Separated suppression of the transverse and longitudinal Josephson flux mobility in a Bi/sub 1.6/Pb/sub 0.4/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub y/ superconductor

We report on very detailed zero-field-cooling and field-cooling magnetization and magnetoresistance measurements in a polycrystalline Bi₁.₆Pb₀.₄Sr₂Ca₂Cu₃Oy superconductor. The results allow us to study the irreversibility line as a function of the magnetic field and temperature. The resistive transition in low fields occurs visibly in two stages, characterizing a granular superconductor. The magnetic irreversibility line as a function of applied field, Tirr(H), reveals a flux dynamics with several regimes. In fields below 0.3 kOe, the Tirr(H) data define an usual de Almeida–Thouless–like line. However, for fields above this value Tirr(H) splits up into two lines of quite different slopes, evidencing a two-step reduction of the flux mobility with decreasing temperatures. We discuss this double onset of irreversibilities in terms of the separated suppression of the transverse and longitudinal fluctuations of the Josephson flux lines. Lacking a specific theoretical approach able to account for the two-step character of the Tirr(H) data, we emphasize its striking analogy with the magnetic irreversibility of CuMn spin glasses and tentatively attribute the upper-temperature line to suppression of the transverse flux mobility along a Gabay-Toulouse-like line and the lower-temperature line to suppression of the longitudinal flux fluctuations along a second de Almeida–Thouless–like line. We suggest that the superconducting glass model including effects from the grain charging energy might be useful to describe these results. We also study the reversible regime of the magnetization above Tirr(H) and up to the bulk critical temperature. These results may be interpreted within the three-dimensional XY model for superconducting thermal fluctuations