2-PHASE MICROSTRUCTURES GENERATING EFFICIENT PINNING CENTERS IN THE HEAVILY PB-SUBSTITUTED BI2SR2CACU2O8-CRYSTALS(DELTA SINGLE)

Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals with a large amount of P h 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 microstructur es are revealed by electron microscopy. The ''single'' crystals consis t of alternating thin (several tens of nanometers) lamellar plates of two phases with the (010) interface, one with lower Pb content (simila r to 0.4) and a modulated structure and the other with higher Pb conte nt (similar to 0.6) and a modulation-free structure. The relative volu me fraction of the latter phase increases systematically with increasi ng starting Pb content. At higher Pb content, lamellas with irregularl y stepped interfaces and bubbles of the latter phase embedded in the m atrix of the former phase are also observed, The growth mechanism is d iscussed in terms of unidirectional solidification of eutectic or eute ctoid. (C) 1998 Academic Press.