  In this paper we present a Monte Carlo study of the critical behavior of the
easy axis anisotropic Heisenberg spin model in two dimensions. Based on the
partial knowledge of the zeros of the energy probability distribution we
determine with good precision the phase diagram of the model obtaining the
critical temperature and exponents for several values of the anisotropy. Our
results indicate that the model is in the Ising universality class for any
anisotropy.
