A MICROSTRUCTURAL INTERPRETATION OF MAGNETIC-RELAXATION IN HIGH-TEMPERATURE SUPERCONDUCTORS - APPLICATION TO YBA2CU3O7-DELTA TWINNED AND DETWINNED SINGLE-CRYSTALS

Magnetic relaxation experiments with YBa2Cu3O7-delta (YBCO) twinned an d detwinned single crystals are interpreted in such a way that a clear correlation between the kinetics of relaxation and microstructure can be proposed. It is shown that theoretical fits to the non-logarithmic kinetics observed at higher temperatures can be used to evaluate the density of isolated strong-pinning regions in these crystals as well a s the rate of escape of vortices from pinning centres. The pinning cen tre density obtained is consistent with the expected density of ordere d oxygen vacancy domains in YBCO, which may act as strong pinning cent res. The time-logarithmic kinetics observed at lower temperatures can be associated with the existence of extended pinning agglomerates (alo ng twin boundaries, for example). The model adopted in the analysis is general and can be applied to other superconductors.