Magnetic and optical properties of spinel FexCo3−xO4 thin films

Magnetic and optical properties of FexCo3−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⩽0.930.76⩽x⩽0.93. The normal-spinel phase is dominant below x=0.76x=0.76 while the inverse-spinel phase above x=0.93x=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 FexCo3−xO4 films containing the inverse-spinel phase exhibit net magnetization that increases with increasing x  . Conversion electron Mössbauer spectrum measured on the x=0.93x=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.