Abstract
An effective Hamiltonian, which includes a coupling between spins and orbitals, is derived for manganites by taking into account the Hund coupling, on-site Coulomb interaction and orbital degeneracy of thee g state. By using the effective Hamiltonian, magnetic structures, spin and orbital excitations are studied in a mean field approximation for the undoped insulating phase and in the exact diagonalization method for the undoped and 50% doped charge-ordered phases. It is shown that the coupling between spins and orbitals is crucial to the magnetic structures and that the so-called A-type AF is realized by self-adjusting the orbital ordering. Implication to the colossal magnetoresistance in manganites is given.
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