THE DC MAGNETIZATION OF YBA2CU3O7-DELTA AG BULK COMPOSITES/

We report dc magnetization measurements on YBa2Cu3O7-delta/Ag composit e samples with different superconducting volume fractions p. All the s amples with 0.35 less than or equal to p less than or equal to 1 displ ay bulk behavior, with the Josephson coupling between the grains dimin ishing with increasing temperature. The p greater than or equal to 0.7 samples exclude flux from the entire volume and have higher values of the Josephson lower and upper critical fields H-c1J and H-c2J, respec tively, indicating stronger coupling between the grains, and greater d egree of pinning at the grain surface. The samples with 0.35 less than or equal to p less than or equal to 0.6 show partial flux exclusion a nd have significantly lower values of H-c1J and H-c2J, and lower degre e of pinning at the surface of the grains. The p<0.35 samples do not e xhibit bulk behavior at any temperature T<T-c indicating that there is no percolative pathway between the grains. The irreversibility lines H(T) display the same p dependent clustering. The H(T*) for the sampl es with p greater than or equal to 0.7 are at higher fields for a give n temperature, indicative of stronger pinning. The values of H-c1G at 10 K extracted from M(H) measurements lie between 200 and 400 Oe for a ll volume fractions, with no discernible p dependence. (C) 1996 Americ an Institute of Physics.