Flux creep and penetration in Fe-dopedYBa2Cu3O7

Measurements of the magnetic-field distribution, flux penetration, and decay of trapped flux were performed for a disc-shaped sample of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_{2.95}$ ${\mathrm{Fe}}_{0.05}$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ at 77 K. Time-dependent magnetic-flux penetration, i.e., a decay of diamagnetic shielding and the Meissner field, was observed for zero-field-cooled (ZFC) and field-cooled (FC) samples. The decays were found to be logarithmic in time. The logarithmic decay rate of diamagnetic shielding depends on an applied magnetic field and reaches its maximum at an applied field of about 30 G. The dependence of a trapped field versus an applied field shows a maximum at an applied field of 100 G for the FC case and 200 G for the ZFC case. The maximum amount of the trapped field is about two times less than that for a pure, undoped ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ sample of similar dimensions. The logarithmic decay rate of the trapped field versus the initial trapped field is described by a linear function with the change of its slope at the trapped field corresponding to an applied field of about 30 G for both the FC and ZFC cases. These results are related to the sample microstructure, i.e., grain size and porosity. Trapped field decays can be explained in terms of the interaction between intergranular vortices and persistent current circulating around normal regions or voids, in the framework of the conventional flux-creep model recently proposed by Hagen, Griessen, and Salomons [Physica 157C, 199 (1989)].