  Theoretical predictions of a semiclassical method - the pure-quantum
self-consistent harmonic approximation - for the correlation length and
staggered susceptibility of the Heisenberg antiferromagnet on the square
lattice (2DQHAF) agree very well with recent quantum Monte Carlo data for S=1,
as well as with experimental data for the S=5/2 compounds Rb2MnF4 and KFeF4.
The theory is parameter-free and can be used to estimate the exchange coupling:
for KFeF4 we find J=2.33 +- 0.33 meV, matching with previous determinations. On
this basis, the adequacy of the quantum nonlinear sigma model approach in
describing the 2DQHAF when S>=1 is discussed.
