Bismuth-induced increase of the magneto-optical effects in iron garnets: a theoretical analysis

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) R 3 Fe 5 O 12 upon doping with Bi 3+ or Pb 2+ ions. The covalent admixture of Bi 3+ , Pb 2+ 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 (FeO 6 ) 9− , (FeO 4 ) 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 Y 3−x Bi x Fe 5 O 12 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.