Short time magnetic relaxation in Bi 2Sr 2Ca 2Ca 3O 10 tapes: a study of thermally activated flux motion and vortex glass transition

Abstract

Short time (0.1–3600 s) magnetic relaxation M( t) after applying small field steps (0–10 mT) has been studied in field (0–0.2 T) cooled Bi 2Sr 2Ca 2Cu 3O 10 tape samples. We plot the magnetization as a function of logarithmic time for a range of temperatures between 20 and 80 K. The absolute creep rates ( dM/dln t) determined at fixed times display maxima at characteristic temperatures which we denote as T ∗(B) . These maxima are related to the crossover between incomplete and complete flux penetration. The T ∗(B) lines are shown to be clearly dependent on the time window, the sample size and the field step amplitude. They resemble in all essentials the irreversibility line usually measured by AC susceptibility experiments. In addition, the relaxation measurements done in flux exit and flux entry mode show the presence of surface/geometrical barriers. For full flux penetration, the relaxation curves can be transformed into V- I characteristics in good agreement with the ones measured by transport in the same sample. The vortex glass transition T G( B) line in both kinds of experiments is found to be the same. In the studied range of temperature and field, we also observe a unique change of curvature in M(ln t) at T G( B) the curvature is found to be negative and above T G( B) the curvature becomes positive. Our results highlight the close relation between the relaxation of the flux generated by a small magnetic field step, AC susceptibility and V- I characteristics.