Nuclear Magnetic Ordering of3He Clusters in Solid4He

The nuclear magnetic ordering of ${}^3$He clusters of a size of about 200 Å was investigated in the confining geometry of a matrix of solid hcp ${}^4$He by heat capacity measurements at $0.7\le T\le 30$ mK and in the pressure range $26.5\le p\le 36.4$ bars. Dependent on the applied pressure, solid and liquid phases in the clusters are in coexistence, and we observe the transition of the solid ${}^3$He phase into the magnetically ordered state at pressures far below the bulk ${}^3$He melting pressure. The ordering temperature is strongly dependent on the history of the sample. Moreover, we observe an additional linear contribution of unknown origin to the heat capacity.