Free energy variational approach for the classical anisotropic XY model   in a crystal field
  A variational approach for the free energy is used to study the three-dimensional anisotropic XY model in the presence of a crystal field. The magnetization and the phase diagrams as a function of the parameters of the Hamiltonian are obtained. Some limiting results for isotropic XY and planar rotator models in two and three dimensions are analyzed and compared to previous results obtained from analytical approximations as well as from those obtained from more reliable approaches such as series expansion and Monte Carlo simulations. It is also shown that from this general variational approach some simple assumptions can drastically simplify the self-consistent implicit equations. The validity of the low temperature region of this approach is analyzed and compared to Monte Carlo results as well. 
L. M. Castro, A. S. T. Pires and J. A. Plascak