Magnetic circular dichroism and Faraday rotation spectra of ${\mathrm{Y}}_3$${\mathrm{Fe}}_5$${\mathrm{O}}_{12}$

The magnetic circular dichroism (MCD), Faraday rotation (FR), and absorption spectra of ${\mathrm{Y}}_3$ ${\mathrm{Fe}}_5$ ${\mathrm{O}}_{12}$ at 77 K are presented between 10 000 and 25 000 ${\mathrm{cm}}^{-1\mathrm{}}$. Comparison of MCD and FR line shapes with absorption-band energies indicates that the ${\mathrm{Fe}}^{3+}$ crystal-field transitions are predominantly paramagnetic in nature. It is shown that spin-orbit mixing of the ${}_{}{}^4{}_{}{}^{}T_{1g}^{}{}_{}{}^{}\mathrm{}\left(G\right)\mathrm{}$ excited crystal-field states into the ground state is sufficient to explain the observed MCD of the ${}_{}{}^6{}_{}{}^{}A_{1g}^{}{}_{}{}^{}\mathrm{}\left(S\right)\mathrm{}\to {}_{}{}^4{}_{}{}^{}T_{1g}^{}{}_{}{}^{}\mathrm{}\left(G\right)\mathrm{}$ band without producing an unacceptable change in the ground-state $g$ factor and is consistent with the observation of paramagnetic line shapes for the other ${\mathrm{Fe}}^{3+}$ crystalfield transitions. It is pointed out that previous analyses of the magneto-optic spectra of iron garnets above 20 000 ${\mathrm{cm}}^{-1\mathrm{}}$ are probably in error.