Excess of cations in the sol-gel synthesis of cobalt ferrite (CoFe2O4): A pathway to switching the inversion degree of spinels

Spinel cobalt ferrites (CoFe 2 O 4 ) with varying inversion degrees were prepared via a citrate-nitrate sol-gel method. The gels, prepared with different cation stoichiometries, were dried at 110 °C, further treated at 850 °C and subjected to structural and magnetic characterization. X-ray diffractograms of the samples confirm the successful synthesis of the spinel. The synthesis conditions displayed a strong influence over the positioning of the cations in the crystal structure of these ferrimagnets. Mössbauer spectra yield inversion degrees of approximately 1.0 and 0.0 for the samples prepared with excess cobalt and excess iron, respectively. These cationic arrangements are also confirmed via Raman spectroscopy, which shows a larger splitting of the A 1g signal in the sample with the highest inversion degree. The magnetic character of the samples at room temperature does not agree with what should be expect from the net magnetic moments per unit cell. However, this phenomenon was explained by the different exchange integrals at play in each of the systems, as shown by the magnetic critical temperature of each arrangement, which differed by approximately 100 °C. The proposed pathway has proven to be an accurate method for completely switching the cationic arrangement of CoFe 2 O 4 , further enabling the application of this material in advanced magnetoresistive and spintronic devices.