  We study the low energy spectrum of the nearest neighbor Heisenberg model on
a square lattice as a function of the total spin S. By quantum Monte Carlo
simulation we compute this spectrum for the s=1/2, s=1 and s=3/2 Heisenberg
models. We conclude that the nonlinear sigma model prediction for the low
energy spectrum is always verified for large enough system size. However the
crossover to the correct scaling regime is particularly slow just for the s=1/2
Heisenberg model. The possibility to detect this unexpected anomaly with finite
temperature experiments on s=1/2 isotropic quantum antiferromagnets is also
discussed.
