Onset of ferromagnetic exchange in adsorbed 3He films.

Nuclear magnetic resonance experiments are reported on multilayer He films adsorbed on the surface of graphite. Initially the second layer exhibits antiferromagnetic exchange, which has been measured as a function of coverage. Measurements of the nuclear magnetic susceptibility and resonant frequency for coverages around the third layer promotion precisely identify the onset of ferromagnetic exchange in the film. The evolution of the structure of the film is discussed in the light of these results and previous heat-capacity and NMR data.