Abstract
We study the planar antiferromagnetic Heisenberg model on a decorated hexagonal lattice, involving both classical spins (occupying the vertices) and quantum spins (occupying the middle of the links). This study is motivated by the description of a recently synthesized molecular magnetic compound. First, we trace out the spin-
degrees of freedom to obtain a fully classical model with an effective ferromagnetic interaction. Then, using high-temperature expansions and Monte Carlo simulations, we analyse its thermal and magnetic properties. We show that it provides a good quantitative description of the magnetic susceptibility of the molecular magnet in its paramagnetic phase.