NUCLEAR MAGNETIC-PROPERTIES OF SOLID HE-3 FILMS

A brief review on the thermodynamic properties of He-3 films adsorbed on graphite is presented. The second solid layer of He-3 adsorbed on g raphite is a particularly interesting two-dimensional system of locali zed identical spin-1/2 fermions, which evolves from antiferromagnetism to ferromagnetism as a function of increasing density. This striking behavior is interpreted within a generalized Heisenberg model includin g competing ring-exchange interactions. While ferromagnetic three-part icle exchange dominates at the highest densities, higher order antifer romagnetic ring-exchange processes become significant at lour densitie s leading to highly frustrated antiferromagnetic or ferromagnetic phas es. At low temperature, the frustrated antiferromagnetic and ferromagn etic states may correspond to unique examples of new quantum phases as suggested by recent theoretical results. The relevance of this ring-e xchange model for other systems of localized fermions is also discusse d.