Flux creep studies in YBa2Cu3O7- superconductors

Author

Yang Ren Sun

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

James R. Thompson

Committee Members

Gerald Much, Marianne Breinig

Abstract

Magnetic relaxation studies were performed on a proton irradiated high J_c YBa₂Cu₃0_7-δ single crystal and a melt-texture-growth YBa₂Cu₃O_7-δ sample. Significant deviation from the logarithmic time decay predicted by conventional Anderson-Kim theory was observed for a wide range of temperature and magnetic field. The flux creep data, with durations up to 3.5 * 10⁵s, could be analyzed using several recent models with comparable accuracies. The difficulty in distinguishing between these models is due to the fact that the time window of observations, which usually covers ~2 or 3 decades, is too short for magnetic relaxation studies.

To complement these studies in the direct time domain, the effects of field-sweep rate K = ∂H/∂t on magnetization hysteresis loops M(H) and on flux-creep M(t) were 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) -[Kt_eff/10] , where dM/dln(t) = ac/30 is the flux-creep rate in a cylindrical sample of radius a, and t_eff is an effective attempt time for vortex hopping. The largest possible M, which corresponds to the critical current density J_c0 in the absence of thermal activation, develops when at high sweep rate K ≥ K_max =ac/[(1+aα)t_eff] with α = ∂J/∂H. Such studies provided a way to measure mathfrak t_eff , whose magnitude has been rather controversial; furthermore, the analysis revealed the time origin of flux creep, t^* = ac/K(1 + a*α) , which is essential in studying the initial stages of relaxation. The theory agrees well with experiments yielding t_eff~0.2 s for the melt-textured-growth sample.

By combining conventional flux creep experiments with measurements of magnetization versus magnetic field sweep rate, we are able to detect the decay of magnetization in both its middle and very early stages, thereby expanding the observational time window up to 5 ~ 6 decades. Among four models tested, only the Vortex-Glass/Collective-Pinning theory gives a consistent description of both stages.

Recommended Citation

Sun, Yang Ren, "Flux creep studies in YBa2Cu3O7- superconductors. " PhD diss., University of Tennessee, 1992.
https://trace.tennessee.edu/utk_graddiss/10783