Enhancement of Coercivity of Nd-Fe-B Ultrafine Powders Comparable with Single-Domain Size by the Grain Boundary Diffusion Process

We describe the enhancement of coercivity of Nd-Fe-B ultrafine powders by the grain boundary diffusion (GBD) process. An effective method for increasing coercivity in Nd-Fe-B sintered magnets is the refinement of the Nd₂Fe₁₄B grains. We decreased the powder size of the grains to <1 μ m by combining hydrogenation-disproportionation-desorption-recombination, hydrogen decrepitation, and helium jet milling (He-JM). The powder size was comparable with the single-domain size of Nd₂Fe₁₄B. However, the coercivity of the ultrafine powders was lower than the value expected from their powder size, which was due to the decrease in the Nd content of the powders and lack of the Nd-rich phase on the powder surface. We performed the GBD process at 700 °C for 30 min before He-JM to increase the amount of Nd-rich phase on the powder surface. The powder showed a high coercivity of μ0H_cJ = 2 T after annealing at 600 °C for 30 min. Therefore, the GBD process is an effective method for increasing the coercivity of ultrafine He-JM powders. The process increases the amount of the Nd-rich phase on the powder surface, which is converted to the liquid phase by annealing, and reduces the number of sites for reverse domain nucleation.