Study of the peak effect in pure, Pb and Pb+Y doped Bi-2212 single crystals

Abstract

The peak effect (PE) in pure, Pb and Pb+Y doped Bi2212 single crystals with different oxygen doping levels was studied by measuring M– H loops over a wide temperature range. The PE in pure Bi2212 crystals was obtained only for crystals with optimal oxygen doping and over-doping but not observed for oxygen under-doped crystals. For Pb doped Bi2212 crystals, the PE appeared at a higher field than in the pure crystals and persisted up to T c. For (Bi 1.64Pb 0.36)Sr 2Ca 1− x Y x Cu 2O 8+ y ( x=0, 0.05, 0.11, 0.33) single crystals, results show that at low temperatures, the peak field is smaller than in solely Pb doped crystals and decreases as x increases ( x>0.1). However, the peak field at high temperature for the x=0.05 sample is higher than for heavily Pb doped Bi2212 crystals, indicative of strong pinning due to the co-doping. The formation of Bi 5+ rich clusters, which cause the reduction of the c-axis lattice parameter and ρ c , is proposed to be responsible for the appearance of the PE in the undoped crystals. The co-existence of Pb 4+- and Bi 5+-rich clusters causes the strong PE in Pb doped Bi2212 crystals. Y 3+ is proposed to be an effective dopant at low doping levels for flux pinning at high temperatures. The PE for all the crystals was characterised by plotting ( H max− H min)/ H max vs T/ T c, where H min and H max represent the fields at which the magnetisation starts to increase ( H min) and reaches a maximum ( H max) at the peak position. Results showed that the evolution of the peak effect with temperature in the Pb and Pb+Y doped crystals was similar to that seen in Y123.