(ESICMM-G8 Symposium on Next Generation Permanent Magnets, Tsukuba, 2015)
First-principles study of microstructure interfaces of Nd-Fe-B magnets


Y. Gohda1,2, H. Misawa2, D. Hirai2, Y. Tatetsu2, H. Tsuchiura3, T. Ozaki4, and S. Tsuneyuki2,4

1Departmentof Materials Science and Engineering, Tokyo Institute of Technology
2ESICMM & Department of Physics, The University of Tokyo
3Department of Applied Physics, Tohoku University
4ISSP, The University of Tokyo

Abstract:

  Optimization of the microstructure is crucial in improving the performance of Nd-Fe-B permanent magnets. In particular, interfaces between microstructural phases are expected to play an important role. Thus, we perform large-scale first-principles calculations of microstructure interfaces on supercomputers such as the K computer to explore realistic interface atomic structures and their magnetic properties. Our focus is on two types of microstructure interfaces: i) interfaces between the main phase and the triple-point phases and ii) interfaces between the main phase and the grain-boundary phases. The first one is of significance in the sense that the demagnetization field comes from the triple-point phases. Motivated by experiments, where the triple-point phase directly in touch with the main phase is Nd oxides with the fcc Nd sublattice (NdOx) [1,2], we calculated Nd2Fe14B-NdOx interfaces such as the structure depicted in Fig. 1 (a). We identified that O atoms at the interface prefer to present with Nd compared with Fe. We have also found that the electronic structure inside the main phase is hardly perturbed by the presence of the interface as shown in Fig. 1 (b). Examining the anisotropy constants of Nd at the interface and inside the main phase, we derived an effective spin Hamiltonian, with which we performed atomistic micromagnetics simulations. Considering the anisotropy of Fe sublattice [3], approximately 10% of estimated coercivity is attributed to the effect of Fe sublattice. In addition, interfaces between the main phase and the grain-boundary phases are also examined.
Fig. 1: (a) The atomic structure of a Nd2Fe14B- NdOx interface and (b) depth profile of the magnetic moment within the main phase.
[1] H. Sepehri-Amin, T. Ohkubo, T. Shima, and K. Hono, Acta Mater. 60, 819 (2012).
[2] T. Fukagawa, T. Ohkubo, S. Hirosawa, and K. Hono, J. Magn. Magn. Mater. 322, 3346 (2010).
[3] Z. Torbatian, T. Ozaki, S. Tsuneyuki, and Y. Gohda, Appl. Phys. Lett. 104, 242403 (2014).