| 1. |
- Garmroudi, Fabian, et al.
(författare)
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Anderson transition in stoichiometric Fe2VAl : high thermoelectric performance from impurity bands
- 2022
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Ingår i: Nature Communications. - : Springer Nature. - 2041-1723. ; 13:1
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Tidskriftsartikel (refereegranskat)abstract
- The mathematical conditions for the best thermoelectric is well known but never realised in real materials. Here, the authors propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for 'the best thermoelectric'-a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.
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| 2. |
- Gerasimov, Arsenii, et al.
(författare)
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Nature of the magnetic moment of cobalt in ordered FeCo alloy
- 2021
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Ingår i: Journal of Physics. - : Institute of Physics Publishing (IOPP). - 0953-8984 .- 1361-648X. ; 33:16
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Tidskriftsartikel (refereegranskat)abstract
- The magnets are typically classified into Stoner and Heisenberg type, depending on the itinerant or localized nature of the constituent magnetic moments. In this work, we investigate theoretically the behaviour of the magnetic moments of iron and cobalt in their B2-ordered alloy. The results based on local spin density approximation for the density functional theory (DFT) suggest that the Co magnetic moment strongly depends on the directions of the surrounding magnetic moments, which usually indicates the Stoner-type mechanism of magnetism. This is consistent with the disordered local moment picture of the paramagnetic state, where the magnetic moment of cobalt gets substantially suppressed. We argue that this is due to the lack of strong on-site electron correlations, which we take into account by employing a combination of DFT and dynamical mean-field theory (DMFT). Within LDA + DMFT, we find a substantial quasiparticle mass renormalization and a non Fermi-liquid behaviour of Fe-3d orbitals. The resulting spectral functions are in very good agreement with measured spin-resolved photoemission spectra. Our results suggest that local correlations play an essential role in stabilizing a robust local moment on Co in the absence of magnetic order at high temperatures.
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| 3. |
- Khmelevskyi, Sergii, et al.
(författare)
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Magnetic ordering and exchange interactions in structural modifications of Mn3Ga alloys : Interplay of frustration, atomic order, and off-stoichiometry
- 2016
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950. ; 93:18
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Tidskriftsartikel (refereegranskat)abstract
- Mn-Ga alloys close to the Mn3Ga stoichiometry can be synthesized in three different crystal modifications: hexagonal, tetragonal, and face-centered cubic, both in bulk and in thin-film forms. The magnetic ordering of these modifications is varying from noncollinear antiferromagnetic in the hexagonal case to ferrimagnetic order in the tetragonal one, whereas it is still unknown for the atomically disordered fcc structure. Here we study the onset of magnetic order at finite temperatures in these systems on a first-principles basis calculating the interatomic magnetic exchange interactions in the high-temperature paramagnetic regime. We employ the disordered local moment formalism and the magnetic force theorem within the framework of the local spin-density approximation and Monte Carlo simulations taking also the effects of atomic disorder in fcc alloys into account. In particular we find the origin of the stabilization of the noncollinear 3k structure in competition between antiferromagnetic inter- and in-plane couplings of frustrated kagome planes in hexagonal Mn3Ga and predict the antiferromagnetic-1 collinear order due to frustration in fcc alloys. Special attention is paid to the effects of the off-stoichiometry and the consequences of atomic disorder. We calculate the site-preference energy of Ga antisite atoms in the tetragonal structures in the range of the compositions from Mn3Ga to Mn2Ga and slightly beyond and confirm the earlier explanation of the effect of magnetization increase due to Ga preferentially occupying one of the Mn sites.
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| 4. |
- Ruban, Andrei V., et al.
(författare)
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Magnetic state, magnetovolume effects, and atomic order in Fe65Ni35 Invar alloy : A first principles study
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:1
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Tidskriftsartikel (refereegranskat)abstract
- We employ the locally self-consistent Green's function technique and exact muffin-tin orbital method to investigate magnetic state and ground state properties of Invar Fe65Ni35 alloy. We show that it is in a chemically disordered state, characterized by a relatively small amount of atomic short-range order, above the magnetic ordering temperature. We speculate that it should remain in this state below the Curie temperature upon applying usual heat treatment for the Invar alloys. The magnetic state at the experimental lattice spacing is shown to be sensitive to the type of approximation for the exchange-correlation functional: While the magnetic ground state is purely ferromagnetic in the generalized gradient approximation, there is a small amount of Fe atoms with magnetic moment antiferromagnetically aligned relative to the global magnetization in the local density approximations. The local spin-density approximation, however, fails to yield correctly the equilibrium lattice spacing, whereas the generalized gradient approximation reproduces it reasonably well. The anomalous spontaneous volume magnetostriction leading to the Invar effect is found to be approximate to 3%, in fair agreement with the experimental estimate of approximate to 2.2%.
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