Ultrasonic Measurement for Nuclear Orderd Solid ^3He

体心立方固体^3He具有独特的磁相图，其中有两个有序相，一个是低场反铁磁相（UUDD），另一个是倾斜反铁磁相（CNAF）。UUDD相具有独特的各向异性自旋结构，并已通过各种方法包括热力学测量、NMR等进行了研究。这种各向异性的有序自旋结构被认为会影响晶体结构，超声波有望成为对此的有力探针。两个实验计划，一个是寻找一个光学自旋波模式，另一个是调查UUDD的各向异性特性，通过使用声音和声学自旋波模式之间的耦合。前者需要200 MHz量级的高频声，而且在样品与声电极之间的声学传播似乎有些困难。后者可以在大约1MHz的众所周知的频率下完成，并且其温度依赖性可用于将声子与声学自旋波模式混合。对于这种实验，具有单磁畴的单晶是必不可少的。晶体应该生长在声音发射器和接收器电极之间，并且在差距上是均匀的。制备如此大的样品，毫米的数量级，不是那么容易的。已经进行了一些试验，改进工作仍在进行中。除此之外，还成功地对高密度体心立方固体^3He进行了第一次绝热退磁。它被证实有一个简化的磁相图类似的低密度之一。对于六方晶系固体，类似的退磁结果强烈地暗示了铁磁基态。

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.