Ferromagnetism and spin waves in double exchange materials

Abstract:

We study the influence of the on-site Hubbard repulsion U between the conduction electrons on the stability of the fully polarized ferromagnetic state in the ferromagnetic Kondo lattice model. Using single spin flip variational wave functions including local correlations in the vicinity of the flipped spin we investigate in detail how each of the two distinct mechanisms Hubbard repulsion and double exchange leads to ferromagnetism in this model, which is relevant for the Colossal Magnetoresistance (CMR) materials. In the ferromagnetic region of the phase diagram variational bounds on the spin wave energies are obtained. In particular the doping dependency of the deviations of the spin wave dispersion from a conventional Heisenberg form is analyzed. The astonishingly good agreement of our variational results in one dimension with results from numerical calculations on finite chains both for the stability region and the spin wave dispersion suggests that our results for the square lattice and the simple cubic lattice give an even better description of the actual properties of the model under consideration.