  The magnetic excitations of the double exchange (DE) model are usually
discussed in terms of an equivalent ferromagnetic Heisenberg model. However
this equivalence is valid only at a quasi--classical level - we show that both
quantum and thermal corrections to the magnetic properties of DE model differ
from an effective Heisenberg model because its spin excitations interact only
indirectly, through the exchange of charge fluctuations. We also find that the
competition between ferromagnetic double exchange and an antiferromagnetic
superexchange provides a new example of an "order from disorder" phenomenon -
an intermediate spin configuration (either a canted or a spiral state) is
selected by quantum and/or thermal fluctuations.
