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

Hiroaki Nishio

doi:10.1109/tmag.2014.2381156

Abstract

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-x La x Fe 12-x Co x O 19 (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.

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