Nuclear Spin Order of High-Pressure Phase Solid ^3He in the Temperature Range of muK

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A large zero point motion of atoms in solid ^3He leads to the direct exchange interactions among atoms, which is responsible for nuclear magnetism. We studied the high-pressure samples of the solid ^3He in the low pressure phase as well as the high-pressure phase of solid ^3He. This study is very important for disclosing the entire profile of the nuclear magnetism in solid ^3He across the low-pressure and high-pressure phases.We measured the magnetic susceptibility of samples with Neel temperatures from 49 to 355muK as a function of temperature and frequency. We discoverd anomalous susceptibility across the nuclear ordering temperatures. We also observed that the relaxation time of the energy flow deduced from the susceptibility decreased drastically by two orders of magnitude across the transition. This fact was explained by the multi-magnon processes of the energy flow in the ordered state.For the high-pressure phase of solid ^3He, we showd by the measurements down to 34muK that it had a ferromagnetic tendency and predicted that its ordering temperature was 12muK,which temperature was formidable to reach. We adopted the mehtod of demagnenizing the nucler spins in solid ^3He to reach that low temperature by taking advantage of the long relaxation time due to the Kapitza thermal resistance. The pressure of solid ^3He decreased 60 Pa during and after demagnetization of solid ^3He from 2T down to 0.01T.The susceptiblity decreased 200 pH after demagnetization. Howeverwe observed no significant signature of the ordering transition. The further trials of observing the ordering transition is in progress. We note that the Neel temperature of 49muK in the low-pressure phase and the lowest temperature of the measurement for the high-pressure phase 34 muK are the record low temperatures for the solid ^3He.