The effects of Sr and Ca on the magnetic irreversibility and fluctuation conductivity of YBCO-123

We have measured the magnetic irreversibility of the YBa2-xDxCu3O7-delta (D = Sr or Ca and x = 0.5 or 1.0) polycrystalline superconductors in fields u p to 7 kOe and of an oxygen depleted YBa(1.75)Sr(0.25)CU3O(6.85) polycrysta lline sample in fields up to 40 kOe as well as the fluctuation magnetocondu ctivity of YBa1.5D0.5Cu3O7-delta (D = Sr or Ca) up to 300 Oe. These partial substitutions of Ba as well as oxygen depletion cause reduction of T, prop ortional to the impurity concentration and an increase of the granular supe rconducting character, maintaining almost unchanged the overall aspect of t he magnetic irreversibility and fluctuation conductivity. Our results show that in a low field region the flux dynamics is dominated by the intergranu lar Josephson flux, that is strongly marked by the conflicting and frustrat ed grain coupling physics, the magnetic irreversibility totally resembling that of the spin glasses. But in fields above 10 kOe the flux dynamics is m ore conventional. (C) 2001 Elsevier Science B.V. All rights reserved.