Study of magnetic and optical transitions in MFe$_2$O$_4$ (M = Co, Zn, Fe, Mn) with spinel structure

Spinel ferrite (MFe$_2$O$_4$) nanoparticles were successfully synthesized by the coprecipitation method. X-ray diffraction technique was employed for structural analysis. Single-phase cubic spinel structure with an average crystallite size ranging from 5 – 20 nm was obtained for the prepared ferrites. The Fourier transform infrared spectra exhibits an absorption band at 550 cm$^{-1}$, which is attributed to metal-oxygen bond vibrations at tetrahedral sites. The thermogravimetric analysis revealed the instability of MFe$_2$O$_4$ and Fe$_3$O$_4$ above 500$^\circ$ C whereas CoFe$_2$O$_4$ is found to be the most stable ferrite. The hysteresis parameters demonstrate the superparamagnetic nature of the prepared nanoparticles with low coercivity except for CoFe$_2$O$_4$. The direct optical band gap energy derived from UV-visible spectra is calculated to be 2.82, 2.83, 2.81, and 2.44 eV for M=Co, Zn, Fe, and Mn respectively. The magnetic and optical properties show a strong dependence on cation site occupancy.