Phase diagram of manganese oxides

We study theoretically the phase diagram of perovskite manganites taking into account the double degeneracy of the e_g orbitals in a Mn³⁺ ion. A rich phase diagram is obtained in the mean-field theory at zero temperature as functions of x (hole concentration) and J_S (antiferromagnetic interaction between t_2g spins). The global features of the phase diagram are understood in terms of the superexchange and double-exchange interactions, which strongly depend on the types of occupied e_g orbitals. The strong electron correlation induces the orbital polarization, which controls the dimension of the conduction band. A sequential change of the spin and orbital structures with doping holes is consistent with the recent experiments. In particular, a metallic A-type (layered) antiferromagnetic state is found for x∼0.5 with the uniform d_x²-y² orbital ordering. In this phase, calculated results suggest two-dimensional conduction and absence of the spin canting, which are observed experimentally. The effects of the Jahn-Teller distortion are also studied.