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

UGe_2在高压和低温下同时表现出铁磁性和超导性。在我们目前的研究之前，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，即使在常压下，UGe_2的铁磁性质也是非常不寻常的。我们还阐明了这种反常行为与超导电性的出现有关，我们所发现的反常行为是在铁磁磁滞曲线上只有在1K以下才出现磁化跳变。磁化跳变的基本特征是：（1）跳变发生在一定的磁场下，即，矫顽场Hc。(2)Hc当温度高于1K时，Hc随温度的降低而增加，而当温度低于1K时，Hc随温度的降低而增加。(3)Hc与场扫描速率无关。这些结果有力地说明磁化强度的跳跃是宏观量子隧穿的结果，并讨论了这种跳跃与超导电性的关系。UGe_2有两个特征压力，Pc = 16 kbar，Px = 12 kbar。铁磁性在Pc处消失，未知的相变在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，表明跳跃是密切相关的未知的相变。这些结果表明，突变对超导性有影响。为了进行这些实验，我们发展了新的静态磁化测量方法。特别是，“霍尔磁强计”使我们能够在低于1 K的温度和高达10 kbar的压力下非常容易地测量静态磁化。

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.