| 1. |
- Abrikosov, Igor, et al.
(författare)
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Importance of Thermally Induced Magnetic Excitations in First-principles Simulations of Elastic Properties of Transition Metal Alloys
- 2012
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Ingår i: Solid State Phenomena. - 1012-0394 .- 1662-9779. ; 190, s. 291-294
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate the importance of accounting for the complex magnetic ground state and finite temperature magnetic excitations in theoretical simulations of structural and elastic properties of transition metal alloys. Considering Fe72Cr16Ni12 face centered cubic (fcc) alloy, we compare results of first-principles calculations carried out for ferromagnetic and non-magnetic states, as well as for the state with disordered local moments. We show that the latter gives much more accurate description of the elastic properties for paramagnetic alloys. We carry out a determination of the magnetic ground state for fcc Fe-Mn alloys, considering collinear, as well as non-collinear states, and show the sensitively of structural and elastic properties in this system to the detailed alignment between magnetic moments. We therefore conclude that it is essential to develop accurate models of the magnetic state for the predictive description of properties of transition metal alloys.
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| 2. |
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| 3. |
- Alling, Björn, 1980-, et al.
(författare)
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Energetics and magnetic impact of 3d-metal doping of the half-metallic ferromagnet NiMnSb
- 2008
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 77:14, s. 144414-
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Tidskriftsartikel (refereegranskat)abstract
- We have performed a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d metals Cr, Mn, Fe, Co, and Ni, with respect to both energetics and magnetic properties. Starting from the formation energies, we discuss the possibility of placing the dopant on different crystallographic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions, and density of states and also outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn have the largest impact on magnetic interactions in the system while preserving its half-metallic property. Therefore, we suggest the possibility that these dopants increase the thermal stability of half-metallicity in NiMnSb, with implications for its possible usage in spintronics applications.
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| 4. |
- Amirabbasi, Mohammad, et al.
(författare)
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Lattice distortions and magnetic interactions in single-layer VOCl
- 2023
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 7:7
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Tidskriftsartikel (refereegranskat)abstract
- Atomically thin layers exfoliated from magnetic van der Waals layered materials are currently of high interest in solid state physics. VOCl is a quasi-two-dimensional layered antiferromagnet which was recently synthesized in monolayer form. Previous theoretical studies have assumed the high-temperature orthorhombic lattice symmetry also in the low-temperature range, where the bulk system is known to be monoclinic due to a strong magnetoelastic coupling. We demonstrate from ab initio calculations that this monoclinic distortion is prevalent also in monolayers, which is in line with recent experimental indications of monoclinic symmetry. Our calculations also show that competing ferromagnetic and antiferromagnetic interactions cause a frustrated twofold magnetic superstructure where higher-order magnetic interactions play a key role.
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| 5. |
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| 6. |
- Dubrovinsky, L., et al.
(författare)
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The most incompressible metal osmium at static pressures above 750 gigapascals
- 2015
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Ingår i: Nature. - : NATURE PUBLISHING GROUP. - 0028-0836 .- 1476-4687. ; 525:7568, s. 226-
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Tidskriftsartikel (refereegranskat)abstract
- Metallic osmium (Os) is one of the most exceptional elemental materials, having, at ambient pressure, the highest known density and one of the highest cohesive energies and melting temperatures(1). It is also very incompressible(2-4), but its high-pressure behaviour is not well understood because it has been studied(2-6) so far only at pressures below 75 gigapascals. Here we report powder X-ray diffraction measurements on Os at multi-megabar pressures using both conventional and double-stage diamond anvil cells(7), with accurate pressure determination ensured by first obtaining self-consistent equations of state of gold, platinum, and tungsten in static experiments up to 500 gigapascals. These measurements allow us to show that Os retains its hexagonal close-packed structure upon compression to over 770 gigapascals. But although its molar volume monotonically decreases with pressure, the unit cell parameter ratio of Os exhibits anomalies at approximately 150 gigapascals and 440 gigapascals. Dynamical mean-field theory calculations suggest that the former anomaly is a signature of the topological change of the Fermi surface for valence electrons. However, the anomaly at 440 gigapascals might be related to an electronic transition associated with pressure-induced interactions between core electrons. The ability to affect the core electrons under static high-pressure experimental conditions, even for incompressible metals such as Os, opens up opportunities to search for new states of matter under extreme compression.
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| 7. |
- Ekholm, Marcus, et al.
(författare)
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Ab initio calculations and synthesis of the off-stoichiometric half-Heusler phase Ni1-xMn1+xSb
- 2010
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Ingår i: JOURNAL OF APPLIED PHYSICS. - : American Institute of Physics. - 0021-8979 .- 1089-7550. ; 108:9
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Tidskriftsartikel (refereegranskat)abstract
- We perform a combined theoretical and experimental study of the phase stability and magnetism of the off-stoichiometric Ni1-xMn1+xSb in the half-Heusler crystal phase. Our work is motivated by the need for strategies to engineer the magnetism of potentially half-metallic materials, such as NiMnSb, for improved performance at elevated temperatures. By means of ab initio calculations we investigate Ni1-xMn1+xSb over the whole composition range 0 andlt;= x andlt;= 1 of Ni replacing Mn and show that at relevant temperatures, the half-Heusler phase should be thermodynamically stable up to at least x=0.20 with respect to the competing C38 structure of Mn2Sb. Furthermore we find that half-Heusler Ni1-xMn1+xSb retains half-metallic band structure over the whole concentration range and that the magnetic moments of substitutional Mn-Ni atoms display magnetic exchange interactions an order of magnitude larger than the Ni-Mn interaction in NiMnSb. We also demonstrate experimentally that the alloys indeed can be created by synthesizing off-stoichiometric Ni1-xMn1+xSb films on MgO substrates by means of magnetron sputtering.
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| 8. |
- Ekholm, Marcus, et al.
(författare)
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Assessing the SCAN functional for itinerant electron ferromagnets
- 2018
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:9
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Tidskriftsartikel (refereegranskat)abstract
- Density functional theory is a standard model for condensed-matter theory and computational material science. The accuracy of density functional theory is limited by the accuracy of the employed approximation to the exchange-correlation functional. Recently, the so-called strongly constrained appropriately normed (SCAN) [Sun, Ruzsinszky, and Perdew, Phys. Rev. Lett. 115, 036402 (2015)] functional has received a lot of attention due to promising results for covalent, metallic, ionic, as well as hydrogen- and van der Waals-bonded systems alike. In this work, we focus on assessing the performance of the SCAN functional for itinerant magnets by calculating basic structural and magnetic properties of the transition metals Fe, Co, and Ni. We find that although structural properties of bcc-Fe seem to be in good agreement with experiment, SCAN performs worse than standard local and semilocal functionals for fcc-Ni and hcp-Co. In all three cases, the magnetic moment is significantly overestimated by SCAN, and the 3d states are shifted to lower energies, as compared to experiments.
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| 9. |
- Ekholm, Marcus, et al.
(författare)
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Configurational thermodynamics of Fe-Ni alloys at Earths core conditions
- 2011
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Ingår i: Earth and Planetary Science Letters. - : Elsevier. - 0012-821X .- 1385-013X. ; 308:1-2, s. 90-96
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Tidskriftsartikel (refereegranskat)abstract
- By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe-Ni alloys at the pressure and temperature conditions of the Earths core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe-Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe-Ni system, we have calculated the fcc-hcp structural free energy difference in a wide pressure-temperature range of 120-360 GPa and 1000-6600K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earths inner core, Ni acts as an agent to stabilise the hcp phase.
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| 10. |
- Ekholm, Marcus, et al.
(författare)
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Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy
- 2010
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 105:16, s. 167208-
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Tidskriftsartikel (refereegranskat)abstract
- In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. We consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100 K below their Curie temperature, to demonstrate from ab initio calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data.
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