SURFACE PINNING AND GRAIN-BOUNDARY FORMATION IN MAGNETIC FLUX-LINE LATTICES OF BI2SR2CACU2O8-T(C) SUPERCONDUCTORS(DELTA HIGH)

Magnetic decoration has been used to investigate the role of surface p inning on the flux-line lattice (FLL) of high-quality Bi2Sr2CaCu2O8+de lta single crystals. We find that surface steps pin flux lines at the sample surface, and that strong pinning along large steps creates grai n boundaries (GBs) in the FLL. Analysis of the competing energetics of pinning versus GB formation leads to a criterion for the formation of extended GBs, and also indicates that these GBs penetrate through the entire sample. Surface pinning, which should occur in all real sample s, will thus significantly affect the bulk FLL structure in these mate rials.