Flux creep and irreversibility line in superconducting Pb-Bi alloys

The critical current density, J/sub c/, in high-temperature superconductors becomes zero at irreversibility field, B/sub irr/, appreciably lower than the upper critical fields, B/sub c2/. The enhancement of J/sub c/ and B/sub irr/ is one of the crucial subjects to be attained. In high-temperature superconductors, conclusive result has not yet been presented because of the complexity of the material structure and in low-temperature superconductors scientific interest has not been paid because of the vanishingly small deviation of B/sub irr/ from B/sub c2/. In this study, we´ll apply a simple system of low-temperature superconductor Pb-Bi, whose pinning characteristics were fully examined and controllable, to understand the relation of B/sub irr/ and the pinning mechanism. Observed irreversibility field, B/sub irr/, is discussed using the pinning characteristics and numerical calculation based on the flux creep theory. Magnetic relaxation caused by the flux creep was also observed by SQUID magnetometer. The relation of the apparent pinning potentials, U*/sub 0/, estimated from the flux creep rate and the irreversibility field, B/sub irr/, are studied.