Low-Temperature Magnetization of SubmonolayerH3eAdsorbed on HD Preplated Graphite

NMR studies of submonolayer ${}^3\mathrm{He}$ adsorbed on a bilayer of HD preplated graphite have been made down to $100\mu \mathrm{K}$, which is more than 1 order of magnitude smaller than the exchange energy ( $J$). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at temperatures around $J$ and then increases gradually down to $100\mu \mathrm{K}$. Nevertheless, it does not show any anomalous behavior corresponding to a spin gap. The normalized magnetization versus the reduced temperature ( $T/J$) is independent of the density just after solidification. This is consistent with the result in the high-temperature region, that the main multiple-spin exchanges have a similar density dependence.