SHIELDING CURRENT-DENSITY IN MG-DOPED YBCO-123 AS DETERMINED BY AC MAGNETIC-SUSCEPTIBILITY MEASUREMENTS

High-resolution a.c. magnetic-susceptibility measurements have been pe rformed on sintered samples of Mg-doped YBCO superconductors YBa2(Cu1- xMgx)(3)O-7-delta (delta less than or equal to 0.08, 0 less than or eq ual to x less than or equal to 0.1). An estimation of the intergranula r shielding currents J(s)(T, H-d.c.) has been obtained by separating t he grains and coupling;junctions contributions to the measured suscept ibility. The results show that Mg doping increases the weak links weig ht and reduces the values of J(s). At H-d.c. = 0, J(s) (T, 0) follows a power law of the type (1 - T/T-c)(beta), with beta varying from 2 (u ndoped sample) to about 5 (x = 2.5%). The dependence of J(s) on the ap plied d.c. magnetic field can be fitted to a Lorentzian lineshape whos e width appears to be independent of the substitution level.