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Interplay between magnetism, thermodynamics and diffusion in bcc Fe-Mn alloys : from first principles to finite temperatures

Abstract : In 3d alloys, magnetic properties of solutes can be extremely sensitive to local chemical environments and can have a crucial impact on various thermodynamic and kinetic properties. In order to properly understand the fundamental properties of these alloys, the first part of this work is dedicated to the study of the effects of local chemical environment on the magnetic state of Mn solutes in bcc Fe-Mn by means of Density Functional Theory. Namely, configurations containing Mn, being isolated or forming a cluster, and in the presence of vacancies or interstitial impurities are investigated and their lowest-energy magnetic configuration is determined. The ab-initio data produced are then used to parameterize an effective interaction model in order to study the properties of Fe-Mn alloys at finite temperature. The key features of Fe-Mn alloys are identified, and the model is validated at low temperature by reproducing ab-initio predictions. Using this model coupled to Monte Carlo simulations, we simulate the chemical evolution of Fe-Mn properties depending on temperature and Mn concentration while relaxing the magnetic structure on-the-fly. In order to illustrate the validity and the applicability of the model, we examine certain finite temperature properties of bcc Fe-Mn alloys such as the concentration dependence of the Curie temperature or the temperature evolution of the mixing energy and the atomic short-range order. Since diffusion in Fe and Fe-Mn alloys is generally ruled by vacancy-mechanism, we also propose a formalism to take explicitly into account the properties of vacancies in the interaction model and the effect of local magnetic state on these properties. Using tracer diffusion Monte Carlo simulations, this approach predicts the temperature dependence of self-diffusion in bcc Fe in excellent agreement with experimental results, including the deviation from Arrhenius law around the Curie temperature and the change of slope between the ferromagnetic and paramagnetic regimes. The accuracy of the widely used Ruch model is discussed in the light of the present results. Finally, we apply this approach to the diffusion of a Mn solute in bcc Fe and compare with experimental results.
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  • HAL Id : tel-02495352, version 1

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Anton Schneider. Interplay between magnetism, thermodynamics and diffusion in bcc Fe-Mn alloys : from first principles to finite temperatures. Atomic Physics [physics.atom-ph]. Université Paris Saclay (COmUE), 2019. English. ⟨NNT : 2019SACLS367⟩. ⟨tel-02495352⟩

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