Non-destructive measurement of volume magnetic properties of large, bulk superconductors

The development of large, single grain bulk superconductors by melt processing techniques has generated a need to characterize samples magnetically over large dimensions, exceeding typically 20 mm in diameter. The usual magnetic characterization gives relies on miniature Hall probe mapping and gives information about the field distribution above the sample surface. If volume properties are required (e.g. magnetization hysteresis loops), the sample needs to be cut in smaller pieces, since the DC magnetic characterization systems for measurements at cryogenic temperatures generally accommodate samples of relatively small size (typically < 5-10 mm diameter). In this work we describe how the hysteresis B(H) loops of large bulk superconducting samples exceeding 10 mm diameter can be determined using home-made sensing coils, either in liquid nitrogen or within the experimental chamber of a Quantum Design Physical Property Measurement System (PPMS). Magnetic properties are measured and compared to those given by several Hall probes attached to both faces of sample. The system is used successfully to measure the DC hysteresis loops of entire (RE)BCO bulk superconducting domains. A careful data acquisition and numerical integration of pick-up coil voltage enables the sweep rate of the magnetic field to be varied from 0.5 to 10 mT/s while keeping an excellent signal/noise ratio. A simple model based on demagnetizing field approach is used to emphasize how the hysteresis loops determined by this technique differ from “true” magnetization loops derived from classical magnetic moment measurements (e.g. SQUID or VSM). These differences are supported with numerical modelling of the average magnetization of the bulk sample using the Brandt method.