Critical Behavior of the Three-Dimensional Random Anisotropy Heisenberg Model

We have studied the critical properties of the three-dimensional random anisotropy Heisenberg model by means of numerical simulations using the Parallel Tempering method. We have simulated the model with two different disorder distributions, cubic and isotropic ones, with two different {anisotropy} strengths for each disorder class. For the case of the anisotropic disorder, we have found evidences of universality by finding critical exponents and universal dimensionless ratios independent of the strength of the disorder. In the case of isotropic disorder distribution the situation is very involved: we have found two phase transitions in the magnetization channel which are merging for larger lattices remaining a zero magnetization low temperature phase. Studying this region using a spin glass order parameter we have found evidences for a spin glass phase transition. We have estimated effective critical exponents for the spin glass phase transition for the different values of the strength of the isotropic disorder, discussing the cross-over regime.
Comment: 19 pages, 17 figures. We have extended the simulations of the isotropic disorder to very low temperatures finding a low temperature phase (for the lattice sizes simulated) with zero magnetization and with spin glass properties. Final version published in PRE