MAGNETIC GRANULARITY, PERCOLATION AND PREFERENTIAL CURRENT FLOW IN A SILVER-SHEATHED BI1.8PB0.4SR2CA2CU3O8+X TAPE

Magneto-optical imaging of the flux penetration into a Ag sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape has been used to extract the current flow paths in a magnetic field applied perpendicular to the c-axis. Using the large aspect ratio of the plate-like grain structure and the slab geometry of the sample to simplify the current-flow geometry, we converted the magneto-optical signal into two-dimensional (2D) field and current distributions. We found that the current patterns were very non-uniform and sensitive to weak magnetic fields of similar to 400-800 Oe, even at 10 K. Current streamlines show that the effective current-carrying cross-section of the tape strongly depends on the field. Magnetization currents flow preferentially near the silver sheath, while the tape center supports mainly percolative and granular current patterns consisting of an array of macroscopic current loops whose long dimensions are of the order of the tape thickness. By comparing contour maps of the local J(c) values with the microstructural images, we found that the high-J(c) regions correlate with colonies of well-aligned long grains which are preferentially located near the silver interface, while the less aligned structure of smaller grains in the central part of the tape is associated with the granular behavior and much lower current-carrying capability. The wide distribution of the local J(c)(x, y) revealed by magneto-optical imaging indicates that the performance of BSCCO-2223 tapes can be significantly improved if a larger fraction of well aligned grains can be produced more uniformly throughout the tape cross-section.