Magnetic phase diagram, critical behaviour and 2D-3D crossover in a   triangular lattice antiferromagnet RbFe(MoO4)2
  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. 
L.E.Svistov, A.I.Smirnov, L.A.Prozorova, O.A.Petrenko, A.Micheler, N.   Buttgen, A.Ya.Shapiro, L.N.Demianets