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

We present magnetization data on a series of Bi/sub 2.2-x/Pb/sub x/(2212) single crystals with x varying from 0.0 to 0.8, confirming for the first time and also extending earlier reports on drastic improvements of the flux pinning with lead doping. The crystals were grown by slow cooling a melt in a temperature gradient. The spontaneous formation of a solid 'capping' layer prevented the evaporation of Pb, which was verified by monitoring weight losses during the whole heat treatment. X-ray and microprobe analysis confirm that Pb 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 improvement of the flux pinning and the critical current density with doping level, especially at temperatures above T=35 K. At T=70 K, this increased pinning 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 effect all the way up to T/sub c/. The marked difference between the pinning behavior at lower and higher temperatures seems to correspond to two distinct pinning regimes.