Unveiling the origin of the large coercivity in (Nd, Dy)-Fe-B sintered magnets

Abstract Nd-Fe-B-based permanent magnets are widely used for energy conversion applications. However, their usage at elevated temperatures is difficult due to the relatively low coercivity (H_c) with respect to the anisotropy field (H_A) of the Nd₂Fe₁₄B compound, which is typically 0.2H_A. In this work, we found that the coercivity of an (Nd_0.8Dy_0.2)-Fe-B sintered magnet could reach 0.4H_A, which was twice as high as the H_c/H_A of its Dy-free counterpart. Detailed microstructural characterizations, density functional theory and micromagnetic simulations showed that the large value of coercivity, H_c = 0.4H_A, originated not only from the enhanced H_A of the main phase (intrinsic factor) but also from the reduced magnetization of the thin intergranular phase (extrinsic factor). The latter was attributed to the dissolution of 4 at.% Dy in the intergranular phase that anti-ferromagnetically coupled with Fe. The reduction in the magnetization of the intergranular phase resulted in a change in the angular dependence of coercivity from the Kondorsky type for the Dy-free magnet to the Stoner–Wohlfarth-like shape for the Dy-containing magnet, indicating that the typical pinning-controlled coercivity mechanism began to show nucleation features as the magnetization of the intergranular phase was reduced by Dy substitution.