The pinning force density in polycrystalline Bi1.8Pb0.4Sr2Ca2−xYxCu3Oy multiphase systems

ac susceptibility measurements as a function of temperature T and the ac magnetic field amplitude, Hac, have been performed on five superconducting Bi1.8Pb0.4Sr2Ca2−xYxCu3Ox polycrystalline samples (x = 0, 0.005, 0.04, 0.15 and 0.36, and with the volume fraction of the Bi2Sr2Ca2Cu3Oy (2223) phase, f ≈ 100%, 100%, 64%, 0% and 0%, respectively. By using the Müller model for granular superconductors, we found that the pinning force density obtained from the imaginary peak of χ″(T) is strongly dependent on the doping level. To describe this dependence we adopted the multilayer model (a stack of superconducting, S, and normal, N, layers) of a superconductor. In the framework of this model, we demonstrated that (a) the yttrium addition influences the degree of coupling between the adjacent S layers (and thereby, the pinning strength) through the modification of the carrier concentration and of the effective thickness of the N layers, and (b) the intergrowth of Bi2Sr2CaCu2Oδ (2212) layers within the 2223 grains has an important influence on the thickness of N.