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 high-resolution transmission electron microscopy (LA-HRTEM), has been developed for nanoscopic observations of the lattice defects in an area as wide as 20 X 20 mu m(2). Utilizing LA-HRTEM, a number of twin boundaries in the YBa2Cu3O7-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. applied 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 exhibiting a pronounced peak effect than for those with less pronounced peak effect. The oxygen-depleted regions, whose width is related to the value of 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 vicinity of somewhat regularly arranged twin boundaries is likely to work towards some sort of flux pinning to cause the peak effect. (C) 2000 Elsevier Science B.V. All rights reserved.