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 ×10⁵ J/m³) and K₂( = 0) of SrFe₁₂O₁₉ sintered ferrites and K₁(5.0 ×10⁶ J/m³) of Nd_13.0Fe_bal.Co_18.3B_6.0Zr_0.1Ga_0.4 highly aligned powder obtained from the torque curves agree very well with the published values of those single crystals. Moreover, K₁ and K₂ of 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, K₁ is too small and K₂ is 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, K₂ cannot be disregarded for Sr_0.7La_0.3Fe_11.7Co_0.3O₁₉ sintered ferrites and Nd-Fe-Co-B magnet powders. It is caused by the angular distribution of the easy axis of the crystal grains.