Temperature dependence of magnetic anisotropy of Ga-substituted cobalt ferrite

Abstract

The temperature dependence of magnetization, magnetic anisotropy, and coercive field of gallium-substituted cobalt ferrite was investigated for a series of compositions of CoGaxFe2−xO4 (0⩽x⩽0.8). Hysteresis loops were measured for each sample over the range of −5T⩽μ0H⩽5T for selected temperatures between 10 and 400K. The magnetization at 5T and low temperatures was found to increase for the lower Ga contents (x=0.2 and 0.4) compared to pure CoFe2O4, indicating that at least initially, Ga3+ substitutes predominantly into the tetrahedral sites of the spinel structure. The high field regions of these loops were modeled using the law of approach to saturation, which represents the rotational process, together with an additional linear forced magnetization term. The first order cubic magnetocrystalline anisotropy coefficient K1 was calculated from curve fitting to these data. It was found that K1 decreased with increasing Ga content at all temperatures. Both anisotropy and coercivity increased substantially as temperature decreased. Below 150K, for certain compositions (x=0, 0.2, 0.4), the maximum applied field of μ0H=5T was less than the anisotropy field and, therefore, insufficient to saturate the magnetization. In these cases, the use of the law of approach method can lead to calculated values of K1 which are lower than the correct value.