Experimental observation of a two dimensional Heisenberg nuclear ferromagnet.

Two dimensional Heisenberg ferromagnetic behavior is observed in sup 3 He layers adsorbed on graphite at very low temperatures. The magnetization is well described by the exact high temperature expansion for T>J=2.1 mK, the highest direct nuclear exchange constant ever reported. At lower temperatures, the exponential increase of the susceptibility followed by a linear approach to saturation can be described qualitatively by present theories. The absence of a phase transition in this two dimensional model system is confirmed for temperatures as low as J/3.