Heat capacity study of the quantum antiferromagnetism of a He-3 monolayer

We have measured the heat capacity of the second layer solid in a He-3 film adsorbed on the surface of graphite to temperatures below 1 mK, in the regime where the nuclear exchange interaction is antiferromagnetic. The temperature dependence of the data is described by high temperature series expansions for a 2D Heisenberg antiferromagnet on a triangular lattice. There is no evidence for a phase transition down to J/3. Comparison with J inferred from the magnetization provides a measure of the frustration due to competing antiferromagnetic and ferromagnetic cyclic exchanges