Effect of Anisotropy Constants on Squareness Ratio in Sr–La–Co High-Performance Sintered Ferrite Magnets at Elevated Temperatures

To decrease the irreversible demagnetization of Sr-La-Co sintered ferrite magnets at elevated temperatures, it is required to remarkably improve the squareness ratio (H_k/H_cJ), where H_k and H_cJ are the knee field and coercivity, respectively. It is very important to find out the main reason behind the effects on H_k/H_cJ at high temperature. Particular attention has been paid to the effect of the ratio of anisotropy constant (K_u2/K_u1) on H_k/H_cJ for Sr_1-xLa_xFe_12-xCo_xO₁₉ (x = 0-0.4) sintered ferrite magnets above room temperature, where K_u2, K_u1, and x are the quartic and quadratic anisotropy constants, and the substituted amount, respectively. The value of H_k/H_cJ linearly decreases as x value increases at every temperature. Furthermore, it decreases with higher temperatures. The value of K_u2/K_u1 remarkably increases with increasing x value at elevated temperatures. The value of H_k/H_cJ linearly decreases in the order of x value as K_u2/K_u1 increases for all the magnets above room temperature. Uniaxial anisotropy term (K_u1 + K_u2) has an effect on the orientation in the direction of easy magnetization. However, biaxial anisotropy term (K_u2/2) has two different directions of easy magnetization. The difference in easy direction of biaxial anisotropy makes an angle of π/4 to each other. An excessive increase in K_u2/K_u1 is not necessarily effective to increase H_k/H_cJ. The decrease in H_k/H_cJ is mainly caused by the increase in K_u2/K_u1 at elevated temperatures.