Magneto-optic spectra and the dielectric tensor elements of bismuth-substituted iron garnets at photon energies between 2.2-5.2 eV

Optical and magneto-optical measurements have been performed at room temperature on both polycrystalline samples and thin single-crystal films of bismuth-substituted iron garnets. Kerr rotation, ellipticity, and reflectivity are given for polycrystalline samples of composition ${\mathrm{Y}}_{3-x}{\mathrm{Bi}}_x{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ ($0<x\le 1$) between 2.0 and 5.2 eV (wavelengths between 0.6 and 0.24 μm). The absorption and the Faraday rotation and ellipticity of epitaxial films with $0<x<\mathrm{}0.5\mathrm{}$ were measured up to 3.5 eV. The experimental results have been used to calculate the diagonal and off-diagonal elements of the dielectric tensor at optical frequencies. Strong magneto-optic active transitions have been found at 2.8, 3.3, 4.1, and 4.9 eV, apart from the weaker crystal-field transitions. The bands at 2.8 and 3.3 have been studied in more detail: The oscillator strengths and splittings due to spin-orbit coupling were calculated, both increase with bismuth substitution. The large splitting of the band at 3.3 eV for the bismuth-substituted compounds is concluded to be the origin of the anomalous Faraday rotation of these compounds. The assignment of these bands in terms of an energy level scheme is discussed.