Low temperature and high pressure magnetization measurements on Ce and U compounds located near quantum critical point

在高压和低温下，UGe_2同时具有铁磁性和超导电性。在本研究之前，铁磁性被认为是正常的，如Fe、Co和Ni。我们目前工作中最重要的成果是，UGe_2的铁磁性即使在常压下也是非常不寻常的。我们还澄清了这一反常行为与超导的出现有关。我们已经发现的反常行为是在铁磁滞后曲线上出现磁化跳跃，其基本特征是：(1)跳跃发生在一个确定的磁场，即矫顽场Hc。(2)在1K以上，Hc随温度降低而增大，低于1K时，Hc趋于平缓；(3)Hc与扫描速度无关。这些结果有力地表明磁化跃迁归因于宏观量子隧道效应。我们研究了这种跃迁与超导电性的出现之间的关系。UGe_2有两个特征压强，即Pc~16kbar和Px~12kbar。铁磁性在Pc处消失，未知相变在Px处消失。超导转变的最佳温度在Px附近，这表明超导电性的重要作用是未知的相变。我们发现跳跃只出现在Px以下，这表明跳跃与未知的相变密切相关。这些结果表明跃迁对超导有影响。为了进行这些实验，我们开发了新的静态磁化测量方法。特别是，‘霍尔磁力计’使我们能够非常容易地测量温度低于1K和压力高达10Kbar的静态磁化强度。

UGe_2 shows ferromagnetism and superconductivity simultaneously under high pressure and at low temperatures. The ferromagnetic nature had believed to be normal such as Fe, Co and Ni before our present research. The most important fruit in our present work is that ferromagnetic nature of UGe_2 is extremely unusual even at ambient pressure. We have also clarified that the unusual behavior is related to the appearance of superconductivity.The unusual behavior, which we have discovered, is that magnetization jump appears in the ferromagnetic hysteresis curve only below about 1 K. The essential features of the magnetization jump are as follows : (1)the jump occurs at a definite field, i.e., coercive field Hc. (2)Hc increases with decreasing temperature above 1 K, while Hc flattens below about 1 K. (3)Hc is independent of field sweeping rate. These results strongly suggest that the magnetization jump is attributed to macroscopic quantum tunneling.We have examined the relation between the jump and the appearance of the superconductivity. There are two characteristic pressures in UGe_2,i.e., Pc〜l6 kbar and Px〜12 kbar. The ferromagnetism disappears at Pc and an unknown phase transition disappears at Px. The superconducting transition has optimum temperature at around Px, suggesting the important role for superconductivity is the unknown phase transition. We found that the jump only appears below Px, indicating the jump is closely related to the unknown phase transition. These results suggest the jump has an influence on superconductivity.In order to perform these experiments, we developed new static magnetization measurement method. In particular, 'Hall magnetometer' enables us to measure static magnetization very easily at temperatures below 1 K and in pressures up to 10 kbar.