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Crystal Field Analysis around Grain-Boundaries in Nd-Fe-B based on First-Principles Calculations

Rare Earth and Furture Permanent Magnets and Their Applications(REPM2014)

2014.8.17-21
(2014.08.02 更新)

Hiroki Tsuchiura(Tohoku University), Takuya Yoshioka(Tohoku University), Pavel Novák(ASCR)

Abstract

 To reveal the coercivity mechanism of sintered Nd-Fe-B magnets is of crucial importance from not only scientific interests but also the viewpoints of sustainable technology. Several recent experimental studies have revealed that local structures around the grain boundaries of sintered Nd-Fe-B magnets significantly affect their coercivity [1]. Theoretically, we have calculated the crystal field parameter (CFP) A20 of Nd ions located on the vacuum surfaces of the crystalline Nd2Fe14B based on first-principles calculations, and have found that the Nd ions exhibit A20 < 0, that is, c-plane local magnetic anisotropy, only if they are exposed on the (001) surface. [2,3]. We have also reported that this surface anisotropy anomaly may cause a reduction of coercivity by half of the anisotropy field HK based on a simple micromagnetic-type simulation [4]. 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 the electronic states and crystal fields of Nd ions of the Nd2Fe14B main phase around grain-boundary-structures based on first-principles calculations. In particular, we focus on the interfacial structure between the main phase and the so-called Nd-rich intergranular oxides such as Nd2O3 and Nd4O.We find that the Nd ions not only on the (001) but also on the (100) or (110) surfaces can exhibit A20 < 0 if the oxygen atoms in the Nd-rich oxides locate close to the surface Nd ions.

References:[1] H. Sepehri-Amin, T. Ohkubo, and K. Hono, Acta Mater. 61, 819 (2012), and references therein.[2] H. Moriya, H. Tsuchiura, and A. Sakuma, J. Appl. Phys. 105, 07A740 (2009).[3] S. Tanaka, H. Moriya, H. Tsuchiura, A. Sakuma, M. Divis, and P. Novák, J. Appl. Phys. 109, 07A702 (2011).[4] C. Mitsumata, H. Tsuchiura, and A. Sakuma, Appl. Phys. Express. 4, 113002 (2011).



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