The Role of Integranular Regions in Sintered Nd-Fe-B Magnets with (B.H)_max>420 kJ/m~3 (52.5 MGOe)

摘要

Anisotropic Nd-Fe-B magnets with a high remanence and energy density were produced by powder metallurgy. In order to obtain an energy density of (B.H)_max >420 kJ/m~3 a minimum coercive field of _JH_c>750 kA/m is necessary. Experimental investigations have shwon that hte improving alignment of the grains decreases the coercive field of the sintered magnets. It was found that abnormal grain growth preferentially occurs in magnets with a higher degree of mislaignment. 3-dimensional micromagnetic simulations show that the intergranular region between the graisn, which is determined by the composition of the magnets and the annealing treatment, plays a significant role determining the magnetic properties. The coercivity is determiend by the long range dipolar interaction and short range exchagne coupling between neighbouring grains. The numerical finite element simulations show that assuming a disturbed intergranular region of about 5 to 10 nm with a reduced magnetocrystalline anisotropy the calculated coercive field is reduced to comparable experimetnal values of _JH_c<1000 kA/m. The micromagnetic simulations further show that in the case of a reduced anisotropy between the hard magnetic grains the nucleation of reversed domains starts at the grain boundaries, and the coercive field increases with decreasing alignment of the hard magnetic grains in the range of 8 deg to 16 deg misalignment.