Ferromagnetic ground state of an orbital degenerate electronic model for transition-metal oxides: Exact solution and physical mechanism

We present an exact ground-state solution of a one-dimensional electronic model for transition-metal oxides in the strong-coupling limit. The model contains doubly degenerated orbit for itinerant electrons and the Hund coupling between the itinerant electrons and localized spins. The ground state is proven to be a full ferromagnet for any density of electrons. Our model provides a rigorous example for metallic ferromagnetism in narrow-band systems. The physical mechanism for ferromagnetism and its relevance to high-dimensional systems, like R1-xXxMnO3, are discussed. Due to the orbital degeneracy of itinerant electrons, the superexchange coupling can be ferromagnetic rather than antiferromagnetic in the half-filled one-band case. [S0163-1829(99)11001-4].