Charge- and magnetic-ordering in a two-orbital double-exchange model for manganites

The phase diagram of half-doped perovskite manganites is studied within the extended double-exchange model. To demonstrate the role of orbital degrees of freedom both one- and two-orbital models are examined. A rich phase dia gram is obtained in the mean-filed theory at zero temperature as a function of J [antiferromagnetic (AFM) superexchange interaction] and V (intersite Coulomb repulsion). For the one-orbital model a charge-ordered (CO) state a ppears at any value of V with different types of magnetic order which chang es with increasing J from ferromagnetic (F) to AFM ones of the types A, C, and G. The orbital degeneracy results in appearance of a new CE-type spin o rder that is favorable due to opening of the "dimerization" gap at the Ferm i surface. In addition, the CO state appears only for V>V-c for F and CE st ates while the C-type AFM state disappears and A-type AFM state is observed only at small values of V as a charge disordered one. The relevance of our results to the experimental data are discussed.