  In this work we apply two different real-space renormalization-group (RSRG)
approaches to the anisotropic antiferromagnetic spin-1/2 Heisenberg model on
the square lattice. Our calculations allow for an approximate evaluation of the
$T$ vs. $\Delta$ phase-diagram: the results suggest the existence of a critical
value of $\Delta>0$, at which the critical temperature goes to zero, and the
presence of reentrant behavior on the critical line between the ordered and
disordered phases. This whole critical line is found to belong to the same
universality class as the Ising model. Our results are in accordance with
previous RSRG approaches but not with numerical simulations and spin-wave
calculations.
