Ferromagnetism in the one-dimensional Kondo lattice model

We investigate the incompletely saturated ferromagnetic phase which occurs at strong coupling in the partially filled one-dimensional (1D) Kondo lattice model. The double-exchange interaction responsible for the ferromagnetic ordering is absent in dilute Kondo systems, and is a missing element in nearly all theoretical treatments of the model. We discuss how double-exchange arises in the system, even though the Kondo coupling is antiferromagnetic, and show that at strong coupling it favors an alignment of the spins of unpaired localized moments. A ground-state phase diagram for the localized moments is given, with double-exchange interactions included, and is in complete qualitative agreement with numerical simulations.