Phase transitions in two-dimensional anisotropic quantum magnets

We consider quantum Heisenberg ferro- and antiferromagnets on the square la ttice with exchange anisotropy of easy-plane or easy-axis type. The thermod ynamics and the critical behaviour of the models are studied by the pure-qu antum self-consistent harmonic approximation, in order to evaluate the spin and anistropy dependence of the critical temperatures. Results for thermod ynamic quantities are reported and comparison with experimental and numeric al simulation data is made. The obtained results allow us to draw a general picture of the subject and, in particular, to estimate the value of the cr itical temperature for any model belonging to the considered class.