EXPERIMENTAL-ANALYSIS OF THE MAGNETIC-FIELD SHIELDING AND TRAPPING PROPERTIES OF BULK MELT-PROCESSED YBA2CU3O7-DELTA

A scanning Hall probe has been used to map the distribution of magnetic field in melt-processed YBa2Cu3O7-delta (YBCO) discs prepared by seeded and controlled molten-zone techniques. Both shielded and trapped fields were studied as a function of applied magnetic field for each specimen. A clear four-fold symmetry has been observed in the field distribution of the seeded melt-processed sample, suggesting that there are two planes in the disc which exhibit a weak response to an applied field. Similar magnetic properties were observed for the molten-zone processed sample and attributed to the presence of cracks in the specimen. It was found that the applied field required to saturate each sample was much higher than the maximum observed trapped field, in contradiction to the Bean model for a slab geometry. This effect, which is probably due to the plate geometry of the specimens and the variation of critical current density with magnetic field, has implications for practical applications of bulk melt-processed YBCO.