Magnetic and optical properties of spinel FexCo3−xO4 thin films

Magnetic and optical properties of Fe_xCo_3−xO₄ thin films grown by sol–gel method have been investigated as the Fe composition (x  ) increases from 0 to 2. X-ray diffraction measurements revealed that the normal- and inverse-spinel phases coexist for 0.76?x?0.93$0.76?x?0.93$. The normal-spinel phase is dominant below x=0.76$x=0.76$ while the inverse-spinel phase above x=0.93$x=0.93$. The lattice constant of the inverse-spinel phase is found to be larger than that of the normal-spinel phase. For both phases the lattice constant increases with increasing x. The Fe_xCo_3−xO₄ films containing the inverse-spinel phase exhibit net magnetization that increases with increasing x  . Conversion electron Mössbauer spectrum measured on the x=0.93$x=0.93$ sample showed that Fe²⁺ ions prefer the octahedral sites, indicating the formation of the inverse-spinel phase. Analysis on the measured optical absorption spectra for the samples by spectroscopic ellipsometry indicates a dominance of the normal-spinel phase for low x in which Fe³⁺ ions mostly occupy the octahedral sites. Observation of a crystal-field transition at 1.6 eV originating from tetrahedral Fe³⁺ ion confirms the existence of the inverse-spinel phase for high x.