A consistent interpretation of the magneto-optical spectra of spinel type ferrites (invited)

Through a systematic investigation of the complete dielectric tensor, betwe en 0.5 and 5.0 eV, of Fe3O4 and of related spinel ferrites, i.e., MgFe2O4,L i0.5Fe2.5O4, NiFe2O4, and CoFe2O4, we have established that intervalence ch arge transfer and intersublattice charge transfer transitions dominate the optical and magneto-optical spectrum (between 0.5 and 5.0 eV) of all spinel ferrites of the general composition MexFe3-xO4. In all cases examined the same set of intersublattice charge transfer transitions was observed. These are the only transitions observed in the cases where Me is a nonmagnetic i on (Mg2+,Li+). In the cases where Me is a magnetic ion (Fe2+,Ni2+,Co2+) add itional intervalence charge transfer transitions are observed. CoFe2O4 is t he only spinel ferrite with a major contribution of crystal field transitio ns to the magneto-optical spectrum. The observed presence of only two inten se crystal field transitions in specifically CoFe2O4 is explained. The obse rved relative strengths of these two transitions in CoFe2O4, in which remar kably the upper transition at 1.82 eV is more intense than the lower transi tion at 0.83 eV is also explained in a crystal field analysis. (C) 1999 Ame rican Institute of Physics. [S0021-8979(99)42508-3].