Spin Wave and Stability of the Double Exchange Ferromagnet

To investigate the spin wave and the stability of the ferromagnetic Kondo lattice, we describe the state with Sz = S − 1 and that with Sz = S − 1/2 in terms of the variational wave function. In this model, it is found that the spin-wave spectrum does not differ appreciably from the RPA result, since the bottom of the individual-particle excitation lies in a relatively high energy region. In this article, the double exchange ferromagnet is discussed with the Nagaoka ferromagnetic state of the U = ∞ Hubbard model, with respect to the stability of the ferromagnetic state.