Design and construction of a magnetic torque measurement system at cryogenic temperature

Abstract

Development of new techniques for the non-destructive characterization of superconducting materials becomes challenging. Several magnetometry techniques exist but do not accommodate sizable samples. Other magnetometers are designed for the characterization of large samples in a non-destructive way but require specific working conditions. In this master thesis we designed and realized a new magnetic torque measurement system able to characterize magnetic samples of approximately 1 cm$^3$ at both room and cryogenic temperatures. This device can be used for torque magnetometry with an intermediate sensitivity between miniature torque magnetometers designed to work at cryogenic temperature and commercial torquemeters poorly adapted to extreme conditions. The torque sensing system is based on four piezoresistive strain gauges glued on a cylindrical transmission shaft. The measurement system was first calibrated at room temperature using a small coil fed with a DC current. Then the system was tested on three different magnetic samples in order to measure their magnetic moment under crossed fields conditions: a neodymium permanent magnet, a bulk superconducting sample and a stack of superconducting tapes. The results were shown to be in a good agreement with those observed from literature. In particular, the demagnetization effect was highlighted under these crossed fields conditions. Such conditions can be encountered for instance in rotating machines applications.