Abstract
We present an exact diagonalization study for perovskite manganites using an effective Hamiltonian which includes a coupling between the spin and orbital degrees of freedomderived from a strong on-site Coulomb interaction and orbital degeneracy of Mn ions. We investigate spin and orbital orderings as well as their excitations for the undoped insulating and the doped charge-ordered phases using three-dimensional 2 × 2 × 2 clusters to maintain proper symmetry of the systems. It is shown that the anisotropic magnetic structures, such as A-type antiferromagnetic ordering, are realized by self-adjustment of the orbital ordering where the orbital degeneracy is lifted by spins, indicating a strong interplay of spins and orbitals.
