Origin of the plateau in the temperature dependence of the normalized magnetization relaxation rate in disordered high-temperature superconductors

The temperature T dependence of the normalized magnetization relaxation rate S in optimally doped YBa₂Cu₃O_7-δ films with the external dc magnetic field H oriented along the c axis exhibits the well-known plateau in the intermediate T range, associated with the presence of elastic (collective) vortex creep. The disappearance of the $S(T)$ plateau in the high-H domain (H≥20 kOe) is not completely understood. We show that in the case of high-temperature superconductors with significant quenched disorder the $S(T)$ plateau is directly related to a crossover in the vortex-creep process generated by the macroscopic currents induced in the sample. In dc magnetization measurements the creep-crossover temperature decreases rapidly with increasing H, reaching the low-T region where the magnetization decay is dominated by micro flux jumps. Consequently, at high H no well-defined elastic-creep domain is present and the $S(T)$ plateau disappears.