Two-dimensional antiferromagnetism of solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on plated graphite

We report measurements of the nuclear magnetic susceptibility of the monolayer two-dimensional solid formed by ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ adsorbed on graphite plated with two atomic layers of HD. A low-density solid is observed, consistent with a √7 × √7 registered structure, which exhibits large antiferromagnetic exchange. A distinct cusp in the susceptibility is observed near 1 mK, suggestive of a magnetic phase transition. The evolution of this and other features with surface density is discussed.