Jahn-Teller distortions and the magnetic order in the perovskite manganites

We introduce an effective model for eg electrons to describe three-dimensional perovskite (La1-xSrxMnO3 and La1-xCaxMnO3) manganites and study the magnetic and orbital order on a 4 x 4 x 4 cluster using correlated wavefunctions. The model includes the kinetic energy, and on-site Coulomb interactions for eg electrons, antiferromagnetic superexchange interaction between S = 3/2 core spins, and the coupling between eg electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: (i) an A-type antiferromagnetic phase in the undoped insulator LaMnO3, with alternating eg orbitals and with small Jahn-Teller distortions, changing to a conducting phase at 32 GPa pressure, and (ii) ferromagnetic order in one-eighth-doped La7/8Sr1/8MnO3 and in quarter-doped La3/4Sr1/4MnO3 compounds. For half-doped La1/2Ca1/2MnO3 one finds a competition between a ferromagnetic conductor and the CE insulating phase; the latter is stabilized by the Jahn-Teller coupling being two times larger than for the strontium-doped compound. Altogether, there is a subtle balance between all Hamiltonian parameters and the phase diagram is quite sensitive to the precise values they take.