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

Through a systematic investigation of the complete dielectric tensor, between 0.5 and 5.0 eV, of Fe3O4 and of related spinel ferrites, i.e., MgFe2O4,Li0.5Fe2.5O4, NiFe2O4, and CoFe2O4, we have established that intervalence charge 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 ion (Mg2+,Li+). In the cases where Me is a magnetic ion (Fe2+,Ni2+,Co2+) additional intervalence charge transfer transitions are observed. CoFe2O4 is the only spinel ferrite with a major contribution of crystal field transitions to the magneto-optical spectrum. The observed presence of only two intense crystal field transitions in specifically CoFe2O4 is explained. The observed relative strengths of these two transitions in CoFe2O4, in which remarkably the upper transition at 1.82 eV is more intense than the lower transition at 0.83 eV is also explained in a crystal field analysis. (C) 1999 American Institute of Physics. [S0021-8979(99)42508-3].