Covalency of Fe3+–O2− bonds and magnetic structure in mixed oxides

Optical absorption and the Faraday rotation spectra of iron(III) in ferrimagnetic oxides may be interpreted using 3d crystal field and the pair excitation processes. Band assignments and ligand–field parameters are in agreement with the results of SCF-Xα-SW molecular orbital calculations of (FeO4)5− and (FeO6)9− clusters. The pair excitation processes are strongly influenced by the nature and strength of the superexchange interactions. The presence of the ferrimagnetism in ferrites and garnets induces greater covalency effects of the Fe3+–O2− bonds. The nephelauxetic ratios β35 of iron(III) in hematite, ferrites, and garnets show that the greater covalency of the Fe3+–O2− chemical bonds in these oxides is related to the magnetic structure.