Investigation of domain wall pinning effect induced by annealing stress in sintered Nd-Fe-B magnet

The magnetization and demagnetization behaviors of a commercial sintered Nd-Fe-B magnet under various post-sintering annealing conditions were investigated. Focus was placed on the various cooling rates after the two-stage post-sintering annealing process. Domain wall pinning effect was observed in the initial magnetization curves in the magnets with rapid cooling rate in the first-stage annealing, and the cooling rate of the second-stage annealing had little effect on the pinning effect. Annealing stress induced by rapid cooling is the origin of domain wall pinning. Stress-induced fringes were observed in the grain boundary region under rapid cooling conditions and the accumulated stress near the grain boundary decreases the magnetic field required to reverse domain nucleation, leading to a relatively low coercivity.