Macroscopic phenomenon of exchange coupling and microscopic effect on magnetization reversal in sintered Nd–Fe–B magnets

Abstract

The permanent magnets of Nd13.5Fe79.18M1.52B5.8 and Nd13.5Fe79.76M0.94B5.8 were prepared, respectively, via strip casting, jet milling and sintering followed by annealing, and adding the non-ferromagnetic elements M (Al, Cu, Ga and Zr) could not only modify the microstructure, but also regulate the exchange coupling effect in the sintered magnets. From the macroscopic view the recoil loops bear spring behavior in Nd13.5Fe79.76M0.94B5.8, indicating that the energy barrier could be overcome by the intergranular exchange coupling. From the microcosmic view, the exchange coupling could increase the domain wall size by suppressing the nucleation of reversed domain, and so the activation volume increases in thermal activation. In Nd13.5Fe79.76M0.94B5.8 the exchange coupling effect is stronger, and both the coercivty of 15.0 kOe and the remanence of 14.3 kGs are a little higher than those of Nd13.5Fe79.18M1.52B5.8 magnets in which the content of non-ferromagnetic elements is a little higher and the exchange coupling effect is weaker. So the exchange coupling must not decrease the coercivity owing to the exchange coupling suppressing the nucleation of reversed domain, though the microstructure is inhomogeneous in the sintered magnets of Nd13.5Fe79.76M0.94B5.8. Reducing the defects size and decreasing the defects concentration should be the practical way to improve the coercivity in Nd-Fe-B permanent magnets.