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砧单元（DAC）和SQUID振动线圈磁强计组成的高压磁化测量系统的性能，以便将磁性和超导性的高压研究的压力范围增加到100 GPa以上。结果如下。(1)通过补偿磁场信号，系统的灵敏度增加了10倍，磁场信号以前阻止我们施加大于3 Oe的磁场。（2）SQUID检测线圈从DAC振动的致动器的热绝缘已经得到改善，导致测量温度增加了两倍以上。(3)利用这个系统，我们研究了CrTe（T_c=330-350 K）的压致磁相变和V在兆巴的超导转变的压力效应。Grazhdankina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的，实验的准确性似乎不足以建立磁状态。我们已经观察到CrTe的铁磁性在6-7 GPa的质量好的样品的消失。钒的超导转变温度T_c在50 GPa以下随压力线性增加。在112 GPa下测量了T_c的压力依赖性，以研究T_c单调增加的程度。在112 GPa下的T_c是常压下的3倍多，并且随压力的增加而增加。这是在单元素中观察到的最大T_c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.