Low temperature magnetization of³He films

We have made NMR studies of thin films of pure³He adsorbed on Graf oil. Such films were cooled to ∼ 75 μK, an order of magnitude colder than previous workers could attain. In particular, we measured the magnetization of the solid second atomic layer, (M₂), which displays antiferromagnetic (AFM) behavior at low densities and ferromagnetic (FM) behavior at high densities. At both AFM and FM densities, we could measure M₂ at temperatures well below the exchange energy (J). Our data at the AFM densities may support a proposed model of that phase. At the FM densities, we have found that M₂ (T = 0) is linear in second layer density (p₂) and rises by a factor of ∼7 with a 6% increase in ρ{variant}₂ and less than a factor of 2 increase in the apparent value of J, suggesting that the FM phase coexists with a lower density AFM phase. Our high temperature (T >J) data also support such a two phase model. We find that M₂ follows the expected behavior for a 2 dimensional Heisenberg ferromagnet down to T ≪ J, the first experimental observation of such behavior. We also confirmed the importance of k = 0 spin waves in determining the low temperature behavior of such a system. The NMR lineshape in the FM phase also displayed shifts and broadening due to demagnetization effects which support the model of two phase coexistence.