The effect of chemical inhomogeneities on the pinning properties of Bi2212 single crystals

We have carried out a detailed characterization of chemical inhomogeneities in Bi2212 single crystals 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 the a-axis, and another with sheet-like structures that lying along the a-axis. The line defects relate to compositional inhomogeneities, such as Ca-enriched or Ca-deficient regions, with widths ranging from a few nanometres to micrometres and lie parallel to the crystal a-axis. From the observation of local flux line arrangement using a Bitter decoration technique and a magneto-optical imaging technique, the flux line arrangement at the 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 hexagonal order 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' flux penetration and trapping behaviour. This result suggests that the samples are composed of two kinds of regions with completely different pinning properties.