Magnetic properties of YBaCuO and BiSrCaCuO crystals: a comparative study of flux creep and irreversibility

We report strong, anisotropic magnetic relaxation for the zero-field-cooled magnetization of a BiSrCaCuO crystal measured with field H = 1 kOe parallel and perpendicular to the c axis. The relaxation rate increases with temperature, peaks at ⋍15 K and drops to zero at ⋍30 K , well below the transition temperature T c ⋍ 84 K . We interpret the low temperature data with a thermally activated flux creep model and derive a pinning energy U 0 | ⋍ 8 × 10 −3 eV . These results show that for Bi-oxides the magnetization is already reversible at moderate temperatures. Similar conclusions are derived from a.c. susceptibility measurements which show an unusually strong frequency dependence of the irreversible temperature T irr for Bi-oxides. These results are compared with those obtained for YBaCuO crystals in which the pinning forces are an order of magnitude larger, the reversible regime is limited to higher temperatures and the frequency dependence of T irr is weaker.