| 1. |
- Herper, Heike C., et al.
(författare)
-
Iron porphyrin molecules on Cu(001) : Influence of adlayers and ligands on the magnetic properties
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 87:17, s. 174425-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Klar, D., et al.
(författare)
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Oxygen-tuned magnetic coupling of Fe-phthalocyanine molecules to ferromagnetic Co films
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:22, s. 224424-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Larsen, Simon R., et al.
(författare)
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Revealing the Magnetic Structure and Properties of Mn(Co,Ge)2
- 2022
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 61:44, s. 17673-17681
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Tidskriftsartikel (refereegranskat)abstract
- 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/mm'c'. 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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| 4. |
- Martinez-Casado, R., et al.
(författare)
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The CeFe11Ti permanent magnet : a closer look at the microstructure of the compound
- 2019
-
Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 31:50
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Nieves, P., et al.
(författare)
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Database of novel magnetic materials for high-performance permanent magnet development
- 2019
-
Ingår i: Computational materials science. - : Elsevier. - 0927-0256 .- 1879-0801. ; 168, s. 188-202
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Schonhobel, A. M., et al.
(författare)
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Intrinsic magnetic properties of SmFe12-xVx alloys with reduced V-concentration
- 2019
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 786, s. 969-974
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Arapan, S., et al.
(författare)
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Computational screening of Fe-Ta hard magnetic phases
- 2020
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:1
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Arapan, S., et al.
(författare)
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Influence of antiphase boundary of the MnAl tau-phase on the energy product
- 2019
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 3:6
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Balatsky, Alexander V., et al.
(författare)
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Functional Dirac Materials : Status and Perspectives
- 2018
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Ingår i: Physica Status Solidi. Rapid Research Letters. - : Wiley-VCH Verlagsgesellschaft. - 1862-6254 .- 1862-6270. ; 12:11
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 10. |
- Brena, Barbara, et al.
(författare)
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Chemical Switching of the Magnetic Coupling in a MnPc Dimer by Means of Chemisorption and Axial Ligands
- 2020
-
Ingår i: The Journal of Physical Chemistry C. - WASHINGTON DC USA : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 124:49, s. 27185-27193
-
Tidskriftsartikel (refereegranskat)abstract
- 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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| 11. |
- Brena, Barbara, et al.
(författare)
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Influence of ligands on the electronic and magnetic properties of Fe porphyrin in gas phase and on Cu(001)
- 2015
-
Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 117:17
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 12. |
- Brumboiu, Iulia Emilia, 1987-, et al.
(författare)
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Elucidating the 3d Electronic Configuration in Manganese Phthalocyanine
- 2014
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 118:5, s. 927-932
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 13. |
- Brumboiu, Iulia Emilia, et al.
(författare)
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Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines
- 2016
-
Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 12:4, s. 1772-1785
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 14. |
- Brumboiu, Iulia, et al.
(författare)
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Ligand Effects on the Linear Response Hubbard U : The Case of Transition Metal Phthalocyanines
- 2019
-
Ingår i: Journal of Physical Chemistry A. - : AMER CHEMICAL SOC. - 1089-5639 .- 1520-5215. ; 123:14, s. 3214-3222
-
Tidskriftsartikel (refereegranskat)abstract
- 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 ligand atoms that are mainly responsible for this difference are 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.
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| 15. |
- Ghorai, Sagar, et al.
(författare)
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Giant magnetocaloric effect in the (Mn,Fe)NiSi-system
- 2023
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 16. |
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| 17. |
- Gruenebohm, Anna, et al.
(författare)
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Ab initio study of the electronic and magnetic structure of the TiO2 rutile (110)/Fe interface
- 2013
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:15, s. 155401-
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 18. |
- Gruenebohm, A., et al.
(författare)
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On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
- 2016
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 49:39
-
Tidskriftsartikel (refereegranskat)abstract
- We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations.
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| 19. |
- Herper, Heike C., et al.
(författare)
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Combining electronic structure and many-body theory with large databases : A method for predicting the nature of 4 f states in Ce compounds
- 2017
-
Ingår i: Physical Review Materials. - : American Physical Society. - 2475-9953. ; 1:3
-
Tidskriftsartikel (refereegranskat)abstract
- 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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| 20. |
- Herper, Heike C., et al.
(författare)
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Fe phthalocyanine on Co(001) : Influence of surface oxidation on structural and electronic properties
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:8, s. 085411-
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 21. |
- Herper, Heike C., et al.
(författare)
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Localized versus itinerant character of 4f-states in cerium oxides
- 2020
-
Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 32:21
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 22. |
- Herper, Heike C., et al.
(författare)
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Magnetic properties of NdFe11Ti and YFe11Ti, from experiment and theory
- 2023
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 242
-
Tidskriftsartikel (refereegranskat)abstract
- 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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| 23. |
- Herper, Heike C.
(författare)
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Ni-based Heusler compounds : How to tune the magnetocrystalline anisotropy
- 2018
-
Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:1
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 24. |
- Herper, Heike C., et al.
(författare)
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Tuning the Magnetic Anisotropy of NiPtMnGa by Substitution and Epitaxial Strain
- 2019
-
Ingår i: IEEE transactions on magnetics. - : IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC. - 0018-9464 .- 1941-0069. ; 55:2
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 25. |
- Kovacs, Alexander, et al.
(författare)
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Computational Design of Rare-Earth Reduced Permanent Magnets
- 2020
-
Ingår i: ENGINEERING. - : ELSEVIER. - 2095-8099 .- 2096-0026. ; 6:2, s. 148-153
-
Tidskriftsartikel (refereegranskat)abstract
- 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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| 26. |
- Krumme, B., et al.
(författare)
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Element-specific electronic structure and magnetic properties of an epitaxial Ni51.6Mn32.9Sn15.5 thin film at the austenite-martensite transition
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 91:21
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 27. |
- Marathe, Madhura, et al.
(författare)
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Exploration of all-3d Heusler alloys for permanent magnets : An ab initio based high-throughput study
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 107:17
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 28. |
- Martinho Vieira, Rafael
(författare)
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Exploring magnetocaloric materials by ab-initio methods
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis explores the characterization of magnetocaloric materials from first-principles calculations, emphasizing entropy variation associated with the magnetocaloric effect. The study happens in the context of the search for new magnetocaloric materials to be applied in domestic magnetic refrigerators, as environmentally friendly and energy-efficient alternatives to conventional vapor-compression devices.The study involves benchmarking entropy calculations in systems like FeRh, which exhibits a first-order metamagnetic transition, and Gd, with a second-order ferromagnetic-paramagnetic transition. Different levels of approximations are examined and compared against experimental data, highlighting the need to distinguish between first-order and second-order transitions in the approach taken. The tests underscore the necessity of calculating vibrational and elastic properties for both phases to accurately calculate the entropy variation. This insight is applied in the study of Mn0.5Fe0.5NiSi0.9Al0.05, with results consistent with experimental data.Furthermore, the relationship between structural changes and magnetic properties is investigated, in particular for pressure-induced polymorphs in Gd and the phase transition in Mn0.5Fe0.5NiSi0.95Al0.05. In the case of Gd, it was shown that variations in magnetic ordering temperature under pressure could be explained through a model based on the formation and accumulation of stacking faults. For the Mn0.5Fe0.5NiSi0.95Al0.05 system, the adoption of a magnetic composite model, in conjunction with experimental data, allowed to determine that the magnetostructural transition in these compounds is predominantly driven by the lattice subsystem. The results positively confirm the feasibility of using first-principles entropy estimates as an effective screening tool in high-throughput studies for magnetocaloric materials. A promising workflow is proposed, demonstrating potential in its initial results. Through comparison with experimental data, the derived routes offer valuable insights for the further refinement of the workflow. This approach aims to enhance accuracy and systematically manage complex systems, highlighting a path forward for future advancements.Lastly, the introduction of a novel scaling scheme in Monte Carlo simulations enhancing accuracy across various temperatures, represents a potential advancement in the field of magnetic simulations.
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| 29. |
- Schröter, M., et al.
(författare)
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Tuning the magnetic phase diagram of Ni-Mn-Ga by Cr and Co substitution
- 2022
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Ingår i: Journal of Physics D. - : IOP Publishing Ltd. - 0022-3727 .- 1361-6463. ; 55:2
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 30. |
- Vekilova, Olga Yu., et al.
(författare)
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Tuning the magnetocrystalline anisotropy of Fe3Sn by alloying
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:2
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 31. |
- Vieira Martinho, Rafael, et al.
(författare)
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The role of pressure-induced stacking faults on the magnetic properties of gadolinium
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 32. |
- Vieira, Rafael Martinho, et al.
(författare)
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High-throughput compatible approach for entropy estimation in magnetocaloric materials : FeRh as a test case
- 2021
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 857
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Tidskriftsartikel (refereegranskat)abstract
- 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.
(författare)
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Realistic first-principles calculations of the magnetocaloric effect : applications to hcp Gd
- 2022
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Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 10:3, s. 156-162
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Tidskriftsartikel (refereegranskat)abstract
- 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.
(författare)
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Ab-initio study of the electronic structure and magnetic properties of Ce2Fe17
- 2021
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 888
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 35. |
- Vishina, Alena, et al.
(författare)
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Data-driven design of a new class of rare-earth free permanent magnets
- 2021
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 212
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Tidskriftsartikel (refereegranskat)abstract
- A new class of rare-earth-free permanent magnets is proposed. The parent compound of this class is Co 3 Mn 2 Ge, 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/m 3 , and the Curie temperature is 700 K. Co 3 Mn 2 Ge samples were then synthesized and characterised with respect to structure and magnetism. The crystal structure was found to be the MgZn 2 -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/m 3 , and the Curie temperature of T C = 359 K. These magnetic properties make Co 3 Mn 2 Ge 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 T 3 Mn 2 X (T = Co or alloys between Fe and Ni; X = Ge, Al or Ga) in the MgZn 2 structure type, form a new class of rare-earth free permanent magnets with very promising performance. (c) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )
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| 36. |
- Vishina, Alena, et al.
(författare)
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Fe2C- and Mn-2(W/Mo)B-4-based rare-earth-free permanent magnets as a result of the high-throughput and data-mining search
- 2023
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Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 11:1, s. 76-83
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Tidskriftsartikel (refereegranskat)abstract
- 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. [GRAPHICS] IMPACT STATEMENT Several promising high-performance rare-earth-free permanent magnets have been found as a result of an ab initio high-throughput search. Alteration proposed to improve their stability and magnetic properties.
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| 37. |
- Vishina, Alena, et al.
(författare)
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High-throughput and data-mining approach to predict new rare-earth free permanent magnets
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 101:9
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 38. |
- Vishina, Alena, et al.
(författare)
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Stable and metastable rare-earth-free permanent magnets from a database of predicted crystal structures
- 2023
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 261
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Tidskriftsartikel (refereegranskat)abstract
- 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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| 39. |
- Vrube, Ivan I., et al.
(författare)
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An ab initio perspective on scanning tunneling microscopy measurements of the tunable Kondo resonance of the TbPc2 molecule on a gold substrate
- 2020
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 101:12
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Tidskriftsartikel (refereegranskat)abstract
- 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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