Effect of DyHx addition on the elemental distribution, magnetization reversal and coercivity in sintered Nd–Fe–B magnets

Abstract

Nd12.2Fe81.8B6 powders were mixed with DyHx powders to prepare sintered magnets for improving the coercivity. The addition of 1 wt% DyHx leads to a significant increase of coercivity from 7.2 kOe to 12.4 kOe, and for 3 wt% DyHx addition the coercivity increases to 17.1 kOe. However, with increasing the addition amount of DyHx the increasing trend of coercivity becomes slower, and the remanence decreases in the sintered magnets. The activation size is a little larger than the theoretical value of domain wall size in Nd–Fe–B, implying that the reversed domain wall involves the defect region at grain outer-layer that promotes the nucleation of reversed domain wall in the interior of grain. Dy atoms prefer to diffuse into Nd–Fe–B and substitute for Nd due to the theomodynamics origin. For 1 wt% DyHx powders addition some of Dy atoms could diffuse into the outer-layer of main phase, which may modify the grain boundary structure and form (Nd, Dy)–Fe–B shell regions. So the nucleation field of reversed domain is improved at the grain outer-layer, and the coercivity increases largely in the sintered magnets. For more amount addition of DyHx powders, most of Dy atoms enter into the interior of Nd–Fe–B phase. The effect of enhancing the magnetocrystalline anisotropy at the grain outer-layer would be weakened, which should be responsible for the decrease of increasing amplitude of coercivity. Optimizing the addition amount of Dy alloys and regulating the elemental distribution are necessary to balance the coercivity, the remanence and energy product in the sintered magnets.