ホーム > 研究活動 > 口頭発表(2014) > First principles calculations of crystal field parameters and atomistic magnetic properties in rare-earth hard magnetic compounds near surfaces and interfaces

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First principles calculations of crystal field parameters and atomistic magnetic properties in rare-earth hard magnetic compounds near surfaces and interfaces

IEEE International Magnetics Conference

Hiroki Tsuchiura ( Tohoku University )

概要/Abstract

Rare-earth ions in permanent magnets such as Nd2Fe14B [1] play a crucial role in enhancing thestability of their magnetization against demagnetizing and external fields. The mechanism of theenhancement is as follows [2]. There is an interaction of their anisotropic 4f electronic cloudswith the crystal electric fields of surrounding charges. This interaction accompanied with spin-orbit coupling produces an energetically favored alignment of the rare-earth magnetic momentsalong a specific crystalline direction. Then the 3d magnetic moments of Fe ions, which providemost of the magnetization in the case of Nd2Fe14B, are aligned via the 4f-3d effective exchangeinteraction. The large anisotropic magnetic energy due to the Nd ions has been strongly believedas a main origin of their large coercivity.To reveal the coercivity mechanism of sintered Nd-Fe-B magnets is of crucial importance fromnot only scientific interests but also the viewpoints of sustainable technology. Several recentexperimental studies [3] have revealed that local structures around the grain boundaries ofsintered Nd-Fe-B magnets significantly affect their coercivity. Theoretically, we have calculatedthe crystal field parameter (CFP) A20 of Nd ions located on the surfaces of the crystallineNd2Fe14B based on first-principles calculations, and have found that the Nd ions exposed on the(001) surface exhibit A20 < 0, that is, c-plane magnetic anisotropy [4,5]. We have also reportedthat this surface anisotropy anomaly may cause a reduction of coercivity by half of the anisotropyfield HK based on a simple micromagnetic-type simulation [6]. It has been still unclear, however,whether this observation can give an explanation of the coercivity mechanism of bulk sintered Nd-Fe-B magnets and other rare-earth hard magnets. Motivated by these backgrounds, we study theelectronic structures and magnetic properties of the Nd2Fe14B main phase around severalsurface- and interfacial-structures based on first-principles calculations by using the WIEN2kcode [7].Figure 1 shows the slab-models of Nd2Fe14B used in this study to mimic the (001) surfaces (Fig.1(a) and (b)), the (100) surface (Fig. 1(c)), and the (110) surface (Fig. 1(d)) structures. We calculatethe CFP A20 and find that any Nd ions exposed on the (100) or the (110) surface do not show c-plane magnetic anisotropy. Also we confirm that, Nd ions on (001) surface can restore the c-axisanisotropy if they are not outcropping on the (001) surface (Fig. 1(b)). In the Nd2Fe14B crystalstructure, the nearest-neighboring ion of an Nd ion is located almost just above the Nd ion, butslightly tilted away from the c-axis direction, as shown in Fig.2. Thus the 5d electrons of the Ndion exchange-couple mainly to the 3d electrons of this Fe, resulting in slight distortion of the 5dvalence orbital to the Fe-direction, that is, almost to the c-direction (Fig.2). The 4f clouds of theNd ion tend to avoid to overlap with the distorted 5d clouds to reduce the electrostatic energybetween them, and extends within the c-plane. This is an intuitive explanation of the c-axisanisotropy of Nd ions in the Nd2Fe14B structure. This scenario can hold also in the cases of Figs1(b), (c), and (d). If an Nd ion is outcropping on the (001) surface (Fig. 1(a)), however, it loses suchnearest-neighboring Fe ion, and also lose the c-axis anisotropy. We will also discuss theelectronic states and local magnetic anisotropy around several interfacial structures between theNd2Fe14B main phase and the so-called Nd-rich intergranular oxides such as Nd2O3.


References

[1] J. F. Herbst, Rev. Mod. Phys. 63, 819 (1991), and references therein.
[2] M. Richiter, J. Phys. D: Appl. Phys. 31, 1017 (1998).
[3] H. Sepehri-Amin, T. Ohkubo, and K. Hono, Acta Mater. 61, 819 (2012), and references therein.
[4] H. Moriya, H. Tsuchiura, and A. Sakuma, J. Appl. Phys. 105, 07A740 (2009).
[5] S. Tanaka, H. Moriya, H. Tsuchiura, A. Sakuma, M. Divis, and P. Novak, J. Appl. Phys. 109,07A702 (2011).
[6] C. Mitsumata, H. Tsuchiura, and A. Sakuma, Appl. Phys. Exp. 4, 113002 (2011).
[7] P. Braha, K. Schwarz, G. Madsen, D. Kvasnicka, and J. Luitz, WIEN2k, an Augmented PlaneWave + Local Orbitals Program for Calculating Crystal Properties, Karlheinz Schwarz, TU Wien,Austria, 2001, ISBN 3-9501031-1-2.


研究活動

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