© 2016 Elsevier B.V. Magnetic cobalt ferrites CoxFe3-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 CoxFe3-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 Fe1.5Co1.5O4 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 Co1.5Fe1.5O4, 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.
Estrada, S. O., Huerta-Aguilar, C. A., Pandiyan, T., Corea, M., Reyes-Domínguez, I. A., & Tavizon, G. (2017). Tuning of the magnetic response in cobalt ferrite Co<inf>x</inf>Fe<inf>3-x</inf>O<inf>4</inf> by varying the Fe<sup>2+</sup> to Co<sup>2+</sup> molar ratios: Rietveld refinement and DFT structural analysis. Journal of Alloys and Compounds, 2706-2716. https://doi.org/10.1016/j.jallcom.2016.11.187