Evolution of structural and magnetic properties and the electronic structure of spinel FexCo3-xO4 thin films

Evolution of structural, optical, and magnetic properties has been investigated for sol-gel-grown spinel Fe_xCo_3-xO₄ thin films as the Fe composition (x) increases from 0 to 2. The crystal structure of Fe_xCo_3-xO₄ is found to remain cubic with the lattice constant increasing with increasing x. Coexistence of two phases is observed by X-ray diffraction for 0.76≤x≤0.93, interpreted as due to normal spinel, dominant for x≤0.55, and inverse spinel, dominant for x≥1.22. Analysis on the measured optical absorption spectra by spectroscopic ellipsometry for the samples indicates the dominance of the normal spinel phase for low x in which Fe³⁺ ions mostly substitute octahedral Co³⁺ sites. X-ray photoelectron spectroscopy measurements revealed that both Fe²⁺ and Fe³⁺ ions exist with similar strength in the x=0.93 sample. Vibrating sample magnetometry measurements revealed that the Fe_xCo_3-xO₄ films start exhibiting magnetic hysteresis behaviors for x≥0.76 with the saturation magnetization increasing with increasing x. Conversion electron Mössbauer spectra measured on the x=0.93 sample showed that Fe²⁺ ions prefer the octahedral sites, indicating the formation of the inverse spinel phase. The remarkable change of the lattice constant for x≥0.76 is derivable from the site preference of the Fe²⁺ and Fe³⁺ ions.