FERROMAGNETIC GROUND-STATE IN THE KONDO-LATTICE MODEL

We present a series of rigorous examples of the Kondo lattice model th at exhibit full ferromagnetism in the ground state. The models are def ined in one-, two- and three-dimensional lattices, and are characteriz ed by a range of hopping terms, specific electron filling, and large f erromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized sp ins could overcome the competition from mobility of a finite density o f electrons and drive the system from a paramagnetic phase to a ferrom agnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous res ults on ferromagnetism in the corresponding Hubbard model are presente d.