significant ferromagnetic coupling between the Mn centers, which decreases by about 20% when a H atom is adsorbed on one of the Mn atoms and is reduced to about 7% when a Cl atom is adsorbed. The magnetic coupling is almost fully quenched when the dimer, bare or with the H ligand, is deposited on the ferromagnetic substrate Co(001). Our calculations indicate that the coupling between the two Mn atoms principally occurs via a superexchange interaction along two possible paths within a Mn-N-Mn-N four-atom loop. When these electrons get involved in chemical bonding outside the dimer itself, an appreciable alteration of the overlap between Mn and N molecular orbitals along the loop occurs, and consequently, the magnetic interaction between the Mn centers varies. We show that this is reflected by the electronic structure of the dimer in various configurations and is also visible in the structure of the atomic loop. The chemical tuning of the magnetic coupling is highly relevant for the design of nanodevices like molecular spin valves, where the molecules need to be anchored to a support.

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  • 7.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Haldar, Soumyajyoti
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Luder, Johann
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines2016In: Journal of Chemical Theory and Computation, Vol. 12, no 4, p. 1772-1785Article in journal (Refereed)
    Abstract [en]

    There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard U term. The choice of U is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of U is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of U for each system before performing any further analysis and shows how this value can influence the final results.

  • 8.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. KTH Royal Inst Technol, Dept Theoret Chem & Biol, S-10691 Stockholm, Sweden; Korea Adv Inst Sci & Technol, Dept Chem, Daejeon 34141, South Korea.
    Haldar, Soumyajyoti
    Lüder, Johann
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ligand effects on the linear response Hubbard U: The case of transition metal phthalocyanines2019In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 123, no 14, p. 3214-3222Article in journal (Refereed)
    Abstract [en]

    It is established that density functional theory (DFT) + U is a better choice compared to DFT for describing the correlated electron metal center in organometallics. The value of the Hubbard U parameter may be determined from linear response, either by considering the response of the metal site alone or by additionally considering the response of other sites in the compound. We analyze here in detail the influence of ligand shells of increasing size on the U parameter calculated from the linear response for five transition metal phthalocyanines. We show that the calculated multiple-site U is larger than the single-site U by as much as 1 eV and the ligand atoms that are mainly responsible for this difference are the isoindole nitrogen atoms directly bonded to the central metal atom. This suggests that a different U value may be required for computations of chemisorbed molecules compared to physisorbed and gas-phase cases.

  • 9.
    Brumboiu, Iulia Emilia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Totani, Roberta
    de Simone, Monica
    Coreno, Marcello
    Grazioli, Cesare
    Lozzi, Luca
    Herper, Heike C
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Puglia, Carla
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and condensed matter physics.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine2014In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 118, no 5, p. 927-932Article in journal (Refereed)
    Abstract [en]

    To shed light on the metal 3d electronic structure of manganese phthalocyanine, so far controversial, we performed photoelectron measurements both in the gas phase and as thin film. With the purpose of explaining the experimental results, three different electronic configurations close in energy to one another were studied by means of density functional theory. The comparison between the calculated valence band density of states and the measured spectra revealed that in the gas phase the molecules exhibit a mixed electronic configuration, while in the thin film, manganese phthalocyanine finds itself in the theoretically computed ground state, namely, the b2g1eg3a1g1b1g0 electronic configuration.

  • 10.
    Ghorai, Sagar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Shtender, Vitalii
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Delczeg-Czirjak, Erna Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vieira, Rafael Martinho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Magneto-structural coupling strength dependent giant magnetocaloric effect in (Mn,Fe)NiSi-systemManuscript (preprint) (Other academic)
  • 11.
    Ghorai, Sagar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Vieira, Rafael Martinho
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Shtender, Vitalii
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Applied Material Science.
    Delczeg-Czirjak, Erna Krisztina
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Björkman, Torbjörn
    Simak, Sergei I.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Analytical Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics I. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Condensed Matter Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Materials Science, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics V. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Inorganic Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Physical and Analytical Chemistry, Surface Biotechnology. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics III. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics IV. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Physics V. Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Materials Chemistry, Structural Chemistry. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics, Theoretical Magnetism. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Materials Science. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Experimental Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Giant magnetocaloric effect in the (Mn,Fe)NiSi-system2023Manuscript (preprint) (Other academic)
    Abstract [en]

    The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn1−xFexNiSi0.95Al0.05, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni2In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.

  • 12. Gruenebohm, Anna
    et al.
    Entel, Peter
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio study of the electronic and magnetic structure of the TiO2 rutile (110)/Fe interface2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 15, p. 155401-Article in journal (Refereed)
    Abstract [en]

    Adsorption of Fe on the rutile (110) surface is investigated by means of ab initio density functional theory calculations. We discuss the deposition of single Fe atoms and increasing Fe coverage, as well as the adsorption of small Fe clusters. It is shown that the different interface structures found in experiment can be understood in terms of the adsorption of the Fe atoms landing first on the rutile surface. Strong interface bonds form if single Fe atoms are deposited. The Fe-Fe bonds in deposited Fe clusters lead to a three-dimensional growth mode. Mainly ionic Fe oxide bonds are formed in both cases and the electronic band gap of the surface is reduced due to interface states. In addition to the structural and electronic properties, we discuss the influence of the interface on the magnetic properties, finding stable Fe moments and induced moments within the interface which lead to a large spin polarization of the Fe atoms at the rutile (110)/Fe interface.

  • 13.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ni-based Heusler compounds: How to tune the magnetocrystalline anisotropy2018In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 98, no 1, article id 014411Article in journal (Refereed)
    Abstract [en]

    Tailoring and controlling magnetic properties is an important factor for materials design. Here, we present a case study for Ni-based Heusler compounds of the type Ni(2)YZ with Y = Mn, Fe, Co and Z = B, Al, Ga, In, Si, Ge, Sn based on first-principles electronic structure calculations. These compounds are interesting since the materials properties can be quite easily tuned by composition and many of them possess a noncubic ground state being a prerequisite for a finite magnetocrystalline anisotropy (MAE). We discuss systematically the influence of doping at the Y and Z sublattices as well as the effect of lattice deformation on the MAE. We show that in case of Ni(2)CoZ the phase stability and the MAE can be improved using quaternary systems with elements from main group III and IV on the Z sublattice whereas changing the Y sublattice occupation by adding Fe does not lead to an increase of the MAE. Furthermore, we studied the influence of the lattice ratio on the MAE. Showing that small deviations can lead to a doubling of the MAE as in case of Ni2FeGe. Even though we demonstrate this for a limited set of systems, the findings may carry over to other related systems.

  • 14.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ahmed, T.
    Los Alamos Natl Lab, Inst Mat Sci, Los Alamos, NM 87545 USA..
    Wills, J. M.
    Los Alamos Natl Lab, Theoret Div, Los Alamos, NM 87545 USA..
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bjorkman, T.
    Abo Akad Univ, Dept Nat Sci, FIN-20500 Turku, Finland..
    Iusan, Diana
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Balatsky, A. V.
    Los Alamos Natl Lab, Inst Mat Sci, Los Alamos, NM 87545 USA.;AlbaNova Univ Ctr, Ctr Nordita, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Örebro, Sweden.
    Combining electronic structure and many-body theory with large databases: A method for predicting the nature of 4 f states in Ce compounds2017In: Physical Review Materials, E-ISSN 2475-9953, Vol. 1, no 3, article id 033802Article in journal (Refereed)
    Abstract [en]

    Recent progress in materials informatics has opened up the possibility of a new approach to accessing properties of materials in which one assays the aggregate properties of a large set of materials within the same class in addition to a detailed investigation of each compound in that class. Here we present a large scale investigation of electronic properties and correlated magnetism in Ce-based compounds accompanied by a systematic study of the electronic structure and 4f-hybridization function of a large body of Ce compounds. We systematically study the electronic structure and 4f-hybridization function of a large body of Ce compounds with the goal of elucidating the nature of the 4f states and their interrelation with the measured Kondo energy in these compounds. The hybridization function has been analyzed for more than 350 data sets (being part of the IMS database) of cubic Ce compounds using electronic structure theory that relies on a full-potential approach. We demonstrate that the strength of the hybridization function, evaluated in this way, allows us to draw precise conclusions about the degree of localization of the 4f states in these compounds. The theoretical results are entirely consistent with all experimental information, relevant to the degree of 4f localization for all investigated materials. Furthermore, a more detailed analysis of the electronic structure and the hybridization function allows us to make precise statements about Kondo correlations in these systems. The calculated hybridization functions, together with the corresponding density of states, reproduce the expected exponential behavior of the observed Kondo temperatures and prove a consistent trend in real materials. This trend allows us to predict which systems may be correctly identified as Kondo systems. A strong anticorrelation between the size of the hybridization function and the volume of the systems has been observed. The information entropy for this set of systems is about 0.42. Our approach demonstrates the predictive power of materials informatics when a large number of materials is used to establish significant trends. This predictive power can be used to design new materials with desired properties. The applicability of this approach for other correlated electron systems is discussed.

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  • 15.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bernien, M.
    Bhandary, S.
    Hermanns, C. F.
    Krueger, A.
    Miguel, J.
    Weis, C.
    Schmitz-Antoniak, C.
    Krumme, B.
    Bovenschen, D.
    Tieg, C.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Weschke, E.
    Czekelius, C.
    Kuch, W.
    Wende, H.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Iron porphyrin molecules on Cu(001): Influence of adlayers and ligands on the magnetic properties2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 17, p. 174425-Article in journal (Refereed)
    Abstract [en]

    The structural and magnetic properties of Fe octaethylporphyrin molecules on Cu(001) have been investigated by means of density functional theory (DFT) methods and x-ray absorption spectroscopy. The molecules have been adsorbed on the bare metal surface and on an oxygen-covered surface, which shows a root 2 x 2 root 2R45 degrees reconstruction. In order to allow for a direct comparison between magnetic moments obtained from sum-rule analysis and DFT, we calculate the spin dipolar term 7T (theta), which is also important in view of the magnetic anisotropy of the molecule. The measured x-ray magnetic circular dichroism shows a strong dependence on the photon incidence angle, which we could relate to a huge value of 7T (theta), e. g., on Cu(001), 7T (theta) amounts to -2.07 mu(B) for normal incidence leading to a reduction of the effective spin moment (m(s) + 7T (theta)). Calculations have also been performed to study the influence of possible ligands such as Cl and O atoms on the magnetic properties of the molecule and the interaction between molecule and surface because the experimental spectra display a clear dependence on the ligand, which is used to stabilize the molecule in the gas phase. Both types of ligands weaken the hybridization between surface and porphyrin molecule and change the magnetic spin state of the molecule, but the changes in the x-ray absorption are clearly related to residual Cl ligands.

  • 16.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bhandary, Sumanta
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fe phthalocyanine on Co(001): Influence of surface oxidation on structural and electronic properties2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 8, p. 085411-Article in journal (Refereed)
    Abstract [en]

    The adsorption of FePc on Co(001) and c(2 x 2)O/Co(001) was studied by means of density functional theory calculations, taking into account the long range van der Waals dispersion forces. Several high symmetry adsorption sites were analyzed, together with two possible orientations of the molecules. For the adsorption of FePc on the bare surface the on-top-of Co position, rotated by 45 degrees relative to the substrate orientation, is most stable, whereas on the surface covered by an O adlayer the on-top-of O position is preferred. This has strong impact on the magnetic coupling but leaves the spin state of S = 1 unaltered. The total energies of the studied adsorption sites on the bare metal differ by at least 0.75 eV and are characterized by a strong hybridization of the carbon atoms in the peripheral benzenic rings with the Co atoms beneath. In the presence of the O adlayer the various sites are closer in energy, which turns out to be related to the screening of the ferromagnetic film by the oxygen atoms.

  • 17.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grünebohm, Anna
    Univ Duisburg Essen, Fac Phys, D-47048 Duisburg, Germany;Univ Duisburg Essen, CENIDE, D-47048 Duisburg, Germany.
    Tuning the Magnetic Anisotropy of NiPtMnGa by Substitution and Epitaxial Strain2019In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 55, no 2, article id 2100804Article in journal (Refereed)
    Abstract [en]

    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L1(0) phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction.

  • 18.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Skokov, Konstantin P.
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Ener, Semih
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Thunström, Patrik
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Diop, Leopold V. B.
    Univ Lorraine, CNRS, IJL, F-54000 Nancy, France..
    Gutfleisch, Oliver
    Tech Univ Darmstadt, Dept Mat Sci, Funct Mat, D-64287 Darmstadt, Germany..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Magnetic properties of NdFe11Ti and YFe11Ti, from experiment and theory2023In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 242, article id 118473Article in journal (Refereed)
    Abstract [en]

    NdFe11Ti and YFe11Ti serve as prototypes for rare-earth (RE) lean or REfree magnets with the ThMn12-type structure. Although NdFe11Ti has been studied for a long time the origin of its complex magnetism at low temperature is so far not well-understood. We present a comprehensive theoretical and experimental study of the magnetic properties of NdFe11Ti and RE-free YFe11Ti to elucidate the influence of the 4f electrons. The partially localized 4 f electrons of Nd are the driving force behind the complex behavior of the magnetocrystalline anisotropy which changes from cone to uniaxial above 170 dK. The spontaneous magnetization and the five leading anisotropy constants were determined from high-quality single crystal samples over a wide temperature range using field dependencies of magnetization measured along the principle crystallographic directions. The experimental data are compared with density functional theory combined with a Hartree-Fock correction (+U) and an approximate dynamical mean-field theory.

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  • 19.
    Herper, Heike C.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vekilova, Olga Yu
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Simak, Sergei I.
    Linköping Univ, Dept Phys Chem & Biol, SE-58183 Linköping, Sweden.
    Di Marco, Igor
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Asia Pacific Ctr Theoret Phys, Pohang 37673, South Korea; POSTECH, Dept Phys, Pohang 37673, South Korea.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Nat Sci & Technol, SE-70182 Örebro, Sweden.
    Localized versus itinerant character of 4f-states in cerium oxides2020In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 32, no 21, article id 215502Article in journal (Refereed)
    Abstract [en]

    The electronic structure of cerium oxide is investigated here using a combination of ab initio one-electron theory and elements from many-body physics, with emphasis on the nature of the 4f electron shell of cerium ions. We propose to use the hybridization function as a convenient measure for the degree of localization of the 4f shell of this material, and observe that changing the oxidation state is related to distinct changes in the hybridization between the 4f shell and ligand states. The theory reveals that CeO2 has essentially itinerant 4f states, and that in the least oxidized form of ceria, Ce2O3, the 4f states are almost (but not fully) localized. This conclusion is supported by additional calculations based on a combination of density functional theory and dynamical mean field theory. Most importantly, our model points to the fact that diffusion of oxygen vacancies in cerium oxide may be seen as polaron hopping, involving a correlated 4f electron cloud, which is located primarily on Ce ions of several atomic shells surrounding the vacancy.

  • 20. Klaer, P.
    et al.
    Herper, Heike
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Faculty of Physics, University Duisburg-Essen, Germany.
    Entel, P.
    Niemann, R.
    Schultz, L.
    Faehler, S.
    Elmers, H. J.
    Electronic structure of the austenitic and martensitic state of magnetocaloric Ni-Mn-In Heusler alloy films2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, p. 174414-Article in journal (Refereed)
    Abstract [en]

    Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys dopedwith Co are investigated by experiment and theory. The nonstoichiometric Ni48Co5Mn35In12 epitaxial film undergoes a transition from a weakly magnetic martensitic phase below T-m = 350 K to a ferromagnetic austenitic phase above T-m. Element-specific magnetic moments and the unoccupied density of states function is investigated using x-raymagnetic circular dichroism.We find an antiparallel alignment ofMnandNi/Comagnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach considering experimental results on the atomic disorder in the sample. The good agreement of experimental and theoretically simulated x-ray absorption spectra in both phases confirms that a band Jahn-Teller like lifting of a degenerate d state is the origin of the energetic preference of the martensitic phase at low temperatures.

  • 21. Klar, D.
    et al.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bhandary, Sumanta
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Weis, C.
    Krumme, B.
    Schmitz-Antoniak, C.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Wende, H.
    Oxygen-tuned magnetic coupling of Fe-phthalocyanine molecules to ferromagnetic Co films2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 88, no 22, p. 224424-Article in journal (Refereed)
    Abstract [en]

    The coupling of submonolayer coverages of Fe-phthalocyanine molecules on bare and oxygen-covered ferromagnetic Co(001) films was studied by x-ray-absorption spectroscopy, especially the x-ray magnetic circular dichroism, in combination with density functional theory. We observe that the magnetic moments of the paramagnetic molecules are aligned even at room temperature, resulting from a magnetic coupling to the substrate. While the magnetization of the Fe ions directly adsorbed on the Co surface is parallel to the magnetization of the Co film, the introduction of an oxygen interlayer leads to an antiparallel alignment. As confirmed by theory, the coupling strength is larger for the system FePc/Co than for FePc/O/Co, causing a stronger temperature dependence of the Fe magnetization for the latter system. Furthermore, the calculations reveal that the coupling mechanism changes due to the O layer from mostly direct exchange to Co of the bare surface to a 180 degrees antiferromagnetic superexchange via the O atoms. Finally, by comparing the experimental x-ray-absorption spectra at the N K edge with the corresponding calculations, the contribution of the individual orbitals has been determined and the two inequivalent N atoms of the molecules could be distinguished.

  • 22.
    Kovacs, Alexander
    et al.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria..
    Fischbacher, Johann
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria..
    Gusenbauer, Markus
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria..
    Oezelt, Harald
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria..
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Nieves, Pablo
    Univ Burgos, Int Res Ctr Crit Raw Mat Adv Ind Technol, Burgos 09001, Spain.;VSB Tech Univ Ostrava, IT4Innovat, Ostrava 70833, Czech Republic..
    Arapan, Sergiu
    Univ Burgos, Int Res Ctr Crit Raw Mat Adv Ind Technol, Burgos 09001, Spain.;VSB Tech Univ Ostrava, IT4Innovat, Ostrava 70833, Czech Republic..
    Schrefl, Thomas
    Danube Univ Krems, Dept Integrated Sensor Syst, A-2700 Wiener Neustadt, Austria..
    Computational Design of Rare-Earth Reduced Permanent Magnets2020In: ENGINEERING, ISSN 2095-8099, Vol. 6, no 2, p. 148-153Article in journal (Refereed)
    Abstract [en]

    Multiscale simulation is a key research tool in the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Iron (Fe)-rich magnetic phases suitable for permanent magnets can be found by means of adaptive genetic algorithms. The intrinsic properties computed by ab intro simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with a realistic structure. Using machine learning techniques, the magnet's structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ.m(-3))) were obtained for iron-tin-antimony (Fe3Sn0.75Sb0.25): (0.49, 290), L1(0) -ordered iron-nickel (L1(0) FeNi): (1, 400), cobalt-iron-tantalum (CoFe6Ta): (0.87, 425), and manganese-aluminum (MnAl): (0.53, 80).

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  • 23. Krumme, B.
    et al.
    Auge, A.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Opahle, I.
    Klar, D.
    Teichert, N.
    Joly, L.
    Ohresser, P.
    Landers, J.
    Kappler, J. P.
    Entel, P.
    Huetten, A.
    Wende, H.
    Element-specific electronic structure and magnetic properties of an epitaxial Ni51.6Mn32.9Sn15.5 thin film at the austenite-martensite transition2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 21, article id 214417Article in journal (Refereed)
    Abstract [en]

    An austenite-martensite transition was observed in a 100-nm-thick Ni51.6Mn32.9Sn15.5 film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of M-S approximate to 260 K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment m(l)/m(S) can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of B = 3 T reverses the change of the electronic structure of Mn and reduces the ratio of m(l)/m(S) from 13.5 to approximate to 1 % indicating a field-induced reverse martensitic transition.

  • 24.
    Larsen, Simon R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Shtender, Vitalii
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Hedlund, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Delczeg-Czirjak, Erna K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Beran, Premysl
    Nuclear Physics Institute, ASCR, Hlavni 130, 25068 Rez, Czech Republic; European Spallation Source ESS ERIC, Box 176, 221 00, Lund, Sweden.
    Cedervall, Johan
    Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden.
    Vishina, Alena
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hansen, Thomas C.
    Institut Laue-Langevin, 71 avenue des Martyrs, 38000 Grenoble, France.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. School of Science and Technology, Örebro University, SE-701 82 Örebro, Sweden.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Revealing the Magnetic Structure and Properties of Mn(Co,Ge)22022In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 61, no 44, p. 17673-17681Article in journal (Refereed)
    Abstract [en]

    The atomic and magnetic structures of Mn(Co,Ge)2 are reported herein. The system crystallizes in the space group P63/mmc as a superstructure of the MgZn2-type structure. The system exhibits two magnetic transitions with associated magnetic structures, a ferromagnetic (FM) structure around room temperature, and an incommensurate structure at lower temperatures. The FM structure, occurring between 193 and 329 K, is found to be a member of the magnetic space group P63/mmc′. The incommensurate structure found below 193 K is helical with propagation vector k = (0 0 0.0483). Crystallographic results are corroborated by magnetic measurements and ab initio calculations.

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  • 25.
    Marathe, Madhura
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Exploration of all-3d Heusler alloys for permanent magnets: An ab initio based high-throughput study2023In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 107, no 17, article id 174402Article in journal (Refereed)
    Abstract [en]

    Heusler alloys have attracted interest in various fields of functional materials since their properties can quite easily be tuned by composition. Here, we have investigated the relatively new class of all-3d Heusler alloys in view of their potential as permanent magnets. To identify suitable candidates, we performed a high-throughput study using an electronic structure database to search for X2Y Z-type Heusler systems with tetragonal symmetry and high magnetization. For the alloys which passed our selection filters, we have used a combination of density functional theory calculations and spin dynamics modeling to investigate their magnetic properties including the magnetocrystalline anisotropy energy and exchange interactions. The candidates which fulfilled all the search criteria served as input for the investigation of the temperature dependence of the magnetization and determination of the Curie temperature. Based on our results, we suggest that Fe2NiZn, Fe2NiTi, and Ni2CoFe are potential candidates for permanent magnets with large out-of-plane magnetic anisotropy (1.23, 0.97, and 0.82 MJ/m3, respectively) and high Curie temperatures lying more than 200 K above the room temperature. We further show that the magnitude and direction of anisotropy are very sensitive to the strain by calculating the values of anisotropy energy for several tetragonal phases. Thus application of strain can be used to tune the anisotropy in these compounds.

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  • 26.
    Martinez-Casado, R.
    et al.
    Univ Complutense Madrid, Fac Phys Sci, Dept Mat Phys, E-28040 Madrid, Spain;Univ Torino, Dipartimento Chim, Via P Giuria 5, I-10125 Turin, Italy.
    Dasmahapatra, A.
    Univ Torino, Dipartimento Chim, Via P Giuria 5, I-10125 Turin, Italy.
    Sgroi, M. F.
    Ctr Ric FIAT, Str Torino 50, I-10043 Orbassano, TO, Italy.
    Romero-Muniz, C.
    Univ Autonoma Madrid, Dept Fis Teor Mat Condensada, E-28049 Madrid, Spain.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ferrari, A. M.
    Univ Torino, Dipartimento Chim, Via P Giuria 5, I-10125 Turin, Italy;Univ Turin, Ctr Excellence, NIS, Turin, Italy.
    Pullini, D.
    Ctr Ric FIAT, Str Torino 50, I-10043 Orbassano, TO, Italy.
    Desmarais, J.
    Univ Torino, Dipartimento Chim, Via P Giuria 5, I-10125 Turin, Italy.
    Maschio, L.
    Univ Torino, Dipartimento Chim, Via P Giuria 5, I-10125 Turin, Italy;Univ Turin, Ctr Excellence, NIS, Turin, Italy.
    The CeFe11Ti permanent magnet: a closer look at the microstructure of the compound2019In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 31, no 50, article id 505505Article in journal (Refereed)
    Abstract [en]

    High-performance permanent magnets (PM) are compounds with outstanding intrinsic magnetic properties. Most PMs are obtained from a favorable combination of rare earth metals (RE = Nd, Pr, Ce) with transition metals (TM = Fe, Co). Amongst them, CeFe11Ti claims considerable attention due to its large Curie temperature, saturation magnetization, and significant magnetocrystalline anisotropic energy. CeFe11Ti has several potential applications, in particular, in the development of electric motors for future automatic electrification. In this work, we shed some light on the mictrostructure of this compound by performing periodic hybrid-exchange density functional theory (DFT) calculations. We use a combined approach of atom-centered local orbitals, plane waves and full-potential linear muffin-tin orbital (LMTO) for our computations. The electronic configuration of the atoms involved in different steps of formation of the crystal structure of CeFe11Ti gives an explanation on the effect of Ce and Ti on its magnetic properties. While Ti stabilizes the structure, atomic orbitals of Ce hybridizes with Fe atomic orbitals to a significant extent and alters the electronic bands. Our calculations confirm a valence of 3(+) for Ce, which has been deemed crucial to obtain a large magnetocrystalline anisotropy. In addition, we analyze several spin configurations, with the ferromagnetic configuration being most stable. We compare and contrast our data to those available and provide an insight for further development of optimized high-performance PMs. Moreover, we compute the Magnetocrystalline Anisotropy of this compound by means of two approaches: the Force Theorem and a full-potential LMTO method.

  • 27.
    Nieves, P.
    et al.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Arapan, S.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain;VSB Tech Univ Ostrava, IT4Innovat, 17 Listopadu 15, CZ-70833 Ostrava, Czech Republic.
    Maudes-Raedo, J.
    Univ Burgos, Dept Civil Engn, Burgos 09006, Spain.
    Marticorena-Sanchez, R.
    Univ Burgos, Dept Civil Engn, Burgos 09006, Spain.
    Del Brio, N. L.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Kovacs, A.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-3500 Krems, Austria.
    Echevarria-Bonet, C.
    Basque Ctr Mat Applicat & Nanostruct, BCMat, UPV EHU Sci Pk, Leioa 48940, Spain.
    Salazar, D.
    Basque Ctr Mat Applicat & Nanostruct, BCMat, UPV EHU Sci Pk, Leioa 48940, Spain.
    Weischenberg, J.
    Tech Univ Darmstadt, Dept Mat & Geosci, D-64287 Darmstadt, Germany.
    Zhang, H.
    Tech Univ Darmstadt, Dept Mat & Geosci, D-64287 Darmstadt, Germany.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Serrano-Lopez, R.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain.
    Barandiaran, J. M.
    Basque Ctr Mat Applicat & Nanostruct, BCMat, UPV EHU Sci Pk, Leioa 48940, Spain.
    Skokov, K.
    Tech Univ Darmstadt, Dept Mat & Geosci, D-64287 Darmstadt, Germany.
    Gutfleisch, O.
    Tech Univ Darmstadt, Dept Mat & Geosci, D-64287 Darmstadt, Germany.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Schrefl, T.
    Danube Univ Krems, Dept Integrated Sensor Syst, A-3500 Krems, Austria.
    Cuesta-Lopez, S.
    Univ Burgos, Int Res Ctr Crit Raw Mat & Adv Ind Technol, ICCRAM, Burgos 09001, Spain;Fdn Int Ctr Adv Mat & Raw Mat Castilla & Leon, ICAMCyL, Leon 24492, Spain.
    Database of novel magnetic materials for high-performance permanent magnet development2019In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 168, p. 188-202Article in journal (Refereed)
    Abstract [en]

    This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. Its main features as software technologies, friendly graphical user interface, advanced search mode, plotting tool and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.

  • 28.
    Schröter, M.
    et al.
    Univ Duisburg Essen, Fac Phys, D-47048 Duisburg, Germany.;Univ Duisburg Essen, Ctr Nanointegrat Duisburg Essen CENIDE, D-47048 Duisburg, Germany..
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Grünebohm, A.
    Ruhr Univ Bochum, Interdisciplinary Ctr Adv Mat Simulat ICAMS, D-44780 Bochum, Germany..
    Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution2022In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 55, no 2, article id 025002Article in journal (Refereed)
    Abstract [en]

    Ni-Mn-based Heusler alloys have a high technical potential related to a large change of magnetization at the structural phase transition. These alloys show a subtle dependence of magnetic properties and structural phase stability on composition and substitution by 3d elements and although they have been extensively investigated, there are still ambiguities in the published results and their interpretation. To shed light on the large spread of reported properties, we perform a comprehensive study by means of density functional theory calculations. We focus on Cr and Co co-substitution whose benefit has been predicted previously for the expensive Ni-Mn-In-based alloy and study the more abundant iso-electronic counterpart Ni-Mn-Ga. We observe that substituting Ni partially by Co and/or Cr enhances the magnetization of the Heusler alloy and at the same time reduces the structural transition temperature. Thereby, Cr turns out to be more efficient to stabilize the ferromagnetic alignment of the Mn spins by strong antiferromagnetic interactions between Mn and Cr atoms. In a second step, we study Cr on the other sublattices and observe that an increase in the structural transition temperature is possible, but depends critically on the short-range order of Mn and Cr atoms. Based on our results, we are able to estimate composition dependent magnetic phase diagrams. In particular, we demonstrate that neither the atomic configuration with the lowest energy nor the results based on the coherent potential approximation are representative for materials with a homogeneous distribution of atoms and we also predict a simple method for fast screening of different concentrations which can be viewed as a blueprint for the study of high entropy alloys. Our results help to explain the large variation of experimentally found materials properties.

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  • 29.
    Schönhöbel, A. M.
    et al.
    Univ Basque Country, BCMat, Sci Pk, E-48940 Leioa, Spain;Univ Basque Country, UPV EHU, Dept Elect & Elect, Leioa 48940, Spain.
    Madugundo, R.
    Univ Basque Country, BCMat, Sci Pk, E-48940 Leioa, Spain.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Barandiaran, J. M.
    Univ Basque Country, BCMat, Sci Pk, E-48940 Leioa, Spain;Univ Basque Country, UPV EHU, Dept Elect & Elect, Leioa 48940, Spain.
    Hadjipanayis, G. C.
    Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA.
    Intrinsic magnetic properties of SmFe12-xVx alloys with reduced V-concentration2019In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 786, p. 969-974Article in journal (Refereed)
    Abstract [en]

    In this work, we present experimental and theoretical results on SmFe12-xVx (x = 0.5 - 2.0) alloys with the ThMn12 (1:12) structure as possible candidates for rare earth-lean permanent magnets. The compound with x = 2 has been previously reported to have a Curie temperature of 330 degrees C, saturation magnetization of about 80 Am-2/kg, and anisotropy field around 9 T. We have synthesized the SmFe11V compound with a nearly pure 1:12 phase; the x = 0.5 compound couldn't be synthesized. The relative stability of the x = 1, 2 compounds was addressed theoretically by enthalpy calculations from first principles. The newly synthesized SmFe11V compound has a Curie temperature of 361 degrees C and saturation magnetization of 115 Am-2/kg (1.12 T). The anisotropy field has been obtained in magnetically-oriented fine powders, and is around 11 T. These parameters make SmFe11V a good candidate for a new kind of high energy, rare earth-lean permanent magnets. 

  • 30.
    Vekilova, Olga Yu.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fayyazi, Bahar
    Tech Univ Darmstadt, Mat Sci, Alarich Weiss Str 16, D-64287 Darmstadt, Germany.
    Skokov, Konstantin P.
    Tech Univ Darmstadt, Mat Sci, Alarich Weiss Str 16, D-64287 Darmstadt, Germany.
    Gutfleisch, Oliver
    Tech Univ Darmstadt, Mat Sci, Alarich Weiss Str 16, D-64287 Darmstadt, Germany.
    Echevarria-Bonet, Cristina
    Univ Basque Country, BCMat, Sci Pk, E-48940 Leioa, Spain.
    Barandiaran, Jose Manuel
    Univ Basque Country, BCMat, Sci Pk, E-48940 Leioa, Spain.
    Kovacs, Alexander
    Danube Univ Krems, Dept Integrated Sensor Syst, Viktor Kaplan Str 2-E, A-2700 Wiener Neustadt, Austria.
    Fischbacher, Johann
    Danube Univ Krems, Dept Integrated Sensor Syst, Viktor Kaplan Str 2-E, A-2700 Wiener Neustadt, Austria.
    Schrefl, Thomas
    Danube Univ Krems, Dept Integrated Sensor Syst, Viktor Kaplan Str 2-E, A-2700 Wiener Neustadt, Austria.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Tuning the magnetocrystalline anisotropy of Fe3Sn by alloying2019In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 99, no 2, article id 024421Article in journal (Refereed)
    Abstract [en]

    The electronic structure, magnetic properties, and phase formation of hexagonal ferromagnetic Fe3Sn-based alloys have been studied from first principles and by experiment. The pristine Fe3Sn compound is known to fulfill all the requirements for a good permanent magnet, except for the magnetocrystalline anisotropy energy (MAE). The latter is large, but planar, i.e., the easy magnetization axis is not along the hexagonal c direction, whereas a good permanent magnet requires the MAE to be uniaxial. Here we consider Fe3Sn0.75M0.25, where M = Si, P, Ga, Ge, As, Se, In, Sb, Te, Pb, and Bi, and show how different dopants affect the MAE and can alter it from planar to uniaxial. The stability of the doped Fe3Sn phases is elucidated theoretically via the calculations of their formation enthalpies. A micromagnetic model is developed to estimate the energy density product (BH)(max) and coercive field mu H-0(c) of a potential magnet made of Fe3Sn0.75M0.25, the most promising candidate from theoretical studies. The phase stability and magnetic properties of the Fe3Sn compound doped with Sb and Mn have been checked experimentally on the samples synthesised using the reactive crucible melting technique as well as by solid state reaction. The Fe3Sn-Sb compound is found to be stable when alloyed with Mn. It is shown that even small structural changes, such as a change of the c/a ratio or volume, that can be induced by, e.g., alloying with Mn, can influence anisotropy and reverse it from planar to uniaxial and back.

  • 31.
    Vieira Martinho, Rafael
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Björkman, Torbjörn
    Šipr, Ondřej
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    The role of pressure-induced stacking faults on the magnetic properties of gadolinium2023Manuscript (preprint) (Other academic)
    Abstract [en]

    Experimental data show that under pressure, Gd goes through a series of structural transitions hcp → Sm-type (close-packed rhombohedral)→ dhcp that is accompanied by a gradual decrease of the Curie temperature and magnetization till the collapse of a finite magnetization close to the dhcp structure. We explore theoretically the pressure-induced changes of the magnetic properties, by describing these structural transitions as the formation of fcc stackings faults. Using this approach, we are able to describe correctly the variation of the Curie temperature with pressure, in contrast to a static structural model using the hcp structure. 

  • 32.
    Vieira, Rafael Martinho
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    High-throughput compatible approach for entropy estimation in magnetocaloric materials: FeRh as a test case2021In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 857, article id 157811Article in journal (Refereed)
    Abstract [en]

    Aiming to predict new materials for magnetic refrigeration from high-throughput calculations asks for an accurate, transferable, and resource-wise balanced approach. Here, we analyze the influence of various approximations on the calculation of key properties of magnetocaloric materials, while revisiting the well-known FeRh system for benchmarking our approach. We focus on the entropy change and its contributions from the electronic, lattice, and magnetic degrees of freedom. All approximations considered are based on first-principles methods and have been tested, and compared for FeRh. In particular, we find that in this context, the Debye approximation for the lattice entropy fails, due to the presence of soft phonon modes in the AFM phase. This approximation is frequently used in the literature as a simple alternative to full phonon calculations. Since soft modes are likely to occur also among promising magnetocaloric materials where structural transformations are common, the use of the Debye approximation should be discarded for these systems treatment. This leaves the calculations of the lattice contribution the most demanding task from the computational point of view, while the remaining contributions can be approximated using more efficient approaches. The entropy change AS shows a peak around 370 K, for which the total entropy change is given by 24.8 JK(-1) kg(-1) (Delta S-ele = 7.38, Delta S-lat = 7.05, Delta S-mag = 10.36 JK(-1) kg(-1)) in good agreement with previous theoretical and experimental findings.

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  • 33.
    Vieira, Rafael Martinho
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Abo Akad Univ, Fac Sci & Engn, Phys, FI-20500 Turku, Finland..
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Physics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics. Örebro Univ, Sch Sci & Technol, Örebro, Sweden..
    Bjorkman, T.
    Abo Akad Univ, Fac Sci & Engn, Phys, FI-20500 Turku, Finland..
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Realistic first-principles calculations of the magnetocaloric effect: applications to hcp Gd2022In: Materials Research Letters, E-ISSN 2166-3831, Vol. 10, no 3, p. 156-162Article in journal (Refereed)
    Abstract [en]

    We present an efficient computational approach to evaluate field-dependent entropy of magnetocaloric materials from ab-initio methods. The temperature dependence is reported for the entropy change, specific heat and magnetization for hcp Gd. To obtain optimal accuracy in the calculations, a mixed-scheme for magnetic Monte Carlo simulations is proposed and found to be superior to using pure quantum or classic statistics. It is demonstrated that lattice and magnetic contributions play a role in the entropy change and that the dominating contribution comes from the magnetic contribution. The total calculated entropy change agrees with measurements at room temperature. IMPACT STATEMENT Demonstration of the accuracy of ab-initio theory, coupled to statistical methods, for accurate calculations of the total entropy variation associated with the magnetic transition of Gd. Reproduction of experimental data of entropy change.

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  • 34.
    Vishina, Alena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, Örebro, Sweden..
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Fe2C- and Mn-2(W/Mo)B-4-based rare-earth-free permanent magnets as a result of the high-throughput and data-mining search2023In: Materials Research Letters, E-ISSN 2166-3831, Vol. 11, no 1, p. 76-83Article in journal (Refereed)
    Abstract [en]

    A high-throughput and data-mining search for rare-earth-free permanent magnets is reported for materials containing a 3d and p-element of the Periodic Table. Three of the most promising compounds, Fe 2 C, Mn2MoB4 , and Mn2WB4, were investigated in detail by ab initio electronic structure theory coupled to atomistic spin-dynamics. For these systems doping protocols were also investigated and, in particular, (Fe0.75X0.25)(2) C (X = Mn, Cr, V, and Ti), Mn2XB4 (X = Mo and W) along with Mn 2 (X0.5Y0.5)B-4 (X,Y = Mo, W, Ta, Cr) are suggested here as promising candidates for applications as permanent magnets.

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  • 35.
    Vishina, Alena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stable and metastable rare-earth-free permanent magnets from a database of predicted crystal structures2023In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 261, article id 119348Article in journal (Refereed)
    Abstract [en]

    With the recent developments in crystal structure prediction, databases of new (not previously synthesized) materials are being created. One of these databases contains more than a million entries with the distance to the Convex Hull predicted by crystal-graph attention networks. Hence, stable and metastable materials can be extracted and then investigated for any desired properties. A high-throughput and data-mining approach we previously developed to search for rare-earth-free permanent magnets was applied to these compounds. As a result, four promising candidates for novel rare-earth-free permanent magnets were discovered with high magnetization, high uniaxial magnetocrystalline anisotropy, and high Curie temperature - Ta3ZnFe8, AlFe2, Co3Ni2, and Fe3Ge. The materials were investigated in more detail and all were verified to be dynamically stable.

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  • 36.
    Vishina, Alena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden..
    Vekilova, Olga Yu
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Stockholm Univ, Dept Mat & Environm Chem, S-10691 Stockholm, Sweden..
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab-initio study of the electronic structure and magnetic properties of Ce2Fe172021In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 888, article id 161521Article in journal (Refereed)
    Abstract [en]

    The Ce2Fe17 intermetallic compound has been studied intensely for several decades; its low-temperature state is reported experimentally either as ferromagnetic or antiferromagnetic by different authors, with a measured ordering temperature ranging within a hundred Kelvin. The existing theoretical investigations overestimate the experimental total magnetic moment of Ce2Fe17 by 20-40% and predict a ferromagnetic ground state. By means of first-principle electronic structure calculations, we show that the total magnetic moment of Ce2Fe17 can be reproduced within the Local Density Approximation while functionals based on the Generalized Gradient Approximation fail. Atomistic spin dynamics simulations are shown to capture the change in the magnetic state of Ce2Fe17 with temperature, and closely replicate the reported helical structure that appears in some of the experimental investigations.

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  • 37.
    Vishina, Alena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Hedlund, Daniel
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Shtender, Vitalii
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Delczeg-Czirjak, Erna K.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Larsen, Simon R.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Stockholm Univ, Dept Mat & Environm Chem, S-10691 Stockholm, Sweden.
    Huang, Shuo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vitos, Levente
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, Appl Mat Phys, SE-10044 Stockholm, Sweden.
    Svedlindh, Peter
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Materials Science and Engineering, Solid State Physics.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Örebro Univ, Sch Sci & Technol, SE-70182 Örebro, Sweden.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Data-driven design of a new class of rare-earth free permanent magnets2021In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 212, article id 116913Article in journal (Refereed)
    Abstract [en]

    A new class of rare-earth-free permanent magnets is proposed. The parent compound of this class is Co3Mn2Ge, and its discovery is the result of first principles theory combined with experimental synthesis and characterisation. The theory is based on a high-throughput/data-mining search among materials listed in the ICSD database. From ab-initio theory of the defect free material it is predicted that the saturation magnetization is 1.71 T, the uniaxial magnetocrystalline anisotropy is 1.44 MJ/m3, and the Curie temperature is 700 K. Co3Mn2Ge samples were then synthesized and characterised with respect to structure and magnetism. The crystal structure was found to be the MgZn2-type, with partial disorder of Co and Ge on the crystallographic lattice sites. From magnetization measurements a saturation polarization of 0.86 T at 10 K was detected, together with a uniaxial magnetocrystalline anisotropy constant of 1.18 MJ/m3, and the Curie temperature of TC = 359 K. These magnetic properties make Co3Mn2Ge a very promising material as a rare-earth free permanent magnet, and since we can demonstrate that magnetism depends critically on the amount of disorder of the Co and Ge atoms, a further improvement of the magnetism is possible. We demonstrate here that the class of compounds based on T3Mn2X (T = Co or alloys between Fe and Ni; X = Ge, Al or Ga) in the MgZn2 structure type, form a new class of rare-earth free permanent magnets with very promising performance.

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  • 38.
    Vishina, Alena
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Vekilova, Olga Yu.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Bjorkman, Torbjorn
    Abo Akad Univ, Dept Nat Sci, FI-20500 Turku, Finland.
    Bergman, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. School of Science and Technology, Örebro University, Örebro, Sweden.
    High-throughput and data-mining approach to predict new rare-earth free permanent magnets2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 9, article id 094407Article in journal (Refereed)
    Abstract [en]

    We present an application of a high-throughput search of new rare-earth free permanent magnets focusing on 3d-5d transition metal compounds. The search involved a part of the Inorganic Crystal Structure Database, together with tailored search criteria and electronic structure calculations of magnetic properties. Our results suggest that it is possible to find candidates for rare-earth free permanent magnets using a data-mining/datafiltering approach. The most promising candidates identified here are Pt2FeNi, Pt2FeCu, and W2FeB2. We suggest these materials to be a good platform for further investigations in the search of novel rare-earth free permanent magnets.

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  • 39.
    Vrube, Ivan I.
    et al.
    Skolkovo Inst Sci & Technol, Moscow 121205, Russia.
    Pervishko, Anastasiia A.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. ITMO Univ, St Petersburg 197101, Russia.
    Herper, Heike C.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Brena, Barbara
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Orebro Univ, Sch Sci & Technol, SE-70182 Orebro, Sweden.
    Yudin, Dmitry
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Skolkovo Inst Sci & Technol, Moscow 121205, Russia.
    An ab initio perspective on scanning tunneling microscopy measurements of the tunable Kondo resonance of the TbPc2 molecule on a gold substrate2020In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 101, no 12, article id 125106Article in journal (Refereed)
    Abstract [en]

    With recent advances in the areas of nanostructure fabrication and molecular spintronics the idea of using single molecule magnets as building blocks for the next generation electronic devices becomes viable. A particular example represents a metal-organic complex in which organic ligands surround a rare-earth element or transition metal. Recently, it was explicitly shown that the relative position of the ligands with respect to each other can be reversibly changed by the external voltage without any need of the chemical modification of the sample. This opens a way of the electrical tuning of the Kondo effect in such metal-organic complexes. In this work, we present a detailed and systematic analysis of this effect in TbPc2 from an ab initio perspective and compare the obtained results with the existing experimental data.

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