In the framework of the concept of charge-transfer transitions, we suggest a
semi-quantitative model which explains the strong increase of the circular
magneto-optical effects in the iron garnets (IG) R3Fe5O12 upon doping with Bi3+
or Pb2+ ions. The covalent admixture of Bi3+, Pb2+ 6p orbitals (with giant
one-electron spin–orbit coupling constant) with oxygen 2p states causes growth of
the oxygen contribution to the spin–orbit coupling constant of (FeO6)9−, (FeO4)5−
complexes (the main magneto-optically active centres of IG). The enhanced
spin–orbit interaction of the oxygen is not the only effect of substitution; it also
gives rise to the effective anisotropic tensor addend to the spin–orbit interaction
and the circular magneto-optical effects in IG as well. A computer simulation of
the magneto-optical spectra of Y3−xBixFe5O12 for the case of an inhomogeneous
bismuth distribution is carried out. Estimates of various contributions to the IG
magneto-optical effects are given. The anisotropic tensor mechanism yields a
contribution to the Faraday effect which is comparable in magnitude with the
‘ordinary’ ferromagnetic contribution.Analysis of experimental data on the magneto-optical effects in
garnets supports our theoretical model.
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