Metallurgical factors determining the coercivity of ND-FE-B magnets

Abstract

In Nd-Fe-B based permanent magnets a high magnetocrystalline anisotropy of the hardmagnetic phase is necessary for a high intrinsic coercivity. In fact, metallurgical parameters (distribution of phases, chemical composition and crystal structures of phases) and processing parameters (alloy preparation, size and shape of particles, sintering and annealing treatments) determine the value of the coercive force of each sintered Nd-Fe-B magnet. Our analytical electron microscope study shows that “melt-spun” and sintered Nd-Fe-B based magnets contain more or less a distribution of nucleation centres for reversed domains, such as iron-rich phases andα-iron precipitates within the hardrnagnetic 2∶14∶1-grains. A continuous non-magnetic layer phase between the hardrnagnetic 2∶14∶1-grains increases the expansion field of reversed domains and increase the coercivity. In “melt-spun” magnets the contribution of the pinning of magnetic domain walls becomes effective during the magnetization reversal process.