ANTIFERROMAGNETIC ORDER AND FRUSTRATION IN SMALL CLUSTERS

Magnetic properties of small antiferromagnetic clusters are studied wi thin the framework of the nearest-neighbor Ising model. We analyze in detail several cluster geometries with different lattice structures an d examine the competing effects due to antiferromagnetic order, frustr ation, and external field. For some geometries in the presence of an e xternal magnetic field, magnetization is: found to increase with incre asing temperature in a considerable temperature range. We also conside r conditions under which antiferromagnetic clusters can display superp aramagnetic behavior. In general, magnetization is found to depend str ongly on the lattice structure of the cluster, but to be rather insens itive to local modifications of the coupling strength. The approach to wards the bulk properties for increasing cluster size and the possibil ity of deducing the lattice structure from the observed magnetic behav ior are discussed.