3He bilayer film adsorbed on graphite plated with a bilayer of 4He:: A new frustrated 2D magnetic system

The heat capacity and NMR response of a He-3 bilayer adsorbed on graphite plated with a bilayer of He-4 have been measured over the temperature range 1-80 mK. We find that the first He-3 layer requires the presence of a He-3 fluid overlayer before it solidifies. Solidification is completed at a total coverage close to 9.85 nm(-2). On further increasing the coverage the heat capacity maximum grows from 'antiferromagnetic-like' (AFM-like) to 'ferromagnetic-like' (FM-like). On the other hand, when the He-3 layer first solidifies, it has a low temperature saturation magnetisation corresponding to a significant fraction of full polarisation, and this increases with increasing coverage. Furthermore the effective exchange constant inferred from the high temperature magnetisation data is always ferromagnetic. The effective exchange constants inferred from the heat capacity and magnetisation are significantly larger than those observed in the second layer of pure He-3 films adsorbed on bare graphite. Otherwise there are strong similarities in the coverage dependence of the heat capacity and magnetisation, providing fresh insights into how the magnetic ground state of such 2D magnets evolves as the frustration is tuned with increasing coverage.