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, χ’ ’(T), in ceramic high-temperature superconductors represents the intergranular bulk pinning loss. The loss peak at ${\mathit{T}}_{\mathit{p}}$ shifts to lower temperature with increasing ac field and to higher temperature with increasing frequency. We have measured the shift of the χ’ ’ 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 ${\mathit{T}}_{\mathit{p}}$ on frequency and ac field. The model allows the determination of the intergranular pinning potential, where pinning occurs at grain-boundary intersection points.