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Gorbatov O.I.,Institute of Quantum Materials Science | Korzhavyi P.A.,KTH Royal Institute of Technology | Ruban A.V.,KTH Royal Institute of Technology | Johansson B.,KTH Royal Institute of Technology | Gornostyrev Yu.N.,RAS Institute of Metal Physics
Journal of Nuclear Materials | Year: 2011

Vacancy-solute interactions play a crucial role in diffusion-controlled processes, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. Detailed knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) as well as for advancing our general understanding of kinetic processes in alloys. Using first-principles calculations based on the density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Sc-Cu), and 4d (Y-Ag) elements. Unrelaxed interactions within the first three coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a strong effect on the vacancy-solute interactions. Implications of the obtained results for interpreting the effects of vacancy trapping and enhanced impurity diffusion are discussed. © 2011 Elsevier B.V. All rights reserved.

Akhukov M.A.,Radboud University Nijmegen | Fasolino A.,Radboud University Nijmegen | Gornostyrev Y.N.,Institute of Quantum Materials Science | Katsnelson M.I.,Radboud University Nijmegen
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Grain boundaries with dangling bonds (DBGB) in graphene are studied by atomistic Monte Carlo and molecular dynamics simulations in combination with density functional (siesta) calculations. The most stable configurations are selected and their structure is analyzed in terms of grain boundary dislocations. It is shown that the grain boundary dislocation with the core consisting of pentagon, octagon, and heptagon (5-8-7 defect) is a typical structural element of DBGB with relatively low energies. The electron energy spectrum and magnetic properties of the obtained DBGB are studied by density functional calculations. It is shown that the 5-8-7 defect is magnetic and that its magnetic moment survives after hydrogenation. The effects of hydrogenation and of out-of-plane deformations on the magnetic properties of DBGB are studied. © 2012 American Physical Society.

Gorbatov O.I.,Institute of Quantum Materials Science | Kuznetsov A.R.,RAS Institute of Metal Physics | Gornostyrev Yu.N.,RAS Institute of Metal Physics | Ruban A.V.,KTH Royal Institute of Technology | And 4 more authors.
Journal of Experimental and Theoretical Physics | Year: 2011

The formation of a short-range order in soft magnetic Fe-Si alloys depending on the annealing temperature has been investigated theoretically and experimentally. The B2-type short-range order has been observed in samples quenched from temperatures T > TC (where TC is the Curie temperature) with the content cSi close to the boundary of the two-phase region. Annealing at temperatures T < TC for the content c Si≥ 0.08 leads to an increase in the fraction of regions with the D03-type short-range order. The mechanism of the formation of the short-range order in Fe-Si solid solutions has been analyzed by the Monte Carlo simulation with the ab initio calculated interatomic interaction parameters. It has been shown that the energy of the effective Si-Si interaction in bcc iron strongly depends on the magnetic state of the matrix. As a result, the B2-type short-range order is formed at T > TC and is fixed at quenching, whereas the D03-type short-range order is equilibrium in the ferromagnetic state. The results reveal the decisive role of magnetism in the formation of the short-range order in Fe-Si alloys and allow the explanation of the observed structural features of the alloys depending on the composition and temperature. © 2011 Pleiades Publishing, Inc.

Singh A.,University of Alabama | Mryasov O.,University of Alabama | Gupta S.,University of Alabama | Okatov S.,Institute of Quantum Materials Science | And 2 more authors.
IEEE Transactions on Magnetics | Year: 2013

We investigate the effect of crystal symmetry driven tensor form of micromagnetic exchange stiffness A on switching properties of L10 FePt composite nano-structures. The exchange interaction term in micromagnetic energy functional have been constructed to capture the results of the first principle calculations for magnetic excitations in [001] and [100] directions. We find that out-of-plane (OP) exchange component (A⊥) is about two times weaker as compared to the in-plane (IP) component (A∥). We study the effect of this difference between principal axis elements of A tensor on the M-H loops using micromagnetic simulations. We compare two cases for IP and OP exchange constants: (a) A⊥ = A∥ (limited case) and (b) A⊥ = 0.5 A∥ (based on ab-initio calculations), and find that the athermal coercive force for 30 × 30 nm bi-layer rectangular dots differs for these two cases. Further, we find that the coercive force difference between two cases decreases as the external field goes from OP to IP. © 1965-2012 IEEE.

Petrik M.V.,Ural Federal University | Gorbatov O.I.,Institute of Quantum Materials Science | Gornostyrev Yu.N.,RAS Institute of Metal Physics
Physics of Metals and Metallography | Year: 2013

The method based on the density-functional theory has been used to study the solubility of 3p (Al, Si) and 4p (Ga, Ge) elements in ferromagnetic and paramagnetic states of bcc iron. To simulate the paramagnetic state, two different approaches have been employed, which were implemented using the SIESTA and LSGF packages. It has been established that the solution energy of all these impurities decreases upon the transition into the paramagnetic state. The solution energies obtained by averaging over the ensemble of unpolarized magnetic configurations agree well with the values obtained in the coherentpotential approximation. At the same time, the allowance for the magnetic polarization in the vicinity of an impurity leads to a decrease in the solution energy, which is most clearly pronounced at temperatures close to TC. The temperature dependence of the solution energies of the impurities in the paramagnetic state is discussed. © Pleiades Publishing, Ltd., 2013.

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