Experimental Observation of a Two-Dimensional Heisenberg Nuclear Ferromagnet

We have measured the magnetization of 2.5 atomic layers of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on graphite to 0.7 mK. The magnetization is well described by the exact high-temperature 2D Heisenberg expansion for $T>J=2.1\mathrm{}$ mK, the highest direct nuclear exchange constant ever reported. At lower temperatures, an exponential increase in susceptibility followed by a linear approach to saturation is seen, consistent with present theories for a 2D Heisenberg ferromagnet. The absence of a phase transition is confirmed for temperatures as low as $\frac{J}{3}$.