Micromagnetic simulations on the grain shape effect in Nd-Fe-B magnets

Micromagnetic simulations were performed to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ∼2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid > circular prism > hexagonal prism > square prism > triangular prism. Sputtered columnar grains and hot-deformed platelet grains with a constant volume were also considered. Results show that the coercivity initially increases with the aspect ratio and then nearly saturates above the ratio of ∼4. Simulations of multigrain ensembles showed that depending on the grain shape, compared to the case of single grain, a further decrease of ∼10%-45% in the coercivity is induced by magnetostatic coupling.