Influence of Grain Boundary Diffusion on the Coercivity, Thermal Stability, and Magnetization Reversal of Sintered Nd–Fe–B Permanent Magnets

Grain boundary diffusion considerably enhances the coercivity of sintered Nd–Fe–B permanent magnets and optimizes their microstructures. Herein, we fabricated these magnets with a nominal composition of Nd30.25Fe68.22M0.59B0.94 (M represents Cu, Co, Zr, Ga, and Al) via powder metallurgy. Terbium fluoride powder served as the diffusion source in grain boundary diffusion. After the diffusion, the coercivity of the magnets substantially increased. Moreover, the temperature coefficient of coercivity ( $\beta $ ) considerably improved in the temperature range of $25~^{\circ }$ C– $150~^{\circ }$ C. The considerable enhancement in coercivity and its $\beta $ is attributed to the formation of Tb-rich (Nd, Tb)2Fe14B phases with high magnetocrystalline anisotropy in the outer layers of the main phase grains. This optimizes the grain boundary structure and reduces surface defects of the main phase grains during grain boundary diffusion, increasing the coercivity, weakening the exchange coupling effect, and suppressing magnetization reversal. The recoil loops and thermal activation reflect this suppression, possibly owing to the exchange coupling effect among the Nd2Fe14B main phase grains or the (Nd, Tb)2Fe14B main phase grains with a core-shell structure as well as between the core and shell of the (Nd, Tb)2Fe14B main phase grains. These synergistic effects enhance the nucleation field for reverse domains on the main phase surface and reduce the activation volume. Consequently, coercivity is enhanced, effectively suppressing magnetization reversal and thermal fluctuations. This ensures high remanence, thermal stability, and demagnetization resistance in the Nd–Fe–B permanent magnets, thereby enhancing the reliability and efficiency of permanent-magnet motors in long-term operation.