Effects of field-sweep rate on the magnetization of melt-textured YBa2Cu3O7-δ

The effects of field-sweep rate K=H/t on magnetization hysteresis loops M(H) and on flux-creep studies M(t) in high-temperature superconductors have been investigated both theoretically and experimentally. We find the basic relation between M and K is, to first order, the following: M=const-{[dM/d ln(t)] ln(K)}-[Kteff/10], where dM/d ln(t)=aC/30 is the flux-creep rate in a cylindrical sample of radius a, and teff is an effective attempt time for vortex hopping. The largest possible M, which corresponds to the critical current density Jc0 in the absence of thermal activation, develops when K Kmax=aC/[(1+aα)teff] with α=J/H. The time origin of flux creep, which is essential in studying the initial stages of relaxation, is given by t*=aC/K(1+aα). The model agrees well with experiments on a melt-textured-growth sample of Y1Ba2Cu3O7-δ, yielding teff∼0. 24±0.03 s at 27 K. By incorporating the calculated time origin into flux-creep studies of M(t), we obtain a very good description in terms of the interpolation formula from vortex-glass-collective pinning theory.