Frustration by multiple spin exchange in 2D solid3He films

Abstract

The heat capacity and the magnetization of the second solid layer of3He adsorbed on graphite have been measured to temperatures below 1mK. Prior to the formation of a fluid overlayer the temperature dependence of both sets of data is well described by high temperature series expansions for a 2D Heisenberg antiferromagnet on a triangular lattice. Differing exchange constants from the two quantities provide a measure of frustration due to multiple spin exchange. The subsequent evolution to ferromagnetic exchange suggests that this arises from variations in the coverage dependence of the frustration. After promotion it is found that an antiferromagnetic heat capacity coexists with a ferromagnetic susceptibility. At the ferromagnetic anomaly, the heat capacity is well described by series expansions for a 2D Heisenberg ferromagnet with effective exchange constant close to that determined from the magnetisation, indicating weak frustration.