Thermodynamic properties of antiferromagnetic clusters

The finite temperature magnetic properties of antiferromagnetic (AF) clusters are investigated in the framework of the Hubbard model at half-band filling. The ground- and excited-state electronic energies En, the corresponding wave functions |Ψn〉, and the total spin Sn are calculated exactly by using a full many-body diagonalization method. A complete optimization and sampling of the topological structures of the cluster is performed. Representative results are presented for the specific heat C(T), magnetic susceptibility χ(T), and spin correlation functions γij(T) between sites i and j of clusters having N=7 atoms. Low-temperature peaks in C(T) are observed and they are interpreted in terms of the spin-resolved excitation spectrum. χ(T) shows a typical AF-like behavior of the form χ≈1/(T+TN) from which the cluster “Néel'' temperature TN is derived. Finally, the effects of temperature-induced fluctuations of the cluster structure are discussed.