The dc magnetization of YBa2Cu3O7-delta/Ag bulk composites

We report dc magnetization measurements on YBa2Cu3O7-delta/Ag composite samples with different superconducting volume fractions p. All the samples with 0.35 less than or equal to p less than or equal to 1 display bulk behavior, with the Josephson coupling between the grains diminishing 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, respectively, indicating stronger coupling between the grains, and greater degree 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 and have significantly lower values of H-c1J and H-c2J, and lower degree of pinning at the surface of the grains. The p<0.35 samples do not exhibit 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 samples with p greater than or equal to 0.7 are at higher fields for a given 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 all volume fractions, with no discernible p dependence. (C) 1996 American Institute of Physics.