Magnetic irreversibility limits of the Abrikosov and Josephson-flux dynamics in doped YBCO-123 superconductors

The granular superconducting character and the magnetic irreversibility of YBa1.75Sr0.25Cu3O7-delta single crystal and oxygen depleted polycrystalline samples are studied in great detail from low field magnetoresistance and z ero-field-cooled and field-cooled DC magnetization as a function of tempera ture, for applied fields from 3 Oe up to 50 kOe. The results are discussed together with those of a very pure and high quality YBCO-123 single crystal . Coarsely the profiles of the irreversibility curves are all similar, but the irreversibility data of the granular superconductors develop in the low field region a structure, that systematically falls off the general trend and is recognized as the signature of the intergrain Josephson-flux dynamic s, that is strongly marked by the frustrated grain coupling physics. (C) 20 01 Elsevier Science B.V. All rights reserved.