MULTIPLE-SPIN EXCHANGE ON A TRIANGULAR LATTICE - A QUANTITATIVE INTERPRETATION OF THERMODYNAMIC PROPERTIES OF 2-DIMENSIONAL SOLID HE-3

We apply exact high-temperature series expansions of the multiple-spin -exchange Hamiltonian for a triangular lattice to describe high precis ion NMR measurements of the nuclear magnetic susceptibility of He-3 fi lms adsorbed on graphite, reported here, as well as all available spec ific heat data. A consistent quantitative description of the unusual t hermodynamic properties of the second layer solid, which provides cano nical examples of two-dimensional magnets, is obtained as a function o f temperature and areal density. We prove that cyclic multiple-spin ex change processes are responsible for the large degree of frustration f ound in the antiferromagnetic phase and that they remain significant i n the ferromagnetic phase.