Ferromagnetism in Electronic Models for Manganites

Ground state properties of the Kondo model for manganese oxides in one dimension are studied using numerical techniques. The large Hund coupling ($J_{H}$) limit is specially analyzed. A robust region of fully saturated ferromagnetism (FM) is identified at all densities. For open boundary conditions it is shown exactly that the ground state is FM at $J_{H} = \infty$. Hole-spin phase separation competing with FM was also observed when a large exchange $J$ between the $Mn^{3+}$ ions is used. As the spin of the transition metal ion grows, the hole mobility decreases providing a tentative explanation for the differences between Cu-oxides and Mn-oxides.