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 ${\mathrm{Fe}}_3{\mathrm{O}}_4$ and of related spinel ferrites, i.e., ${\mathrm{MgFe}}_2{\mathrm{O}}_4,$ ${\mathrm{Li}}_{\text{0.5}}{\mathrm{Fe}}_{\text{2.5}}{\mathrm{O}}_4,$ ${\mathrm{NiFe}}_2{\mathrm{O}}_4,$ and ${\mathrm{CoFe}}_2{\mathrm{O}}_4,$ 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 ${\mathrm{Me}}_x{\mathrm{Fe}}_{\text{3−x}}{\mathrm{O}}_4.$ 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 (${\mathrm{Mg}}^{\text{2+}},$ ${\mathrm{Li}}^{+}\mathrm{).}$ In the cases where Me is a magnetic ion (${\mathrm{Fe}}^{\text{2+}},$ ${\mathrm{Ni}}^{\text{2+}},$ ${\mathrm{Co}}^{\text{2+}})$ additional intervalence charge transfer transitions are observed. ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ 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 ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ is explained. The observed relative strengths of these two transitions in ${\mathrm{CoFe}}_2{\mathrm{O}}_4,$ 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.