Evaluation of the microstructural contribution to the coercivity of fine-grained Nd-Fe-B sintered magnets

The relationship between coercivity and the microstructural parameters α and N _eff for grain refined Nd-Fe-B sintered magnets was examined. The parameter α describes the effect of grain misalignment and a decrease in the magnetocrystalline anisotropy of the Nd ₂Fe ₁₄B grains on the coercivity. The parameter N _eff describes the influence of the demagnetizing field on the coercivity. These parameters were evaluated for Nd-Fe-B sintered magnets with three different grain sizes and the same composition. N _eff decreased with grain refinement, while α varied only slightly with grain size. This implies that the decrease in the demagnetizing field increased the coercivity through grain refinement. Grain shape and asperity were then evaluated by the aspect ratio, angle of the grain to the direction of magnetization, and reverse circularity. Although the aspect ratio and angle of the grains were not related to the changes in N _eff, variation of the average reverse circularity corresponded to the changes in N _eff with grain refinement. Therefore, improving the asperity should increase the coercivity through grain refinement.