Ultrasonic Measurement for Nuclear Orderd Solid ^3He

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Bcc solid ^3He is known to have a unique magnetic phase diagram with two ordered phases, one is a low field antiferromagnetic one called UUDD and the other is a canted antiferromagnetic one (CNAF). The UUDD phase has a unique anisotropic spin structure and has been investigated by various methods including thermodynamic measurements, NMR so on so. Such an anisotropic ordered spin structure is supposed to affect a crystal structure, and ultrasonic is expected to be a powerful probe for that. Two experiments were planned, one is to search for a optical spin wave mode and the other is to investigate anisotropic properties of the UUDD by use of coupling between sound and an acoustic spin wave mode. The former requires a high frequency sound of the order of 200 MHz and seems to have some difficulty in acoustic propergation between the sample and the sound electrode. The latter can be done at the well-known frequency of around 1 MHz and its temperature dependence ia available to mix phonon with the acoustic spin wave mode. For this kind of experiment, a single crystal with a single magnetic domain is essential. The crystal should be grown between the sound transmitter and the receiver electrode, and be homogeneous over the gap. Preparation of such a large sample, the order of mm, was not so easy. Some trials have been made and improvement is still under way. Besides these, a first adiabatic demagnetization has successfully been done for a high density bcc solid ^3He. It is confirmed to have a reduced magnetic phase diagram similar to that of a low density one. A similar demagnetization for the hcp solid gives us the results which strongly suggest a ferromagnetic ground state.