The Origin of Coercivity Enhancement of Sintered NdFeB Magnets Prepared by Dy Addition

The effect of Dy addition on the microstructure and magnetic properties of the sintered NdFeB magnets was investigated. The results of the microstructure analysis showed that Dy-free and Dy-doped samples are composed of Nd2Fe14B (P42/mnm) and a trace of Nd-rich phase. Dy addition reduces significantly the pole density factor of (004), (006) and (008) crystal faces as estimated by the Horta formula. Accordingly, the coercivity of the Dy-doped sample increases from 2038 kA.m(-1) up to 2288 kA.m(-1). The H-cj(T)/M-s(T) versus H-N(min)/M-s(T) (Kronmuller-plot) behavior shows that the nucleation is the dominating mechanism for the magnetization reversal in these two kinds of magnets, and two microstructural parameters of alpha(k) and N-eff are obtained. The Kronmuller-Plot gives evidence for an increase of the alpha(k) responsible for an increase of the coercivity as the result of the increase of the magnetic field as the magnetic domain reversed.