Magnetic granularity, percolation and preferential current flow in a silver-sheathed Bi1.8Pb0.4Sr2Ca2Cu3O8+x tape

Suppression of magnetic granularity by transport current in (Bi,Pb)2Sr2Ca2Cu3Ox tapes

“…10,11 It was shown that J"r… could be deduced from the maps of B"r… and that the J c was markedly higher near the Ag-BSCCO interfaces, where colonies of well-aligned longer grains are mostly located. This observation verifies the inferences of previous microstructural 12,13 and electromagnetic 7,14 studies.…”

“…4 Extrinsic factors, such as irregularities of the Ag-BSCCO interface, 5 the grain-to-grain alignment, distributions of low-T c intergrowths 6 and cracks 7 play a significant role too. This results in strong inhomogeneities in J c (r), giving rise to percolative current flow [7][8][9][10] and an effective current-carrying cross section smaller than the real cross section of the tape. Thus the identification of current paths in BSCCO/Ag tapes is of great importance to understanding the real potential of high-temperature superconductors.…”

“…Central parts of the tapes having smaller grains and more second phase exhibited a pronounced magnetic granularity characterized by percolative current patterns consisting of an array of current loops much larger than the crystallites themselves. 10 An important question is whether such pictures are equally valid in the presence of transport current ͑see also, Refs. 15 and 16͒.…”

“…10 for which J c (4.2 K, 0 T͒ϭ90 000 A/cm 2 , and J c (77 K, 0 T͒ϭ18 000 A/cm 2 , at E c ϭ1 V/cm. The sample was cut into a 10ϫ2 ϫ0.2 mm slab.…”

“…MO imaging of B z (r) from both magnetization and transport currents was performed by placing a 2 m thick Bi-doped Y 3 Fe 5 O 12 film directly onto the polished central part of the 10ϫ0.2 mm surface of the sample. 10,16 MO images were taken for the magnetic field H a applied along the broad face of the tape parallel to the ab planes and perpendicular to the indicator film ͑see Fig. 2͔͒ at 14, 30, and 77 K, first in the zero-field cooled regime at Iϭ0 and then after passing both dc and pulsed I(t).…”