Electronic states and magnetic properties around the grain boundary in Nd-Fe-B sintered magnets studied by first-principles calculations

Abstract

The coercivity in Ga-doped Nd-Fe-B magnets is enhanced about 80 % recovering from the non-Ga-doped sintered magnets after annealing processes. From experimental analyses, Ga is the key to the wettability improvement of the Nd-rich phase, and the coercivity increases due to Nd6Fe13Ga present at triple junctions. In order to examine the reason why the coercivity is improved by Nd6Fe13Ga, we performed first-principles calculations for Nd2Fe14B/Nd6Fe13Ga grain-boundary model structures. Practically, large-scale simulations are necessary for understanding the coercivity improvement in Nd-Fe-B magnets from nanoscale. In this study, however, we chose smaller unit cells to seek the termination layer of Nd6Fe13Ga at the interstitial regions of Nd-Fe-B magnets for large grain-boundary model systems. All atomic positions and cell parameters were optimized. After the structural optimization, the atomic positions of the inner region of the main phase and subphase are barely changed. On the other hand, the interface structures are complicated. In the Nd2Fe14B surface system, K1 of Nd at the surface shows strong in-plane anisotropy that can cause the coercivity decrease of Nd-Fe-B magnets. In our grain-boundary model systems, K1 of Nd is improved compared with that of the surface system thanks to Nd6Fe13Ga present at the interface. This is due to Nd 5d electrons distributing in the longitudinal direction around interstitial regions, which improves K1 of Nd. In conclusion, Ga doping works effectively in terms of improving K1 of Nd at the interface.