  We derive the low-temperature properties of spin-S quantum Heisenberg magnets
from the Gibbs free energy G(M) for fixed order parameter M. Assuming that the
low-lying elementary excitations of the system are renormalized spin waves, we
show that a straightforward 1/S expansion of G(M) yields qualitatively correct
results for the low-temperature thermodynamics, even in the absence of
long-range magnetic order. We explicitly calculate the two-loop correction to
the susceptibility of the ferromagnetic Heisenberg chain and show that it
quantitatively modifies the mean-field result.
