Detection of Ferromagnetic Domains in a Two-Dimensional Nuclear-Spin System

We observe magnetic ordering of the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ spins at the surface of exfoliated graphite. In very low applied magnetic fields (≤3 G) we find new modes in the NMR resonance spectrum with the amplitude of these lines increasing sharply below 0.8 mK. We interpret our results in terms of a ferromagnetically aligned spin system, and conclude that there must be an anisotropy in the graphite planes.