| 1. |
- Khmelevska, T., et al.
(författare)
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Dependence of magnetism of VAu4 alloy on the state of chemical order : A first principles study
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 76:5
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Tidskriftsartikel (refereegranskat)abstract
- The strong dependence of the magnetic properties of the alloy VAu4 upon the degree of chemical order has been a subject of intense experimental studies and controversial theoretical interpretations. In the framework of density functional theory using the coherent potential approximation embodied in the Korringa-Kohn-Rostoker method, we perform first principles calculations of VAu4 varying the degree of atomic chemical order from a disordered fcc alloy to the fully ordered MoNi4-type structure. In contrast to the conventional point of view, partially also based on earlier first principles studies of the ordered structure, our results suggest a localized character of the vanadium moments rather than being weakly itinerant. Moreover, we find that in the fully ordered alloy an antiferromagnetic state is more stable than the ferromagnetic. This finding leads to a significant revision of the earlier descriptions of magnetism in VAu4, which were based either on itinerant or local moment pictures. Investigating fcc Au-V alloys richer in vanadium, we also study the role of local environment effects on the stabilization of the magnetic moments at the V atoms and advocate a ferrimagnetic character of the experimentally observed state with a small spontaneous magnetization.
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| 2. |
- Khmelevska, T., et al.
(författare)
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Magnetism and origin of non-monotonous concentration dependence of the bulk modulus in Fe-rich alloys with Si, Ge and Sn : a first-principles study
- 2006
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 18:29, s. 6677-6689
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Tidskriftsartikel (refereegranskat)abstract
- Using the first-principles calculations based on the coherent potential approximation, we study the electronic structure, magnetic moments and the bulk modulus of FeX alloys with IVB group elements ( X = Si, Ge, Sn) in the Fe-rich concentration range (x = 0.0 - 0.25), which form a stability region of bcc-related phases. In agreement with experiment, our calculations reproduce well a peculiar non-monotonous behaviour of the bulk modulus in Fe - Si alloys with increasing Si concentration. Such a dependence is found for all bcc-related disordered and partially ordered Fe - Si phases A2, B2 and D0(3), which is in contrast with an earlier suggestion that the non-monotonous bulk modulus behaviour is related to partial ordering in Fe - Si. In addition, our results predict a similar behaviour in Fe - Ge and Fe - Sn alloys. It is shown that the observed behaviour of the bulk modulus is entirely related to the changes of the magnetic properties with chemical composition.
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| 3. |
- Khmelevskyi, S., et al.
(författare)
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Ab initio investigation of the Invar anomaly in ordered fcc Fe-Pt alloys
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:6
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Tidskriftsartikel (refereegranskat)abstract
- The magnetovolume anomaly in ordered fcc Fe-Pt Invar alloys is studied by ab inito Korringa-Kohn-Rostoker atomic-sphere approximation technique in the framework of density-functional theory. The paramagnetic state above the magnetic ordering transition temperature is treated using the disordered local moment (DLM) approach. We show that the DLM approach is able to describe and explain the Invar anomaly in ordered Fe-rich Fe-Pt alloys similarly to our previous work for the disordered modification. In particular, our calculations predict the weakening of the Invar anomaly in chemically ordered compositions as compared to the disordered case as well as a decreasing magnetovolume anomaly with increasing Pt concentrations for both ordered and disordered cases in full agreement with experimental observations. The detailed study of the Fe3Pt compound with varying chemical long-range order has revealed a continuously decreasing spontaneous volume magnetostriction with increasing degree of order. This can be understood from the corresponding band structure of random and ordered alloys.
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| 4. |
- Khmelevskyi, S., et al.
(författare)
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Electronic structure analysis of the pressure induced metamagnetic transition and magnetovolume anomaly in Fe3C - Cementite
- 2005
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 17:46, s. 7345-7352
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Tidskriftsartikel (refereegranskat)abstract
- Within the framework of ab initio electronic structure calculations we study a mechanism for the recently discovered pressure induced metamagnetic transition in Fe3C-cementite. It is shown that the transition occurs between a high moment ferromagnetic state and a non-magnetic (non-spin-polarized) state without stabilization of any intermediate antiferromagnetic-like state, which may explain the experimentally observed difference in character of the transitions in Fe3C and fcc Fe-Pt. Although both materials are Invar-type systems we find considerable differences between their electronic structures and the degrees of localization, or itinerancy, of the Fe magnetic moments. The possibilities for describing the thermal expansion anomaly in Fe3C within existing approaches to the Invar problem are discussed in detail.
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| 5. |
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| 6. |
- Khmelevskyi, S., et al.
(författare)
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Magnetic exchange interactions in the paramagnetic state of hcp Gd
- 2007
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 19:32
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Tidskriftsartikel (refereegranskat)abstract
- We use the first-principles magnetic force theorem embodied in the Korringa-Kohn-Rostoker method to calculate pair magnetic exchange interactions in pure hexagonal close packed ( hcp) Gd metal in the ferromagnetic as well as in the paramagnetic state with disordered local 4f-moments. It is found that the exchange interactions between the localized 4f-moments, in particular also distant ones, depend on the state of magnetic disorder. Such a dependence is a consequence of the electronic structure changes of the conduction band that mediates the interaction between the local moments. The magnetic ordering temperature has been calculated using a Monte Carlo simulation technique and the results are compared with mean-field based studies.
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| 7. |
- Ruban, Andrei V., et al.
(författare)
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Temperature-induced longitudinal spin fluctuations in Fe and Ni
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:5
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Tidskriftsartikel (refereegranskat)abstract
- We have developed an ab initio framework for calculating parameters of a high-temperature magnetic Hamiltonian. In the adiabatic approximation, this includes transverse and longitudinal magnetic excitation spectra on equal footing. The exchange interaction parameters of the Hamiltonian for bcc Fe and fcc Ni are determined from constrained local spin-density approximation calculations. Finite temperature magnetic properties of the resulting model Hamiltonian are then investigated by a Monte Carlo simulation technique. The calculated Curie temperatures and paramagnetic susceptibilities are in good agreement with experimental data for both metals. We demonstrate that the temperature-induced longitudinal spin fluctuations are important for high temperature properties such as susceptibility and magnetic specific heat.
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