Vortex creep characteristics via magnetic relaxation of Bi < inf > 2 < /inf > Sr < inf > 2 < /inf > CaCu < inf > 2 < /inf > O < inf > 8+δ < /inf > crystals

Abstract

The vortex activation energy U(B,J)∝BvJ-μ ∼ HvM-μ, where J and M are the current and the magnetization, respectively, in Bi2Sr2CaCu2O8+δ is investigated via magnetic relaxation measured by a miniature Hall sensor. The exponent μ at the low-field solid phase is definitively observed to be ∼ 1/7, which is consistent with the single-vortex collective creep. With increasing H, plastic creep is observed just above Hon if the measurement is performed after a sufficiently long waiting time. On the other hand, with increasing T below Hon, the μ decreases abruptly at 4πMdp ∼ 5 G near the depinning temperature Tdp. The kink at Mdp is attributed to a change in the creep character from that controlled by the bulk pinning into that controlled by the surface pinning. © 2001 Elsevier Science B.V. All rights reserved.