PHASE-DIAGRAM OF MANGANESE OXIDES

We study theoretically the phase diagram of perovskite manganites taki ng into account the double degeneracy of the e(g) orbitals in Mn3+ ion . A rich phase diagram is obtained in the mean-field theory at zero te mperature as functions of x (hole concentration) and J(S) (antiferroma gnetic interaction between t(2g) spins). The global features of the ph ase diagram are understood in terms of the superexchange and double-ex change interactions, which strongly depend on the types of occupied e( g) orbitals. The strong electron correlation induces the orbital polar ization, which controls the dimension of the conduction band. A sequen tial change of the spin and orbital structures with doping holes is co nsistent with the recent experiments. In particular, a metallic A-type (layered) antiferromagnetic state is found for x similar to 0.5 with the uniform d(x2-y2) orbital ordering. In this phase, calculated resul ts suggest two-dimensional conduction and absence of the spin canting, which are observed experimentally. The effects of the Jahn-Teller dis tortion are also studied. [S0163-1829(98)01242-9].