Study of Nuclear Spin Ordering

在大量的纯金属中，只有铜中的核自旋被冷却到其自发有序温度以下。虽然人们已经付出了巨大的努力来实现核自旋有序，但到目前为止，还没有在其他金属中观察到明显的空位证据。我们感兴趣的是Sc金属中的核自旋有序，这种金属被称为高交换增强型Pauli顺磁金属。为了实现Sc金属中的核自旋有序，我们构造了两级退磁低温恒温器。在进行两级退磁冷却实验之前，我们测量了Sc金属在0.25mK以上的单级退磁冷却磁化率的温度依赖关系。这一结果可以用Sc金属中铁杂质的自旋玻璃现象来解释。然后我们进行了两个阶段的磁化实验。通过第一阶段的退磁冷却，我们将Sc金属冷却到0.25MK。在这个温度和7T的初始磁场下，Sc金属中的核自旋被退磁冷却到50nK以下。在Sc样品本身退磁冷却后，测量了交流磁化率的升温行为。观察到类反铁磁的磁化率极大值。核自转在有序状态下停留了大约45分钟。这是第二次观察到金属元素中的核自旋有序化。但我们还没有确定有序温度T_N，我们将做进一步的实验来明确Sc金属的核磁性。

Among a large number of pure metals, only in Cu the nuclear spins have been cooled down below their spontaneous ordering temperature. Though great efforts have been devoted to achieve the nuclear spin order, the clear evidence of oraering has not yet been observed in other metals so far. We are interested in the nuclear spin ordering in Sc metal which is known as the highly exchange enhanced Pauli paramagnetic metals. In order to achieve the nuclear spin ordering in Sc metal, we constructed two stage demagnetiziaton cryostat. Before doing the two stage demagnetized cooling experiment, we measured a temperature dependance of the magnetic susceptibility of Sc metal above 0.25 mk by the one stage demagnetized cooling. Result can be understood by the spin glass phenomenon of iron impurity in Sc metal. Then we performed the two stage semagnetized experiment. By a first stage demagntization cooling, we cooled down the Sc metal to 0.25 mk. From this temperature and 7 T initial magnetic field, nuclear spins in Sc metal was cooled down below 50 nk by the demagnetization. The warming up behaviour of the ac magnetic susceptibility has been measured following demagnetization cooling of Sc specimen itself. Amtiferromagnetic-like maximum of the magnetic susceptibility was observed. Nuclear spins stayed in the ordered state about 45 minutes. This is the second observation of the nuclear spin ordering in metal elements. But we have not yet deter mined the ordering temperature T_N. We are going to make further experiment to clear the properties of the nuclear magnetism of Sc metal.