  We have studied the magnetic and thermodynamic properties as well as the NMR
spectra of the Heisenberg antiferromagnet RbFe(MoO4)2. The observed temperature
dependence of the order parameter, the critical indices and the overall
magnetic H-T phase diagram are all in a good agreement with the theoretical
predictions for a 2D XY model. The temperature dependence of the specific heat
at low temperature demonstrates a crossover from a T^2 law characteristic of a
two-dimensional antiferromagnet to a three-dimensional T^3 law.
