LOWER CRITICAL-FIELD, REMANENT MAGNETIZATION AND IRREVERSIBILITY LINE OF CERAMIC (BI,PB)-2223-HTSC

A detailed magnetic study of the (Bi, Pb)-2223 superconductors was carried out over the temperature range 4-120 K. At temperatures below 25 K the lower critical field H(c1) was drastically enhanced. This enhancement can probably be explained either by a theory based on an intrinsic proximity effect in the normal layers of the Bi-2223 unit cell or by a blocking of the flux penetration because of a surface barrier effect. An analysis of the anomalies in the irreversibility line has been undertaken in the framework of the proximity effect allowing us to obtain also plausible values for xi-0, lambda-0 and the Fermi velocities in the superconducting and normal layers. The temperature dependence of the large remanent magnetization at 4 K with a drastic decrease above 25 K offered evidence for a strong depinning of the vortex lattice around 25 K. Below this temperature the active pinning centers have a rather small pinning energy. Above 25 K a smaller group of pinning centers with larger activation energy gives rise to a small and weakly temperature-dependent remanent magnetization. A scaling behavior of the magnetization curves was obeyed in the temperature range between 40 and 60 K in good agreement with the predictions of an extended Bean model.