Molecular-orbital analysis of magneto-optical Bi-O-Fe hybrid excited states

Molecular-orbital estimates of the excited-state spin-orbit coupling multiplet splitting are presented to support the theory that enhanced magneto-optical effects in Y3−xBixFe5O12 originate from covalent interactions between Bi3+ and Fe3+ ions. A self-consistent approximation is applied to a two-level bonding-antibonding hybrid formed from the excited 4P term of Fe3+ and the excited 3P of Bi3+. For term energy values chosen from spectroscopic data, overlap integrals s<0.05 are sufficient to account for bonding-state depressions that match those reported for the tetrahedral and octahedral site-enhanced transitions at 2.6 and 3.15 eV. The hybrid eigenfunctions of the upper states can contain more than 30% of the Bi3+ 3P orbital term. Multiplet splittings that are enhanced by the large Bi3+ spin-orbit coupling constant (λ≊2 eV) are found to be on the order of 0.5 eV with a full complement of Bi surrounding Fe in the garnet lattice, in agreement with estimates based on experiment.