2016 Volume 63 Issue 13 Pages 1042-1046
To decrease the irreversible demagnetization of anisotropic ferrite magnets at elevated temperatures, it is required to outstandingly improve the squareness ratio (Hk/HcJ), where Hk and HcJ are the knee field and coercivity, respectively. The effect of ratio of anisotropy constant (Ku2/Ku1) on Hk/HcJ for Sr1-xLaxFe12-xCoxO19 (x = 0–0.4) sintered magnets was studied, where Ku2, Ku1, and x are the quartic and quadratic anisotropic constants, and the substituted amount, respectively. The accurate values of anisotropy constants were obtained from a Fourier analysis of torque curves. The value of Hk/HcJ decreased as the temperature rose. Above room temperature, Ku2/Ku1 increased rapidly with the rise of the temperature and Hk/HcJ decreased in order of x = 0, 0.1–0.2, and 0.3–0.4 as Ku2/Ku1 increased. Uniaxial anisotropy term (Ku1 + Ku2) has an effect on the orientation in the direction of easy magnetization. On the other hand, biaxial anisotropy term Ku2 has two different directions of easy magnetization. According to the calculation using the equation of magnetic torque for a hexagonal crystal, the direction of the easy magnetization is widely distributed as the value of Ku2/Ku1 increases. An excessive increase in Ku2/Ku1 is not necessarily effective to increase Hk/HcJ.
