COERCIVITY MECHANISM OF SINTERED PR17FE75B8 AND PR17FE53B30 PERMANENT-MAGNETS

Two coercivity models, a phenomenological model developed by Givord el al. and a micromagnetic model developed by Kronmuller et al., have be en used to analyze the temperature dependence of the coercive field of sintered Pr17Fe75B8 and Pr17Fe53B30 permanent magnets. A general comp arison of these two models is made. The micromagnetic model takes into account the misalignment of grains and the anisotropy imperfections a t the grain surface. From the analysis based on this model it follows that the coercivity in sintered Pr17Fe75B8 and Pr17Fe53B30 permanent m agnets is controlled by a nucleation mechanism occurring preferentiall y in the grain surface where the magnetic anisotropy is reduced and th e local demagnetizing field is the highest. A simple proportional rela tionship between the micromagnetic parameters and N(eff) is found. In addition, the effect of magnetic coupling among grains can be estimate d with this model. The phenomenological model takes into account the g eometrical effect of nucleated domains, the effect of the disturbance of domain-wall energy, and the thermal activation. From the analysis b ased on this model, it follows that the expansion of reversed domains takes place preferentially in regions where the domain-wall energy is reduced and where a spikelike reversed domain is energetically favorab le. It is demonstrated in the present investigation that the phenomeno logical model corresponds approximately to the micromagnetic model whe n the nucleation process dominates the magnetization reversal process and the magnetic coupling between grains is weak.