A possible cause of the peak effect in Y-Ba-Cu-O melt-grown bulks: modulation in oxygen distribution

A novel transmission electron microscopic technique, termed as large-area h igh-resolution transmission electron microscopy (LA-HRTEM), has been develo ped for nanoscopic observations of the lattice defects in an area as wide a s 20 X 20 mu m(2). Utilizing LA-HRTEM, a number of twin boundaries in the Y Ba2Cu3O7-d (Y-123) matrix are observed so that statistical analyses may be made on the inter-twin-boundary distance (d(tw)). After the annealing of a sample exhibiting no peaks in the critical current density (J(c)) vs. appli ed magnetic field (H) curve, the peak effect re-appeared as the value of d( tw) decreased. This fact implies that d(tw) is undoubtedly correlated with the degree of peak effect. Some HRTEM images have revealed that the oxygen- depleted regions in the vicinity of twin boundaries are wider for samples e xhibiting a pronounced peak effect than for those with less pronounced peak effect. The oxygen-depleted regions, whose width is related to the value o f d(tw), may be thought to have lower critical temperatures (T-c) than the rest. Then, the variation in T-c due to the oxygen-depleted region in the v icinity of somewhat regularly arranged twin boundaries is likely to work to wards some sort of flux pinning to cause the peak effect. (C) 2000 Elsevier Science B.V. All rights reserved.