LOW-FIELD VORTEX DYNAMICS OVER 7 TIME DECADES IN A BI2SR2CACU2O8-CRYSTAL FOR TEMPERATURES THAN-OR-EQUAL-TO-T-LESS-THAN-OR-EQUAL-TO-83K(DELTA SINGLE)

Using a custom-made de superconducting quantum interference device (de -SQUID) magnetometer, we have measured the time relaxation of the rema nent magnetization M(rem) of a Bi2Sr2CaCu2O8+delta single crystal from the fully critical state for temperatures 13 K less than or equal to T less than or equal to 83 K. The measurements cover a time window of seven decades 10(-2) s less than or similar to t less than or similar to 10(5) s, so that the current density j can be studied from values v ery close to j(c) down to values considerably smaller than j(c). From the data we have obtained (i) the flux-creep activation barriers U as a function of current density j, (ii) the current-voltage characterist ics E(j) in a typical range of 10(-7)-10(-15) V/cm, and (iii) the crit ical current density j(c)(0) at T=0. Three different regimes of vortex dynamics are observed: For temperatures T less than or similar to 20 K the activation barrier U(j) is logarithmic, no unique functional dep endence U(j) could be found for the intermediate-temperature interval 20 K less than or similar to T less than or similar to 40 K, and final ly for T greater than or similar to 40 K the activation barrier U(j) f ollows a power-law behavior with an exponent mu similar or equal to 0. 6. From the analysis of the data within the weak collective pinning th eory for strongly layered superconductors, it is argued that for tempe ratures T less than or similar to 20 K pancake vortices are pinned ind ividually, while for temperatures T greater than or similar to 40 K pi nning involves large collectively pinned vortex bundles. A description of the vortex dynamics in the intermediate-temperature interval 20 K less than or similar to T less than or similar to 40 K is given on the basis of a qualitative low-field phase diagram of the vortex state in Bi2Sr2CaCu2O8+delta. Within this description a second peak in the mag netization loop should occur for temperatures between 20 and 40 K, as has been observed in several magnetization measurements in the literat ure.