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Atomic scale simulations(LLG and MC) of magnetization reversal at finite temperatures.

IEEE MAGNETICS Intermag Europe 2017

2017年4月27日(木)

S. Miyashita1,2, M. Nishino2,3, Y. Toga2, A. Sakuma4, S. Hirosawa2, T. Miyake5,2, H. Akai6,2and S. Doi6,2, 1. Physics, University of Tokyo, Tokyo, Japan; 2. ESCIMM, NIMS, Tsukuba,Japan; 3. MANA, NIMS, Tsukuba, Japan; 4. Applied Physics, Tohoku University, Sendai, Japan;5. CD DMat, AIST, Tsukuba, Japan; 6. ISSP, University of Tokyo, Kashiwa, Japan

Abstract

To clarify coercive mechanism of permanent magnets from a microscopic viewpoint at finitetemperatures, we studied temperature dependence of theprocesses of magnetization reversalwith a finite temperature LLG equation[1,2] and also with Monte Carlo method[3]. First, westudied threshold of magnetic fields for nucleation and domain wall propagation in a sandwichstructure of hard-soft-hard magnet. The thresholds have been obtained at zero temperatureby an analytical method for the continuum magnets. We revealed general features of thereduction of the threshold with the temperature.There we found novel aspect due to thenarrow domain wall effectinherent to the discreteness of the lattice structure and also to reduction of the exchangeenergy at finite temperatures. We also studied magnetic properties of Nd2Fe14B in a realisticatomic scale modeling of magnets by making use of real structure and exchange constantsobtained from thefirst-principle calculations based on the Korringa-Kohn-Rostoker Green’s function method,and anisotropy constants (the Stevens factor) determined in previous studies [3]. The unitcell of the material is 0.88×0.88×1.22nm3 and contains 68 atoms. There we investigatedthermodynamic properties such as temperature dependences of magnetization M(T) and alsothe anisotropy constant K(T), and we successfully reproduced the reorientation at a lowtemperature(Fig.1). Moreover, we obtained temperature dependence of magnetic properties ofindividual atoms. For example, we find the reduction of magnetization amplitude of Nd atom isfaster than that of Fe, etc. The temperature dependence of free energy barrier for themagnetization reversal of the material was also obtained[3]. Temperature dependent ofstructure of domain walls parallel and perpendicular to the Nd plane in microscopic realisticscale[4]. The width is consistent with the experimental estimations. The domain wall along thec-axis (perpendicular to the Nd plane) is shorter than that along the a-axis. The width of theDomain wall was found to increase with the temperature. Furthermore, we demonstrate LLGdynamics of nucleation process and domain wall formation from it in a lattice of 12x12x9 unitcells, in which we found the revered magnetization propagates fast in the a-axis, and a domainwall is formed parallel to the Nd-plane(Fig.2). We willalso discuss on effect of the dipole-dipole interaction on the domain wall formation.
[1] M. Nishino and S. Miyashita, Phys. Rev. B91, 134411(1-13) (2015).[2] S. Mohakud, S. Andraus, M. Nishino, A. Sakuma, S. M., Phys. Rev.B 94, 054430 (2016).[3] Y. Toga, M. Matsumoto, S. Miyashita, H. Akai, S. Doi, T. Miyake, and A. Sakuma: Phys. Rev.B 94, 174433 (2016). &Phys. Rev. B 94, 219901 (2016).[4] M. Nishino, Y. Toga, S. Miyashita, H. Akai, A. Sakuma, and S. Hirosawa, submitted.


研究活動

文部科学省

文部科学省
元素戦略プロジェクト(活動紹介)
NIMS磁石パートナーシップ

元素戦略拠点

触媒・電池元素戦略拠点
触媒・電池元素戦略研究拠点 (京都大学)
東工大元素戦略拠点
東工大元素戦略拠点 (東京工業大学)
構造材料元素戦略研究拠点
構造材料元素戦略研究拠点 (京都大学)
高効率モーター用磁性材料技術研究組合
高効率モーター用 磁性材料技術研究組合