The Critical Current Density of Yttrium BARIUM(2) COPPER(3) OXYGEN(7-DELTA) Single Crystals

The results of an extensive electromagnetic characterization of single and polycrystalline YBa_2Cu_3O _{7-delta} are presented. Anomalous magnetization peaks are observed in all flux-grown single crystals studied. Such peaks are observed in polycrystalline samples in which the size and aspect ratio of the grains approach those of flux-grown crystals. We propose that this effect is a consequence of inhomogeneities in the oxygen sublattice due to incomplete oxygenation. A strong enhancement of this effect at temperatures around 60 K is observed in high quality flux-grown crystals, suggesting the presence of oxygen deficient domains of lower T _{c} (OII and/or OIII) even though the T_{c} measured by AC susceptibility is 93 K and has a breadth of only 25 mK. Measurements of the magnetic field interval necessary to fully invert the critical state of the crystals do not indicate the presence of intragrain weak links. These results indicate that as larger, thicker and more perfect crystals are grown, the slow oxygen in-diffusion coupled with the vacancy ordering that occurs in the basal plane will lead to inhomogeneities in the microstructure which add extra flux pinning at high fields.