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

我们改进了由金刚石压腔(DAC)和SQUID振动线圈磁强计组成的高压磁化测量系统的性能，以便将磁学和超导高压研究的压力范围提高到100 Gpa以上。研究结果如下。(1)通过对磁场信号进行补偿，使系统的灵敏度提高了10倍，该磁场信号以前阻止我们施加大于3Oe的磁场。(2)改进了DAC对鱿鱼检测线圈振动的致动器的隔热性能，使测量温度提高了两倍以上。(3)利用该系统研究了CrTe(T_c=330-350K)的压力感生磁相变和兆巴压强对钒超导相变的影响。Grazhdanina等人首先报道了CrTe在3.5 Gpa左右的磁相变，但他们的结果是有争议的，因为他们使用的样品不是单相，而且实验精度似乎不足以确定磁态。我们观察到，对于质量较好的样品，CrTe的铁磁性在6-7 GPa时消失。钒的超导转变温度T_c随压力增加而线性增加，最高可达50 Gpa。T_c的压力依赖关系已被测量到112 Gpa，以便研究T_c的这种单调增加能维持到什么程度。在112 GPa时，T_c是常压的三倍多，并且仍随着压力的增加而增大。这个T_c值是在单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.