LOW-FIELD AC SUSCEPTIBILITY STUDIES OF GAMMA-IRRADIATED GRANULAR YBCOSUPERCONDUCTORS - MAGNETIC HISTORY EFFECTS

We report here the temperature dependence of the out-of-phase suscepti bility of Co-60 gamma-irradiated granular YBa2Cu3O7-delta samples unde r the zero field cooling (ZFC) and field cooling (FC) configurations. DC field of different magnitudes was superimposed on a fixed 1 Oe ac f ield of frequency 111.1 Hz. A modified form of the Anderson-Kim model has been invoked to compute the critical current density (J(c)) and th e pinning force density (f(p)). J(c) has been seen to be a non-linear function of the increasing radiation dosage. Both J(c) and f(p) are fo und to be higher for the FC case than for the ZFC case. Attempts have been made to account for the experimental results in terms of the form ation of point defects like vacancies, interstitials and their recombi nation during irradiation. (C) 1997 Elsevier Science B.V.