CRITICAL CURRENTS OF CERAMIC SUPERCONDUCTORS INDUCED BY PULSED MAGNETIC-FIELDS

We attempted to measure the critical current density J(c) of ceramic s uperconductors with pulsed magnetic fields. Using a pair of search coi ls coaxially mounted just below a ring sample, we measured induced sig nals that were considered to be proportional to the time derivative of the sample magnetization. Four samples were used: YBa2Cu3Oy, YBa2Cu4O y, Bi2Sr2CaCu2Oy, and Bi1.6Pb0.4Sr2Ca2Cu3Oy. These were all synthesize d by the polymerized complex method. It was found that the induced sig nals from the paired search coils are typically of two types. One show s a monotonic decrease to zero as a function of time. The other has a minimum at a certain time and then approaches zero. The observed signa ls were simulated in the framework of the Bean model and the Kim-Ander son model for critical current density. Theoretical profiles by these two models reproduce the observed results well and suggest that the tw o types of induced signals are the same in appropriate limits. We disc uss how to evaluate J(c) from the observed signals.