The variations in magnetization and magnetic anisotropy of Ge4+/Co2+ cosubstituted cobalt ferrite with temperature were investigated for a series of compositions Co1+xGexFe2−2xO4 (0 ≤ x ≤ 0.4). The magnetization at 5 T and low temperature were observed to increase for all Ge/Co cosubstituted samples compared to pure CoFe2O4. Hysteresis loops were measured for each sample over the magnetic field range of −5 T to +5 T for temperatures in the range of 10–400 K. The high field regions of these loops were modeled using Law of Approach to saturation, which represents the rotational and forced magnetization processes. The first order cubic magnetocrystalline anisotropy coefficient K1 was calculated from these fits. K1 decreased with increasing Ge content at all temperatures. Anisotropy increased substantially as temperature decreased. Below 150 K, for certain compositions (x = 0, 0.1, 0.2, and 0.3), the maximum applied field of μ0H = 5 T was less than the anisotropy field and therefore insufficient to saturate the magnetization. In these cases, the use of the Law of Approach model can give values of K1 that are lower than the correct values and this method cannot be used to estimate anisotropy accurately under these conditions.
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