CRITICAL-BEHAVIOR OF THE 2-DIMENSIONAL EASY-PLANE FERROMAGNET

The critical behavior of 2D magnetic easy-plane systems has mainly bee n studied by the classical XY model. However, the z components of the spins have to be considered in order to describe real systems, and the ir fluctuations cannot positively be neglected when quantum effects ar e to be included, quantum spins being intrinsically three-component ob jects. Therefore, Monte Carlo simulations are performed for the Heisen berg ferromagnet with easy-plane anisotropy (XXZ model) on a two-dimen sional square lattice with a twofold aim: first, to obtain accurate qu antitative results about the critical behavior of the classical model, showing the relevant role played by the out-of-plane fluctuations; se cond, to open the way for approaching the quantum thermodynamics by me ans of the effective Hamiltonian method that reduces the quantum therm odynamics of the XXZ ferromagnet to the investigation of an effective classical model with temperature-dependent renormalized interaction pa rameters. Specific heat, magnetic susceptibility, and correlation leng th are calculated in the critical region for lattice sizes up to 128x1 28. These quantities preserve the Kosterlitz-Thouless behavior of the XY model. For the transition temperature of the classical XXO model we obtain the estimate k(B)T(c)/(J ($) over tilde S-2)=0.69+/-0.01.