Anisotropy effects in a mixed quantum-classical Heisenberg model in two dimensions

We analyse a specific two-dimensional mixed-spin Heisenberg model with exchange anisotropy, by means of high-temperature expansions and Monte Carlo simulations. The goat is to describe the magnetic properties of the compound (NBu4)(2)Mn[Cu(opba)]3 . 6DMSO . H2O which exhibits a ferromagnetic transition at T-c = 15 K. Extrapolating our analysis on the basis of renormalization group arguments, we find that this transition may result from a very-weak-anisotropy effect.