On the relation of the `double-exchange' model to low-temperature magnetic properties of doped manganites

Exact calculations of energy eigenstates of the double-exchange (DE) model are carried out for rings with N sites and electrons for a variety of even N and odd values, the ground states being ferromagnetic. The shapes of the spin-wave spectra (energy versus wave vector) differ markedly from that of the Heisenberg model for general concentration and localized spin S, as expected from earlier work, in contrast to a recent theory. But there is a range of x, including 0.3, for which the dispersion shapes are close to Heisenberg for realistic S. This suggests that the Heisenberg spectrum observed recently in might still be explained by the DE model. Also, interesting asymptotic behaviour of the dispersion width for large S, large N, is clarified.