Investigation of the valence states of Fe and Co in Fe1-xCoxOy (0<x<1) thin films by x-ray absorption spectroscopy

The unusual magnetic properties of thin films of Fe1-xCoxOy {0≤x≤1} ferrite solid solutions have been explained through an investigation of their electronic structure. The x-ray absorption near-edge structure (XANES) spectra of the Fe L3-edge and K-edges show that Co replaces Fe3+ in the tetrahedral (Td) sites up to x = 0.34 and octahedral (Oh) sites from 0.39<x<0.89. The Co L3-edge and O K-edge results indicate mixed valence states of Co as it replaces Fe at dissimilar sites. From the results, it is concluded that the magnetic properties of the thin films of the spinel Fe1-xCoxOy arise from the Fe3+ at the Td sites. The replacement of Fe3+ and Fe2+ by Co at the Oh sites results in a crossover to the rocksalt structure and a loss of the magnetic properties