Magnetic Ordering, Orbital State and Lattice Distortion in Perovskite Manganites

Possible ordered phases in perovskite manganites are investigated in the framework of an effective spin Hamiltonian for the atomic limit. Using the mean-field approximation combined with a group-theoretical consideration, we determine the stable orbital and spin ordered phases at finite temperatures. It is found that the magnetic ordering with low-symmetry spatial arrangement is stabilized due to the mixing interaction between different multipoles. The resulting orbital states are discussed by taking into account the effect of lattice distortion from the cubic symmetry. The spin and orbital states in LaMnO 3 are discussed on the basis of the present theory.