LOW-FIELD VORTEX DYNAMICS OVER 7 TIME DECADES IN A BI2212 SINGLE-CRYSTAL

Measurements of the relaxation of the remanent magnetization M(rem) in a Bi2212 single crystal have been performed in a time window of seven decades 10(-2)s less than or similar to t less than or similar to 10( 5)s and for temperatures 13 K less than or equal to T less than or equ al to 83 K. From the relaxation data the functional dependence of the activation barrier U on the current density j is obtained. Three diffe rent regimes of vortex dynamics are observed: The activation barrier U (j) is logarithmic for temperatures T less than or similar to 20 K, no unique functional dependence U(j) could be found for the temperature interval 20 K less than or similar to T less than or similar to 40 K, and finally for temperatures T greater than or similar to 40 K the bar rier U(j) follows a power-law with an exponent mu similar or equal to 0.6. A description of the vortex dynamics in the temperature interval 20 K less than or similar to T less than or similar to 40 K is given o n the basis of qualitative considerations about the low-field vortex s tate in Bi2212.