Anisotropic double exchange in orbitally degenerate mixed valence systems

The problem of the double exchange is considered for the mixed valence dimers in which one or both transition metal ions possess orbitally degenerate ground states. In the pseudo-angular momentum representation, the general formula is deduced for the matrix elements of double exchange involving the transfer integrals and all spin and orbital quantum numbers. The pairs T-3(1) (t(2)(2))-T-2(2)(t(2)(1)) and T-3(1) (t(2)(2))-(4)A(2) (t(2)(3)) are considered in three high-symmetric topologies: edge-shared D-2h, corner-shared D-4h, and face-shared D-3h bioctahedra. The double exchange in orbitally degenerate systems is shown to produce strong magnetic anisotropy of an orbital nature. The character of the anisotropy proved to be dependent on the overall symmetry of the system and on the ground terms of the metal ions. (C) 2000 Elsevier Science B.V. All rights reserved.