  A numerical method is described for evaluating transverse spin correlations
in the random phase approximation. Quantum, spin-fluctuation corrections to
sublattice magnetization are evaluated for the half-filled Hubbard
antiferromagnet in two and three dimensions in the whole U/t range. Extension
to the case of defects in the Hubbard AF is discussed, and the vacancy-induced
enhancement in transverse spin fluctuation is obtained in the infinite-U limit
for the spin vacancy problem in two dimensions, for vacancy concentration up to
the percolation threshold. For low-U impurities, the overall spin fluctuation
correction is found to be strongly suppressed, although surprisingly spin
fluctuations are locally enhanced at the low-U sites.
