On the origin of the magneto-optical effects in Li, Mg, Ni, and Co ferrite

A reexamination of the magneto-optical (MO) spectra of ${\mathrm{Li}}_{\mathrm{0.5}}{\mathrm{Fe}}_{\mathrm{2.5}}{\mathrm{O}}_{\mathrm{4}},$ ${\mathrm{MgFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}},$ ${\mathrm{NiFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}},$ and ${\mathrm{CoFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ is presented. Thus far the MO spectra of these compounds have been explained by either orbital promotion processes, oxygen to iron charge transfer transitions or crystal field transitions and, in the case of ${\mathrm{CoFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}},$ a single intervalence charge transfer (IVCT) transition. For ${\mathrm{Li}}_{\mathrm{0.5}}{\mathrm{Fe}}_{\mathrm{2.5}}{\mathrm{O}}_{\mathrm{4}}$ and ${\mathrm{MgFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}},$ the major transitions in the MO–Kerr spectra between 0.5 and 5.0 eV are assigned to intersublattice charge transfer (ISCT) transitions. The MO–Kerr spectrum of ${\mathrm{NiFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ is more complex as in this case both ${\mathrm{Fe}}^{\mathrm{3+}}$ and ${\mathrm{Ni}}^{\mathrm{2+}}$ contribute to the spectrum. In ${\mathrm{NiFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ an additional IVCT transition is observed at 2.9 eV. The complex MO spectrum of ${\mathrm{CoFe}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}}$ could be resolved using the dielectric tensors of ${\mathrm{Co}}^{\mathrm{2+}}{\mathrm{Al}}_x^{\text{3+}}{\mathrm{Fe}}_{\text{2−x}}^{\text{3+}}{\mathrm{O}}_{\mathrm{4}}$ $\text{(x=0,}$ 0.1, 0.6, and 1) and rigorously fitting both the diagonal and the off-diagonal elements of the dielectric tensor with one set of transitions which was consistent for all degrees of ${\mathrm{Al}}^{\mathrm{3+}}$ substitution. Thus, in these ferrites a consistent assignment of all the major transitions in the 2.0 to 5.0 eV energy range to either IVCT or ISCT transitions is made.