Vortex creep characteristics via magnetic relaxation of Bi2Sr2CaCu2O8+delta crystals

The vortex activation energy U(B,J) proportional to B(v)J(-mu) similar to ( HM-mu)-M-v, where J and M are the current and the magnetization, respective ly, in Bi,Sr2CaCu2O8+delta is investigated via magnetic relaxation measured by a miniature Hall sensor. The exponent mu at the low-field solid phase i s definitively observed to be -1/7, which is consistent with the single-vor tex collective creep. With increasing H, plastic creep is observed just abo ve H,,, if the measurement is performed after a sufficiently long waiting t ime. On the other hand, with increasing T below H-on, the mu decreases abru ptly at 4 piM(dp) similar to 5 G near the depinning temperature T-dp. The k ink at M-dp is attributed to a change in the creep character from that cont rolled by the bulk pinning into that controlled by the surface pinning. (C) 2001 Elsevier Science B.V. All rights reserved.