Development of High-Sensitive Magnetic Measurement System under High Pressures

研制了一种金刚石砧槽极小样品的精密磁化测量系统。该系统基于位置微分检测（PDD）和SQUID磁力计的使用。PDD是一种VCM（振动线圈磁强计），适用于需要对样品进行高灵敏度检测的情况，而检测线圈周围的空间很小。目前的系统使我们能够在1.4到30K的温度范围内，在高达12GPa的压力下进行磁化测量。灵敏度约为10^<-9>至10^<-10> emu。在液氦温度下，使用三个双晶型压电致动器，使检测线圈在金刚石砧尖周围稳定振动，振幅为90mum。VCM系统的特点之一是如何减小磁体的磁场贡献。由于磁体中磁场的梯度与检测线圈的位置有很大的关系，为了估计磁化磁场的贡献，提出了一种低温下的位置传感技术。使用两个复合磁铁和检测线圈作为位置传感器来确定位置。通过对EuTe在高压下的磁化测量，说明了该系统的性能。在7.9GPa及以上的压力下，首次观察到EuTe的压力诱导铁磁性。

A precise magnetization measurement system for extremely small samples in the diamond anvil cell has been developed. The system is based on position differentiating detection (PDD) combined with the use of a SQUID magnetometer. The PDD is a kind of the VCM (Vibrating Coil Magnetometer) and is suitable for the case where high sensitivity detection is required for a sample around which there is little space for the detection coil. The present system enables us to make magnetization measurements at pressures up to about 12GPa in the temperature range from 1.4 to 30K.The sensitivity is about 10^<-9> to 10^<-10> emu. Three piezoelectric actuators of a bimorph type are used at liquid helium temperature for stable vibration of the detection coil with an amplitude of 90mum around the tips of the diamond anvils. One of the problems characteristic of the VCM system is how to reduce the contribution of the field from a magnet. A position sensing technique at low temperature has been developed so as to estimate the contribution of the magnetizing field because the gradient of the magnetic field in the magnet strongly depends on the position of the detection coil. Determination of position is made using two composite magnets and the detection coil as a position sensor. The performance of the system is illustrated by magnetization measurements of EuTe under high pressures. Pressure induced ferromagnetism of EuTe was observed for the first time at pressures of 7.9GPa and above.