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

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 Tc. For (Bi1.64Pb0.36)Sr2Ca1-xYxCu2O8+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 Bi5+ rich clusters, which cause the reduction of the c-axis lattice parameter and ρc, is proposed to be responsible for the appearance of the PE in the undoped crystals. The co-existence of Pb4+- and Bi5+-rich clusters causes the strong PE in Pb doped Bi2212 crystals. Y3+ 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 (Hmax -Hmin)/Hmax vs T/Tc, where Hmin and Hmax represent the fields at which the magnetisation starts to increase (Hmin) and reaches a maximum (Hmax) 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.