Determination of flux-density gradients in YBa₂Cu₃O_7-δ superconductors using the high-resolution Faraday effect

Domain patterns of magnetic flux penetrated in YBa₂Cu₃O_7-δ superconductors have been observed by means of a high-resolution Faraday technique. The experimental conditions for observing domain patterns with the high-resolution Faraday effect are discussed. Flux-density profiles and the related flux-density gradients were determined from an analysis of the domain patterns. An analysis of the measured flux-density gradients in single-crystals shows that twin-boundaries are not the strongest acting pinning mechanism at low temperatures. Our results support the assumption that intrinsic pinning determines the flux-density gradients and governs the critical currents. It is shown that the flux-density gradients in grains of the sintered specimens are larger than in single-crystals. The resulting thirty times greater pinning forces are measured both magneto-optically and with a SQUID-magnetometer. This behaviour may be explained by superimposed extrinsic pinning effects such as grain-boundaries.