THERMODYNAMICS OF A MIXED QUANTUM-CLASSICAL HEISENBERG-MODEL IN 2 DIMENSIONS

We study the planar antiferromagnetic Heisenberg model on a decorated hexagonal lattice, involving both classical spins (occupying the verti ces) and quantum spins (occupying the middle of the links). This study is motivated by the description of a recently synthesized molecular m agnetic compound. First, we trace out the spin-1/2 degrees of freedom to obtain a fully classical model with an effective ferromagnetic inte raction. Then, using high-temperature expansions and Monte Carlo simul ations, we analyse its thermal and magnetic properties. We show that i t provides a good quantitative description of the magnetic susceptibil ity of the molecular magnet in its paramagnetic phase.