Systematic variation of flux pinning in Bi,Pb(2212) crystals with Pb doping

We present magnetization data on a series of Bi2.2-xPbx(2212) single crysta ls with x varying from 0.0 to 0.8, confirming for the first time and also e xtending earlier reports on drastic improvements of the flux pinning with l ead doping. The crystals were grown by slow cooling a melt in a temperature gradient. The spontaneous formation of a solid 'capping' layer prevented t he evaporation of ph, which was verified by monitoring weight losses during the whole heat treatment. X-ray and microprobe analysis confirm that ph is incorporated in the 2212 structure up to a solubility Limit of x=0.8. The magnetization loops measured at various temperatures show a systematic impr ovement of the flair pinning and the critical current density with doping l evel, especially st temperatures above T=35K. At T=70K, this increased pinn ing leads to irreversibility fields which are a factor > 50 higher in the x =0.8 crystals than in the undoped crystals, with the appearance of a peak e ffect all the way up to T-c, The marked difference between the pinning beha vior at lower and higher temperatures seems to correspond to two distinct p inning regimes.