FLUX PINNING AT GRAIN-BOUNDARIES IN BI-(PB)-SR-CA-CU-O CERAMIC SUPERCONDUCTORS

For low magnetic-field amplitudes the imaginary part of the ac susceptibility, chi"(T), in ceramic high-temperature superconductors represents the intergranular bulk pinning loss. The loss peak at T(p) shifts to lower temperature with increasing ac field and to higher temperature with increasing frequency. We have measured the shift of the chi" peak in ceramic Bi-(Pb)-Sr-Ca-Cu-O samples. The Anderson flux-creep model in conjunction with the critical-state model for Josephson vortices is employed to explain quantitatively the dependence of T(p) on frequency and ac field. The model allows the determination of the intergranular pinning potential, where pinning occurs at grain-boundary intersection points.