Order from disorder in the double-exchange model

Abstract

The magnetic excitations of the double-exchange (DE) model are usually discussed in terms of an equivalent ferromagneticHeisenberg model. However this equivalence is valid onlyat a quasi-classical level - we show that both quantum andthermal corrections to the magnetic properties of the DE modeldiffer from any effective Heisenberg model because itsspin excitations interact only indirectly, through the exchange of charge fluctuations. We also find that the competitionbetween ferromagnetic DE and an antiferromagneticsuperexchange provides a new example of an `order fromdisorder' phenomenon - an intermediate spin configuration (either a canted or a spiral state) is selected by quantumand/or thermal fluctuations.