SHORT-TIME MAGNETIC-RELAXATION IN BI2SR2CA2CU3O10 TAPES - A STUDY OF THERMALLY ACTIVATED FLUX MOTION AND VORTEX-GLASS TRANSITION

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 Bi2Sr 2Ca2Cu3O10 tape samples, We plot the magnetization as a function of lo garithmic time for a range of temperatures between 20 and 80 K. The ab solute 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 pen etration, The T(B) lines are shown to be clearly dependent on the tim e window, the sample size and the field step amplitude, They resemble in all essentials the irreversibility line usually measured by AC susc eptibility experiments. In addition, the relaxation measurements done in flux exit and flux entry mode show the presence of surface/geometri cal barriers, For full flux penetration, the relaxation curves can be transformed into V-I characteristics in good agreement with the ones m easured 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 th e studied range of temperature and field, we also observe a unique cha nge of curvature in M(ln t) at T-G(B). Below T-G(B) the curvature is f ound to be negative and above T-G(B) the curvature becomes positive. O ur results highlight the close relation between the relaxation of the flux generated by a small magnetic field step, AC susceptibility and V -I characteristics.