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We consider the double exchange for systems in which doped electrons occupy degenerate orbitals, treating the realistic situation with double degenerate e₆ orbitals. We show that the orbital degeneracy leads in general to the formation of anisotropic magnetic structures and that, in particular, depending on the doping concentration, the layered magnetic structures of the A-type and the chainlike structures of the C-type are stabilized. The phase diagram that we obtain provides an explanation for the experimentally observed magnetic structures of some overdoped (electron-doped) manganites of the type Nd₁-ₗSrₗMnO₃, Pr₁-ₗSrₗMnO₃, and Sm₁-ₗCaₗMnO₃, with x>0. 5.
Brink et al. (Mon,) studied this question.