Two-phase microstructures generating efficient pinning centers in the heavily Pb-substituted Bi2Sr2CaCu2O8+δ single crystals

Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals with a large amount of Ph substituted for Ri were grown by the floating zone method, and their magnetic properties and microstructures have been studied by means of SQUID magnetometory and electron microscopy. Critical current density (J(c)) increases dramatically beyond a critical Pb content of similar to 0.4 per formula unit, where characteristic two-phase microstructures are revealed by electron microscopy. The "single" crystals consist of alternating thin (several tens of nanometers) lamellar plates of two phases with the (010) interface, one with lower Pb content (similar to 0.4) and a modulated structure and the other with higher Pb content (similar to 0.6) and a modulation-free structure. The relative volume fraction of the latter phase increases systematically with increasing starting Pb content. At higher Pb content, lamellas with irregularly stepped interfaces and bubbles of the latter phase embedded in the matrix of the former phase are also observed, The growth mechanism is discussed in terms of unidirectional solidification of eutectic or eutectoid. (C) 1998 Academic Press.