Temperature dependence of the Faraday rotation of lead- and bismuth-substituted gadolinium iron garnet films at 633 nm

The Faraday rotation of epitaxial garnet films of composition ${\mathrm{Gd}}_{3-x}{\mathrm{Pb}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ with $x<~0.36$ and ${\mathrm{Gd}}_{3-x}{\mathrm{Bi}}_x{\mathrm{Fe}}_{5-y}{\mathrm{Ga}}_y{\mathrm{O}}_{12}$ with $0<~x$, $y<~1$ has been investigated in the temperature range $4.2 \mathrm{K}<~T<~T_C$ at $\lambda =633\mathrm{}$ nm. The experimental data can be described in terms of the corresponding sublattice magnetizations which were deduced from the fit of the molecular-field theory to the measured saturation magnetizations. The tetrahedral and octahedral magneto-optical coefficients turned out to be temperature independent. The negative tetrahedral coefficient is increased in magnitude, and the octahedral coefficient exhibited a strong reduction with increasing Pb and Bi content. The contribution to the Faraday rotation per formula unit $\frac{\Delta {\theta }_F}{x}$ for Pb and Bi at $T=295\mathrm{}$ K was found to be -7700 and -18 000 deg ${\mathrm{cm}}^{-1\mathrm{}}$, respectively. The measured wavelength dependence of the Faraday rotation and the Faraday ellipticity showed similar characteristics for both systems, indicating that the basic mechanisms causing the magneto-optical behavior via the increased superexchange are of the same type.