The effect of Cr-substitution on the magnetic anisotropy and its temperature dependence in Cr-substituted cobalt ferrite

The temperature dependence of the magnetic anisotropy of magnetoelastic chromium-substituted cobalt ferrites (CoCrxFe2-xO4 with 0 <= x <= 0.8) was investigated over the temperature range 10-400 K. The first-order cubic anisotropy coefficient K, was calculated by fitting the high-field regimes of the major hysteresis loops to the law of approach to saturation, which is based on the assumption that at sufficiently high field only rotational processes remain. It was found that anisotropy increases substantially with decreasing temperature from 400 to 150 K. Below 150 K, it appears that even under a maximum applied field of 5 T, the anisotropy of CoFe2O4 and CoCr0.2Fe1.8O4 is so high that it prevents complete approach to saturation. In general, at fixed temperature the magnetic anisotropy decreases with increasing Cr content, which could be explained based on the one-ion model and the results of Mossbauer studies: According to the one-ion anisotropy model, Co2+ ions in octahedral sites of the spinel structure are responsible for the high anisotropy of cobalt ferrite, and substitution of Cr3+ for Fe3+ appears to displace Co2+ ions from octahedral sites to tetrahedral sites.