Muon-spin-relaxation studies of flux pinning in Bi2Sr2CaCu2O8 and Pb0.7Bi1.3Sr2CaCu2O8

We have studied flux-depinning phenomena in the Bi2Sr2CaCu2O8 (Bi 2:2:1:2) and Pb-doped Pb0.7Bi1.3Sr2CaCu2O8 (Pb-Bi 2:2:1:2) systems using the transverse-field muon-spin-relaxation (μSR) technique. Comparison of field-cooled (FC) and zero-field-cooled (ZFC) results with external fields applied along the c axis of single-crystal specimens defines an irreversibility temperature (depinning temperature) Tirr: the FC and ZFC relaxation rates are essentially identical above Tirr, while the relaxation rate in the ZFC measurements is larger than that in the FC measurements below Tirr, reflecting the increased inhomogeneity of the local fields in the ZFC measurements due to flux pinning. The irreversibility line Tirr(H) in the H-T phase diagram for Bi 2:2:1:2, obtained by μSR measurements for several fields, is compared with previous results from ac-susceptibility and mechanical-oscillator measurements. Using a superconducting-quantum-interference-device (SQUID) magnetometer, the time-dependent diamagnetic magnetization has been measured in the same Bi 2:2:1:2 crystals. We show that the results from μSR and these other techniques can be explained consistently within a framework of the flux-creep model. The irreversibility temperature in Pb-Bi 2:2:1:2, determined by μSR measurements, is significantly higher than that in the pure Bi 2:2:1:2 system. This result, together with the larger critical current and the higher activation energy U0 in the Pb-Bi 2:2:1:2 crystals as found by the SQUID magnetization measurements, suggests an enhancement of flux pinning by the Pb doping. We also compare the μSR results in sintered ceramic, oriented film, and single-crystal specimens in the pure Bi 2:2:1:2 system, and discuss possible effects of sample morphology on μSR measurements.