Anisotropy Distribution of Nd-Fe-B Magnet Powder Prepared by the HDDR Process

The anisotropy constants K1 and (K2-K3) of anisotropic Nd-Fe-B magnet powders prepared by the modified HDDR process were obtained from torque measurements in magnetic fields of up to 7.2 MA/m. A value of K1=5.2×106 J/m3 was obtained by Fourier analysis of an unsaturated torque curve. In the case of quasi-single crystal powders, K1 was 5.9×106 J/m3, which was closer to the results for Nd2Fe14B sintered magnets. The magnitude of the anisotropy field HA distribution in each direction for 2 kinds of anisotropic and isotropic Nd-Fe-B magnet powders prepared by the HDDR process was also determined by Flanders and Shtrikman's method. In powders prepared by the modified HDDR process, the quantity of particles with higher HA was greater than in those prepared by the ordinary HDDR process in the easy direction, and the angular distribution of HA was sharp. The easy-axis distribution of particles was less than ±20° for anisotropic powders. In the case of isotropic powders, the easy axes were equally distributed in all directions. The results of experiments showed that Flanders and Shtrikman's method was appropriate for determining the magnitude of HA and the angular distributions of permanent magnets with high anisotropy.