Coercivity and its thermal stability of Nd[sbnd]Fe[sbnd]B hot-deformed magnets enhanced by the eutectic grain boundary diffusion process

Abstract

Eutectic grain boundary diffusion process was applied to NdFeB hot-deformed magnet using Nd60Tb20Cu20 alloy, which resulted in a large coercivity enhancement from 0.87 T to 2.57 T with a relatively small decrease in remanent magnetization from 1.50 T to 1.38 T. Improved temperature coefficient of coercivity from −0.493%°C−1 to −0.328%/°C−1 led to a high coercivity of 1.47 T at 150 °C. The partial formation of Tb-rich shell on the surface of platelet shaped Nd2Fe14B grains while maintaining their ultra-fine grain size is the reasons for the substantial enhancement of the coercivity. Micromagnetic simulations suggested that a higher coercivity can be obtained when Tb-rich shell covers the c-plane surface interface of the grains than that covering the side surface interfaces. Improvement of the thermal stability of coercivity was found to be due to the exchange decoupling of Nd2Fe14B grains and the formation of (Nd,Tb)2Fe14B shell. In the frame of Kronmüller equation and based on the micromagnetic simulations, the improvement of the thermal stability of coercivity is attributed to the decrease of Neff and increase of α induced by exchange decoupling of grains, as well as the additional decrease of Neff induced by the formation of high-Ha shell.