Understanding demagnetization behaviors of heavy rare earth diffused and macro-composite Nd–Fe–B magnets

The demagnetization mechanism of heavy rare earth grain-boundary diffusion magnets was studied by experiment and micromagnetic simulation. The Tb70Cu30 alloy was employed as the diffusion source. With an increase in the Tb70Cu30 alloy content from 0.8 to 3.0 wt. %, the coercivity and demagnetization nucleation field of the magnets first increase, followed by a decrease. The highest coercivity enhancement was achieved by 1.5 wt. % Tb70Cu30 diffusion. In order to replicate the different anisotropic field regions inside the grain-boundary diffused magnet, the magnets with various coercivities were assembled into macro-composite magnets, and they were analyzed and compared with grain-boundary diffused magnets. It is found that the composite magnets exhibited nearly identical nucleation fields despite different anisotropy fields of the end magnets, whereas their coercivity and the squareness of the demagnetization curves showed similar variation to those of the grain-boundary diffused magnets. Based on micromagnetic simulations, the difference in the magnetostatic interaction between high and low-anisotropy-field regions can explain the significant different demagnetization nucleation field between composite and diffused magnets, as well as the variation in the demagnetization curve observed in the magnets diffused by Tb70Cu30 with different contents. These results highlight the importance of selecting the diffusion source content for enhancing the diffused magnets.