Separated suppression of the transverse and longitudinal Josephson flux mobility in a Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor - art. no. 174502

We report on very detailed zero-field-cooling and field-cooling magnetizati on and magnetoresistance measurements in a polycrystalline Bi1.6Pb0.4Sr2Ca2 Cu3Oy superconductor. The results allow us to study the irreversibility lin e as a function of the magnetic field and temperature. The resistive transi tion in low fields occurs visibly in two stages, characterizing a granular superconductor. The magnetic irreversibility line as a function of applied field. T-irr(H), reveals a flux dynamics with several regimes. In fields be low 0.3 kOe, the T-irr(H) data define an usual de Almeida-Thouless-like lin e. However, for fields above this value T-irr(H) splits up into two lines o f quite different slopes, evidencing a two-step reduction of the flux mobil ity with decreasing temperatures. We discuss this double onset of irreversi bilities in terms of the separated suppression of the transverse and longit udinal fluctuations of the Josephson flux lines. Lacking a specific theoret ical approach able to account for the two-step character of the T-irr(H) da ta, we emphasize its striking analogy with the magnetic irreversibility of CuMn spin glasses and tentatively attribute the upper-temperature line to s uppression of the transverse flux mobility along a Gabay-Toulouse-like line and the lower-temperature line to suppression of the longitudinal flux flu ctuations along a second de Almeida-Thouless-like line. We suggest that the superconducting glass model including effects from the grain charging ener gy might be useful to describe these results. We also study the reversible regime of the magnetization above T-irr(H) and up to the bulk critical temp erature. These results may be interpreted within the three-dimensional XY m odel for superconducting thermal fluctuations.