Simulation of atomic migration in CoPt ordered alloys

Tounsia Bouzar 2Christine Goyhenex 1Romaric V. Montsouka 1Hamid Bouzar 2Véronique Pierron-Bohnes 1

1. Institut de Physique et Chimie des Materiaux de Strasbourg, UMR7504, CNRS - ULP, 23, rue du Loess, BP 43, Strasbourg CEDEX 2 67034, France
2. Laboratoire de Physique et Chimie Quantique, Université Mouloud Mammeri (LPCQ), Faculté des Sciences, Département de Physique, Tizi-Ouzou 15000, Algeria

Abstract

The L10 systems are extensively studied as good candidates for high density magnetic storage media due to their high magnetic anisotropy, which is related to their chemical order anisotropy. In recent experiments, epitaxied thin bilayers NiPt/FePt/MgO(001) grown at 700K were annealed at 800K and 900K. At 800K, the L10 long range order increases without measurable interdiffusion. Surprisingly further annealing at 900K leads to interdiffusion that takes place without destroying the L10 long range order [1]. The same phenomenon has been observed by Rennhofer et al. [2] in the case of L10 FePt multilayers suggesting that interdiffusion can occur through a series of atomic mechanisms while keeping the L10 structure. As this point is important for the growth of high quality layers of alloys, we have investigated the possible atomic processes through numerical simulations for such diffusion without order change. We have used Molecular Dynamics in the second moment approximation of the tight binding method taking as reference CoPt that orders in the L10 tetragonal structure like FePt and NiPt. In a first stage, molecular statics calculations were performed in order to study the vacancy migration and to determine in particular if some jump cycles are feasible with an energy barrier low enough to make possible a diffusion without order change. For instance we find that a 6 jump cycle mechanism is favourable from an energy point of view relatively to a second nearest neighbour jump that has a much higher energy barrier. We have also checked mechanisms involving two or three vacancies. The static results are supported by a statistical study for which large series of constant energy simulations are performed at the temperatures of interest (900-1000 K) in order to check the actual occurrence of each elementary step of the previously identified jump cycles.

[1] R. V. Montsouka et al., Phys. Rev. B 74, 144409 (2006)
[2] M. Rennhofer et al., Phys. Rev. B 74, 104301 (2006)

 

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Presentation: Oral at E-MRS Fall Meeting 2008, Symposium G, by Christine Goyhenex
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-05-19 11:20
Revised:   2009-06-07 00:48