A simple torque magnetometer for magnetic moment measurement of large samples: Application to permanent magnets and bulk superconductors.

Abstract

The development of large size magnetic materials requires nondestructive measurement techniques to characterize their magnetic moment. In this work, we report the design and construction of a torque magnetometer able to accommodate sizable magnetic samples (>1 cm3) both at room temperature and cryogenic temperature. This device has an intermediate sensitivity between miniature torque magnetometers designed to work at cryogenic temperature and industrial torquemeters poorly adapted to extreme conditions. We show that torque sensing in the range 10-3-100 Nm can be achieved with piezoresistive metallic strain gages cemented on a cylindrical aluminum shaft with external temperature control. An absolute calibration of the device, carried out with a coil fed by a DC current, shows that magnetic moments down to 5 × 10-3 A m2 can be measured by this technique. The magnetometer is used to characterize a Nd-Fe-B permanent magnet and a permanently magnetized bulk, large grain superconductor at liquid nitrogen temperature (77 K). Results are in excellent agreement with data obtained with a flux extraction magnetometer for large samples. The device is able to measure magnetic moments in excess of 1.5 A m2, i.e., two orders of magnitude above the maximum magnetic moment of commercial magnetometers. The sample can be inserted in the air-gap of an electromagnet to measure the decrease in magnetic moment in the presence of a transverse applied field. The device was used to characterize the magnetic moment of "quasibulk" superconductors made of stacked coated conductor tapes (12 mm width) in such "crossed field" conditions.