Spectral origins of giant Faraday rotation and ellipticity in Bi-substituted magnetic garnets

Electrical permittivity theory has been applied to the interpretation of data describing the anomalously high Faraday rotation and ellipticity in the Y3−xBixFe5O12 ferrimagnetic system. By the use of exact forms of the basic phenomenological equations for the off-diagonal tensor element ε1 = ε′1 + iε″1, the measured Bi3+ influence on the separate component ε′1 and ε″1 spectra from 1 to 5 eV has been closely fitted to theory by the superposition of three Fe3+ (diamagnetic) electric dipole transitions occurring at 2.6, 3.15, and 3.9 eV, respectively. The strong transition at 3.15 eV that is believed to originate from Fe-O6 molecular complexes of the octahedral sublattice may also have a smaller companion peak in the vicinity of 5 eV. Transition bandwidths and excited-state splittings determined from the matching of theory to experiment both indicate that strong Bi covalent interactions exist with the Fe-O6 and Fe-O4 complexes. These results also confirm that the enhanced magneto-optical effects in the longer-wavelength region (λ∼1 μm) are dominated by the tail of the ε′1 diamagnetic peak at 3.15 eV.