HIGH-FIELD TRAPPING AND FLUX CREEP OF LARGE QMG (QUENCH AND MELT GROWTH) CRYSTAL

Magnetic flux density trapped in large QMG (Quench and Melt Growth) crystal was measured at 77K and 63K. QMG crystals which had no high-angle grain boundaries were enlarged by seeding. Distribution of magnetic flux density was measured on the surface of the QMG crystal using several Hall probes. Field cooling was performed to investigate trapping of the magnetic field. According to the critical state model, it is expected that trapped flux density increases with increasing sample size. A QMG crystal which size was 45mm in diameter 15mm thick trapped 0.72T at the center of the surface. Double piled QMG crystals trapped 1.35T at the center at 77K. The overall critical current density which was estimated by Bean type critical state model was about 7 x 10(3) A/cm2. Flux creep rate (normalized decay rate) was about 2.5 x 10(-2) at 77K. Moreover, a QMG crystal trapped 1.07T in triplet point of nitrogen (63K). Flux jump was not observed during these measurements. From these results, it is found that QMG crystal could stably trap high magnetic field without stabilization at 77K and 63K unlike conventional metal superconductors at 4.2K.