Development of High-Pressure Precise Magnetization Measurement System and Studies of Magnetic Phase Transitions under High Pressure

为了将磁性和超导高压研究的压力范围提高到100 GPa以上，我们改进了由金刚石砧池（DAC）和SQUID振动线圈磁强计组成的高压磁化测量系统的性能。结果如下：(1)通过补偿磁场信号，使系统的灵敏度提高了10倍，因为磁场信号以前使我们不能施加大于3欧耳的磁场。(2)来自DAC的SQUID检测线圈的振动致动器的隔热性能得到了改进，导致测量温度增加了两倍以上。(3)利用该系统研究了CrTe （T_c=330 ~ 350k）中压力诱导的磁相变和压力对钒在兆巴超导相变的影响。Grazhdankina等人首先报道了3.5 GPa左右CrTe的磁相变，但他们的结果存在争议，因为他们使用的样品不是单相，实验精度似乎不足以建立磁性状态。我们观察到质量好的样品在6-7 GPa时CrTe的铁磁性消失。钒的超导转变温度T_c随压力升高而线性升高，直至50gpa。为了研究T_c的单调增加能维持到什么程度，我们在112 GPa以下测量了T_c的压力依赖性。在112 GPa时，T_c是环境压力下的3倍多，且仍随压力的增加而增加。T_c的这个值是在单个元素中观察到的最大值。

We have improved the performance of the high-pressure magnetization measurement system consisting of a diamond-anvil cell (DAC) and a SQUID vibrating coil magnetometer in order to increase the pressure range of a high-pressure research in magnetism and superconductivity to more than 100 GPa. Results are as follows. (1) The sensitivity of the system has been increased by a factor of 10 by compensating the magnetic-field signal which previously prevented us from applying larger magnetic field than 3 Oe. (2) Thermal insulation of the actuators for vibration of the SQUID detection coil from the DAC has been improved, resulting in an increase in the temperature of the measurements by a factor of more than two. (3) Using this system we have investigated a pressure-induced magnetic phase transition in CrTe (T_c=330-350 K) and pressure effect on superconducting transition of vanadium at megabar. A magnetic phase transition in CrTe at around 3.5 GPa was first reported by Grazhdankina et al., but their results are controversial because the sample they used was not single phase and the experimental accuracy seems not to be enough for establishing the magnetic state. We have observed a disappearance of the ferromagnetism of CrTe at 6-7 GPa for samples with good quality. The superconducting transition temperature T_c of vanadium increases linearly with pressure up to 50 GPa. Pressure dependence of T_c has been measured up to 112 GPa so as to investigate to what extent such a monotonic increase in T_c would hold. T_c at 112 GPa was found to be more than three times as large as that at ambient pressure and to still increase with pressure. This value of T_c is the largest one observed in single elements.