  We compute, by means of exact diagonalization of systems of N=16 and 18
spins, the correlation function <\sigma^z_0\sigma^z_n> at nonzero temperature
for the XXZ model with anisotropy \Delta. In the gapless ferromagnetic region
-1<\Delta<0 for fixed separation the temperature can always be made
sufficiently low so that the correlation is always negative for n \neq 0.
However we find that for sufficiently large temperatures and fixed separation
or for fixed temperature greater than some T0(\DElta) and sufficiently large
separations the correlations are always positive. This sign changing effect has
not been previously seen and we interpret it as a crossover from quantum to
classical behavior.
