Flux penetration in granular YBa₂Cu₃O_7-δ samples. A magneto-optical study

Using the high-resolution Faraday effect (HRF) technique, flux distributions of sintered, granular YBa₂Cu₃O_7-δ samples are obtained. The flux penetration is found to occur stepwise and only the last step of the penetration of Abrikosov vortices into individual grains can be observed directly. The high spatial resolution of the HRF technique enables one to determine the flux-density profiles inside individual grains. From these profiles, the intragranular critical current densities and acting local pinning forces are obtained. The intragranular critical current densities and the corresponding volume pinning forces are found to be reciprocally dependent on the grain size. This behavior could be explained by pinning at the grain boundaries between superconducting YBa₂Cu₃O_7-δ grains and non-superconducting phases like BaCuO₂ assuming a disturbed layer Δx with a thickness of approximately 0.1 μm. Furthermore, the influence of melt processing on the weak-link behavior can be directly observed in the flux distributions. Melt-processed samples are found to show an intermediate behavior of flux penetration between sintered, polycrystalline samples and single-crystalline materials like single crystals and epitaxial thin films. It is shown that the parameter γ=H^g_cl/H^J_cl, linking the lower critical field of the grains, H^g_cl, to the lower critical field of the matrix, H^J_cl, has a significant influence on the flux-penetration behavior in type-II superconductors.