Magnetization of He3 on grafoil in the low-temperature limit

We have measured the magnetization of the ferromagnetic second layer of pure He3 films adsorbed on Grafoil down to 0.075 mK. The zero-temperature magnetization is linear in second layer density (ρ2) and rises by a factor of ∼7 with a 6% increase in ρ2 and less than a factor of 2 increase in the apparent exchange energy, indicating that the ferromagnetic phase coexists with a lower density phase. This is the first experiment to measure the low-temperature limiting magnetization of a 2D Heisenberg ferromagnet. Our data may also support Elser's model of a registered antiferromagnetic phase at lower ρ2.