MAGNETIC-FLUX PENETRATION IN LI DOPED AND UNDOPED BI2SR2CACU2OZ CAST TAPES BEFORE AND AFTER FAST-NEUTRON IRRADIATION

In a study of the effects of Li fission on magnetic-flux pinning in Bi 2Sr2CaCu2Oz (2212), a high-resolution, magneto-optical imaging techniq ue (MOT) has been employed to conduct the first direct observations of microscopic flux flow in Li doped and undoped 2212 polycrystalline ta pes before and after fast-neutron irradiation. The studies were perfor med at applied magnetic fields in the range of 0 to +/-65 mT and at te mperatures of 20, 30 and 40 K. The local magnetization behavior was co rrelated with bulk magnetization measurements obtained by superconduct ing quantum interference device (SQUID) magnetometry at the same appli ed fields and temperatures. The MOT studies showed that flux penetrati on in both the Li doped and undoped tapes was highly non-uniform. Meas urements by the two techniques demonstrated that the neutron-irradiati on treatment caused a significant increase in the magnetic-flux pinnin g in both tapes. SQUID measurements performed at applied fields up to +/-5 T showed that the irradiation treatment increased pinning by the same factor (2.2 at 20 K and 30 K) in the two tapes.