Antiferromagnetism and phase separation in electronic models for doped transition-metal oxides

We investigate the ground state properties of electronic models for doped manganites and nickelates. An effective t - J like Hamiltonian is derived from the case of strong Hund coupling between the conduction electrons and localized spins by means of the projection technique. An attractive interaction for conduction electrons and an anti-ferromagnetic coupling of the localized spin are obtained. A large ratio of the attraction to effective electron hopping, which is modulated by the spin background, will lead to the phase separation. The anti-ferromagnetic phase and the phase separation appear in the case of either high or low density of electrons. The possible relevance of the phase separation to the charge stripe phase in the manganites and nickelates is discussed.