Accurate Determination of Magnetic Anisotropy Constants for High-Performance Permanent Magnets

The quadratic (K1) and quartic (K2) anisotropy constants obtained from a Fourier analysis of the torque curve were compared with the modified Sucksmith-Thompson (ST) method for anisotropic Sr-La-Co sintered ferrite magnets, Nd-Fe-Co-B magnet powders with high coercivity, and those of highly aligned samples. K1(3.5 - 3.6 ×105J/m3) and K2( = 0) of SrFe12O19sintered ferrites and K1(5.0 ×106J/m3) of Nd13.0Febal.Co18.3B6.0Zr0.1Ga0.4highly aligned powder obtained from the torque curves agree very well with the published values of those single crystals. Moreover, K1and K2of Sr-La-Co sintered ferrites obtained from the torque method have little effect on the volume fraction of the aligned particle ( α). On the contrary, K1is too small and K2is much larger than the published values and these are affected by α in the modified ST method. The results from the torque measurements were found to be superior to those from the modified ST method. Comparing with K1, K2cannot be disregarded for Sr0.7La0.3Fe11.7Co0.3O19sintered ferrites and Nd-Fe-Co-B magnet powders. It is caused by the angular distribution of the easy axis of the crystal grains.