Coercivity mechanism of Nd-Fe-B sintered magnets

We first study the electronic structure of several surfaces of crystalline Nd₂Fe₁₄B and estimate the crystal field parameters at the Nd sites based on first principles calculations. It is found that Nd ions exposed on the (001) surface exhibit an c-plane magnetic anisotropy. On the basis of this observation, we propose a mechanism that exhibits the coercivity reduction of Nd-Fe-B magnets. The model employed here is a micromagnetic model involving the anisotropy- and exchange-fields obtained by the first-principles calculations; it has a c-plane magnetic anisotropy around the surface, in contrast to the bulk c-axis anisotropy. We find that the local c-plane anisotropy drastically affects the coercivity, resulting in a reduction by half of the bulk anisotropy field H_c.