Development of High-Sensitive Magnetic Measurement System under High Pressures

研制了一套适用于金刚石压腔中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磁强计相结合的使用。PDD是VCM(振动线圈磁强计)的一种，适用于检测线圈周围空间很小的样品需要高灵敏度检测的情况。该系统能在1.4~30K的温度范围内，在约12 Gpa的压力下进行磁化强度测量，灵敏度约为10^~10~10。在液氦温度下使用三个双压电片作动器，使检测线圈在金刚石顶端附近以90微米的幅度稳定振动。VCM系统的特点之一是如何减小磁体对磁场的贡献。由于磁体中磁场的梯度强烈依赖于检测线圈的位置，因此发展了一种低温位置传感技术，以估计磁化场的贡献。位置的确定使用两个复合磁体和检测线圈作为位置传感器。高压下测量了EuTe的磁化强度，说明了该系统的性能。首次观察到了EuTe在7.9 Gpa及以上压力下的压致铁磁性。

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.