Influence of the lattice geometry on the thermodynamical properties of two-dimensional spin systems

Various types of mixed-spin two-dimensional Heisenberg network are investigated by means of Monte Carlo simulations. This study aims at interpreting quantitatively the thermodynamical properties of two-dimensional molecule-based magnets that have been recently synthesized. The proposed model requires that: (i) one of the two magnetic centres 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 the magnetic susceptibility are calculated. The effects of the lattice geometry are analysed. We obtain for the specific heat a typical curve which is independent of these effects.