  The two-dimensional (2D) Heisenberg magnet with a weak easy-plane anisotropy
is considered. A renormalization group (RG) analysis in this model is performed
for both quantum and classical cases. A crossover from the Heisenberg to 2D XY
model is discussed. The magnetic transition owing to the interlayer coupling is
considered. Analytical results for the Kosterlitz-Thouless and Curie (Neel)
temperatures are derived with account of two-loop corrections. The results are
compared with experimental data, e.g. on K2CuF4, and turn out to provide a
quantitative description, unlike the standard one-loop results.
