| 1. |
- Peil, Oleg E., et al.
(författare)
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Ab initio investigation of magnetic ordering and spin-glass transition in Cu-rich Cu-Mn systems
- 2007
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 310:2, s. 1561-1563
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Tidskriftsartikel (refereegranskat)abstract
- Manganese-noble metal alloys exhibit a large variety of magnetic structures as a function of Mn concentration. Ab initio studies of these structures are practically absent due to difficulties in handling simultaneously atomic and magnetic ordering effects in disordered alloys. In this work we use the generalized perturbation method (GPM) A.V.Ruban, S. Shallcross, S.I. Simak H.L. Skriver, Phys. Rev. B 70 (2004) 125115 in the framework of the KKR method and the coherent potential approximation (CPA) in order to determine both effective chemical interactions and magnetic exchange interaction parameters of the classical Heisenberg Hamiltonian in Cu-Mn alloys. The effective chemical interactions have been used in Ising Monte Carlo simulations to determine the equilibrium distribution of atoms on the FCC lattice at the experimental aging temperatures. The obtained atomic short-range order for Cu-Mn alloys is in excellent agreement with the existing experimental data. The underlying atomic distribution has subsequently been used in Heisenberg Monte Carlo simulations with the exchange interaction parameters calculated for the corresponding alloy composition by the GPM method. For alloys with low Mn content, up to 20 at%, we find a transition into a spin-glass state with a specific magnetic short order.
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| 2. |
- Peil, O. E., et al.
(författare)
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Ab initio study of atomic ordering and spin-glass transition in dilute CuMn alloys
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:2
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Tidskriftsartikel (refereegranskat)abstract
- An archetypical spin-glass metallic alloy, Cu0.83Mn0.17, is studied by means of an ab initio based approach. First-principles calculations are employed to obtain effective chemical, strain-induced, and magnetic exchange interactions, as well as static atomic displacements, and the interactions are subsequently used in thermodynamic simulations. It is shown that the calculated atomic and magnetic short-range order accurately reproduces the results of neutron-scattering experiments. In particular, it is confirmed that the alloy exhibits a tendency toward ordering and the corresponding ordered phase is revealed. The magnetic structure is represented by spin-spiral clusters accompanied by weaker ferromagnetic short-range correlations. The spin-glass transition temperature obtained in Monte Carlo simulations by a finite-size scaling technique, 57 K, is in reasonable agreement with experimental data, 78 K.
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| 3. |
- Peil, O. E., et al.
(författare)
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Detailed ab initio calculations of the structure and magnetic state of a metallic spin glass
- 2008
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Ingår i: New Journal of Physics. - : IOP Publishing. - 1367-2630. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- We present for the first time a long-sought atomic and magnetic structure of one of the most well-known and studied spin-glass systems: the Cu83Mn17 alloy, which is the prototype for a large class of metallic spin-glass systems. The structure has been determined from simulations based on atomic and magnetic interactions obtained from first-principles calculations. Being in perfect agreement with recent neutron scattering experiments, our results reveal the atomic and magnetic structure that exhibits a specific short-range order (SRO) and is believed to be responsible for a complicated magnetic cluster dynamics leading to the spin-glass behaviour in this alloy. The ordered phase underlying the atomic SRO is shown to have a Cu3Mn-type structure with 16 atoms per unit cell. The magnetic SRO is associated with incommensurate spin-spiral correlations.
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| 4. |
- Pourovskii, L. V., et al.
(författare)
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Fully relativistic spin-polarized exact muffin-tin-orbital method
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:9
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Tidskriftsartikel (refereegranskat)abstract
- The exact muffin-tin-orbital (EMTO) method is generalized for fully relativistic (FR) spin-polarized calculations. In the present implementation we solve self-consistently the four-component Dirac equation by using the Green's function formalism. Substitutional disorder is treated within the coherent potential approximation. To obtain accurate total energies we use the full-charge density technique. We apply the FR-EMTO Green's function method to calculate the ground-state properties of delta-Pu. We also calculate spin and orbital magnetic moments in random bcc, fcc, and hcp Fe-Co alloys, as well as in the B2 ordered and partially ordered phase.
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| 5. |
- Ruban, Andrei V., et al.
(författare)
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Theoretical investigation of bulk ordering and surface segregation in Ag-Pd and other isoelectornic alloys
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75, s. 054113-1-05411316
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Tidskriftsartikel (refereegranskat)abstract
- Bulk ordering in Ag-Pd and other isoelectronic alloys is investigated theoretically by a number of first-principles techniques. The electronic structure and total energy have been calculated by the Green's function Korringa-Kohn-Rostocker and full-potential plane wave methods. The effective cluster interactions of the Ising-type Hamiltonian have been obtained by the screened generalized perturbation method. They reveal a complex concentration-dependent ordering behavior in these alloys due to band filling and Fermi surface effects. In particular we show that long-period superstructures are gradually stabilized by a great number of relatively weak long-range effective pair- and three-site interactions, which can be seen as collective effect. A similar complex concentration dependence is also found for surfaces of Ag-Pd alloys. The surface composition of the (111) and (100) surface of Ag75Pd25, Ag50Pd50, and Ag33Pd67 alloys have been then investigated by the surface Green's function technique and the screened generalized perturbation method for the effective interactions of the Ising-type Hamiltonian and the grand canonical Monte Carlo method for statistical thermodynamic simulations at finite temperatures. We compare our results with experimental data and other theoretical calculations.
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| 6. |
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| 7. |
- 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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| 8. |
- Korzhavyi, Pavel A., 1966-, et al.
(författare)
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First-principles calculations as a new tool in steel research
- 2008
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Ingår i: 6th European Stainless Steel Conference, Science and Market Helsinki, Finland, June 10-13, 2008. - : Jernkontoret. - 9197413194 ; , s. 279-284
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Konferensbidrag (refereegranskat)abstract
- Great scepticism over the earliest applications of the first-principles theory to the calculation of the ground state properties of real systems in the 70’s and 80’s has gradually turned into a widespread belief in its quantitative accuracy and predictive power, due to a great number of successful applications to very different systems and problems, so that several leading materials manufacturing companies have initiated first-principles based research programs aimed at design of new materials. However, the role of first-principles calculations in the design process, and, most importantly, their potential, remain quite obscure. We therefore try to give a “balanced” view on the possibilities of first-principles calculations, not only telling success stories, but also discussing their problems and limitations. Then, we demonstrate uniqueness of the information that can be obtained using first-principles methods. Thus we determine their place among the traditional research methods and tools in materials science. In order to show the capabilities of first-principles calculations, we present some recent applications of first-principles methods to investigation of the thermodynamic and kinetic properties of austenitic (including high-nitrogen) and ferritic steels, at the atomic level. Finally, we indicate problems, related to steel research and development, where first-principles methods (together with other experimental or theoretical techniques) can lead to substantial progress or even breakthroughs.
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| 9. |
- Razumovskii, I. M., et al.
(författare)
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New generation of Ni-based superalloys designed on the basis of first-principles calculations
- 2008
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Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 497:1-2, s. 18-24
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to the design of Ni-based single crystal superalloys is proposed. It is based on a concept that under given structural conditions, the creep-rupture characteristics of superalloys are mainly determined by interatomic bonding given by the cohesive energy. In order to characterize the individual contribution of each alloying element to the strength properties at high temperature, we introduce a parameter, X, which is the partial molar cohesive energy of an alloy component. This parameter is then obtained in the total energy first-principles calculations for a usual set of alloying elements. We demonstrate that creep-rupture characteristics of alloys indeed correlate with the total gain partial molar cohesive energy due to alloying and find that W, Ta, and Re have the highest values of X, and should therefore play the major role in providing high-temperature strength of superalloys. Based on this finding, we design three new superalloys with a high content of W and show that they have superior creep-rupture properties compared not only with their counterparts with the lower content of W, but also with the best Ru-bearing Ni-based superalloys.
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| 10. |
- Ruban, Andrei V., et al.
(författare)
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First-principles theory of magnetically driven anomalous ordering in bcc Fe-Cr alloys
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:9, s. 094436-
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio perturbation-theory techniques, such as the generalized perturbation method and magnetic force theorem, are used to determine the Heisenberg exchange interaction parameters and the effective cluster interactions in Fe-rich bcc Fe-Cr alloys in different magnetic states. We establish a direct connection between chemical and magnetic exchange interactions, as well as their dependence on the global magnetic state of the alloy. These findings have important implications for phase equilibria in magnetic alloys. In particular, we demonstrate that the experimentally reported concentration interval of anomalous ordering in Fe-Cr alloys is determined by the thermal history of the alloys through the value of global magnetization at the annealing temperature.
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