Flux pinning and irreversibility in YBa2Cu3O7 superconducting crystal.

Abstract

Extensive ac-magnetic-permeability studies have been carried out on the irreversible behavior and flux dynamics in superconducting ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals in external magnetic fields up to 7.5 T. From the analysis the irreversibility line ${\mathit{B}}^{\mathrm{*}}$(${\mathit{T}}^{\mathrm{*}}$) is found to scale as (1-${\mathit{T}}^{\mathrm{*}}$/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$, where n\ensuremath{\simeq}1.5 for fields higher than 1.5 T, and n\ensuremath{\simeq}1 for lower fields. The exponents are sensitive to the choice of ${\mathit{T}}_{\mathit{c}}$. The frequency dependence of the irreversibility temperature is carefully studied in the low-frequency region from 10 to ${10}^{5}$ Hz and is found to be logarithmic in the whole frequency range. The small logarithmic slope increases with increasing external field. The analysis is done within a flux-creep picture using a single-relaxation-time form for the complex permeability, and a vortex-glass model. The peak in the imaginary part of the permeability in different dc fields is successfully fitted to the predictions of the flux-creep model. A dynamical screening length is needed to account for the observed frequency dependence of the irreversibility temperature.