Effect of synthesis parameters on cobalt ferrite saturation magnetization

Cobalt substituted magnetite, Co_xFe_1−xFe₂O₄ was synthesized via co-precipitation method for 0.0≤x≤1.0 and annealed in argon from 600 to 800°C to investigate the effect of synthesis parameters e.g. cation substitution and thermal treatment on its saturation magnetization. Particle size variation, crystallinity and cationic distribution between sublattices which resulted from the varying synthesis parameters are expected to alter the saturation magnetization of cobalt ferrite. Both thermal treatment and cation substitution are affecting the value of saturation magnetization. Correlation of microstructural properties with magnetization were analysed using TG-DTA, XRD, FTIR and VSM. Crystallinity, average crystallite size and lattice constant of the as-annealed samples is increasing with annealing temperature. The presence of maghemite and cobalt oxide as secondary phases have caused reduction in the saturation magnetization. Interestingly, increasing cobalt concentration in the samples has dramatically shifted the temperature at which magnetite transformed to maghemite. We have found that x = 0.4 is the optimum cobalt ratio at which all reactants are converted into reaction product with no incomplete reactions that contribute to the formation of impurities. Preferential occupancy of Co²⁺ ions on the octahedral sites is evident in the FTIR spectra with intensified octahedral band splitting in the non-stoichiometric Co_xFe_1−xFe₂O₄ (0.2≤x≤0.8), whereby the structures formed are in metastable state. It is also evident in the FTIR spectra that distinct absorption bands appeared only when the cations in both sites are well ordered. By substituting Co²+ ions, saturation magnetization keeps on increasing until x=0.4, and gradually reduced beyond that. Highest saturation magnetization (78.86 emu/g) was obtained in Co_0.4Fe_0.6Fe₂O₄ after annealing at 800°C. In conclusion, heat treatment and substitution of ferrous/ferric ions with cobalt of lower magnetic moment has amplified the super-exchange interaction between octahedral and tetrahedral sites in magnetite spinel structure for greater saturation magnetization....