  Various types of mixed spin two-dimensional Heisenberg networks are
investigated by means of Monte Carlo simulations. This study aims at
interpreting quantitatively the thermodynamical properties of two-dimensional
molecule-based magnets recently synthesized. The proposed model requires that:
(i) one of the two magnetic centers has a spin large enough to be treated as a
classical spin; (ii) the zero field Hamiltonian is isotropic; (iii) the quantum
spins have only classical spins as neighbours. The quantum Hamiltonian is then
replaced by a classical one with effective ferromagnetic interactions. The
temperature dependence of both the specific heat and magnetic susceptibility
are calculated. The effect of the lattice geometry is analysed.
