Tuning of the magnetic response in cobalt ferrite Co_xFe_3-xO₄ by varying the Fe²⁺ to Co²⁺ molar ratios: Rietveld refinement and DFT structural analysis

Magnetic cobalt ferrites Co_xFe_3-xO₄ (x = 3, 2.25, 1.5, 0.75, 0.0) were synthesized and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Different molar ratios of Fe²⁺ to Co²⁺ ions in the initial salt solutions were used to yield cobalt ferrites, and the results show that the ratio Co²⁺:Fe²⁺ (50:50) yields smaller size (<100 nm) of cobalt ferrite than those from other proportional mixtures. The XRD and Rietveld refinement data reveal that a characteristic peak, corresponding to the Fe₂O₃ (α-hematite) almost disappeared in Co_xFe_3-xO₄ (x = 0.75), suggesting that Co ions are notably favoring the spinel structure formation of the cobalt ferrite; this is consistent with the Density functional theory (DFT) study where a lower total energy was observed for Co₃O₄ than for Fe₃O₄, thus the latter favors a substitution reaction to yield a stable cobalt ferrite. Thus a reduction in the bandgap (HOMO-LUMO) as well as a higher density of electronic states is observed for Fe_1.5Co_1.5O₄ in the Molecular Oribital (MO) frontier region. This synergistic effect, i.e., the incorporation of Co ions in Fe sites of Fe₃O₄, considerably increases magnetization saturation (Ms) to 104 emu/g and reduces coercivity (0.70 kOe), for Co_1.5Fe_1.5O₄, exhibiting a trend towards to a superparamagnetic behavior. The influence of the molar ratios Fe²⁺/Co²⁺ and thermal treatments on the magnetization saturation and coercivity are also studied.