Absorption spectra of ${\mathrm{Y}}_3$${\mathrm{Fe}}_5$${\mathrm{O}}_{12}$ (YIG) and ${\mathrm{Y}}_3$${\mathrm{Ga}}_5$${\mathrm{O}}_{12}$: ${\mathrm{Fe}}^{3+}$

The absorption spectra of ${\mathrm{Y}}_3{\mathrm{Fe}}_x{\mathrm{Ga}}_{5-x}{\mathrm{O}}_{12}$ ($x=5, 3.9, 0.29, 0.09$) at 300, 77, and 4.2 K have been recorded and least-squares fits to the spectra have been employed using a series of Gaussians whose energies, widths, and heights were adjustable parameters. This allowed a separation of overlapping contributions to the spectra arising from (i) crystal-field transitions with typical oscillator strengths of ${10}^{-5\mathrm{}}$ and (ii) more broad intense bands occurring above 20 000 ${\mathrm{cm}}^{-1\mathrm{}}$ with typical oscillator strengths of ${10}^{-3\mathrm{}}$. The crystal-field band energies were fitted using the cubic crystal-field matrices of Tanabe and Sugano. The oscillator strengths of bands of types (i) and (ii) which were resolved in both dilute and concentrated samples were found to be dependent on ${\mathrm{Fe}}^{3+}$ concentration implying ${\mathrm{Fe}}^{3+}$ pair interaction. This is consistent with type-(i) transitions being exciton-magnon bands; type-(ii) transitions may be double-exciton bands. A series of antiresonancelike lines observed in YIG at 4.2 K appear to be magnon and phonon sidebands of ${}_{}{}^6{}_{}{}^{}A_{1g}^{}{}_{}{}^{}\left({}_{}{}^6{}_{}{}^{}S\right)\to {}_{}{}^4{}_{}{}^{}E_g^{}{}_{}{}^{}\left({}_{}{}^4{}_{}{}^{}G\right)$ and ${}_{}{}^6{}_{}{}^{}A_1^{}{}_{}{}^{}\left({}_{}{}^6{}_{}{}^{}S\right)\to {}_{}{}^4{}_{}{}^{}E\left({}_{}{}^4{}_{}{}^{}G\right)$ transitions.