The effect of chemical inhomogeneities on the pinning properties of Bi2212 singlecrystals

Abstract

We have carried out a detailed characterization of chemical inhomogeneities in Bi2212 singlecrystals grown by a floating zone method. According to the chemical analyses using WDS,there are two types of inhomogeneities; one with linear structures extending toward thea-axis, and another with sheet-like structures that lying along thea-axis.The line defects relate to compositional inhomogeneities, such as Ca-enriched or Ca-deficientregions, with widths ranging from a few nanometres to micrometres and lie parallel to the crystala-axis. From the observation of local flux line arrangement using a Bitter decorationtechnique and a magneto-optical imaging technique, the flux line arrangement atthe defect region is largely affected by the type of nonstoichiometry involved.We found that the flux-line lattice at the Ca-enriched region has a hexagonalorder with small scattering, while that at the Ca-deficient region is amorphous.On the other hand, the crystal with sheet-like inhomogeneities showed a ‘zebra-like’ fluxpenetration and trapping behaviour. This result suggests that the samples arecomposed of two kinds of regions with completely different pinning properties.