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

Eutectic grain boundary diffusion process was applied to Nd–Fe–B 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 N_eff and increase of α induced by exchange decoupling of grains, as well as the additional decrease of N_eff induced by the formation of high-H_a shell.