Flux-creep studies of vortex pinning in an aligned YBa2Cu3O7- delta superconductor with oxygen deficiencies delta

Abstract

Flux-creep measurements were carried out in magnetically aligned, sintered samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ with various oxygen deficiencies \ensuremath{\delta} (in the range 0\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.2), for temperatures T in the range from 5 to 75 K, with an applied magnetic field H\ensuremath{\parallel}c axis of 1 T. The normalized flux-creep rate S=d lnM/d lnt was determined as a function of T and \ensuremath{\delta}. The effective vortex-pinning potential ${\mathit{U}}_{\mathrm{eff}}$(J) and the pinning energy ${\mathit{U}}_{0}$ were calculated both in the linear Anderson-Kim approximation as well as in the nonlinear formalism of vortex-glass and collective-pinning theory. The applicability of the nonlinear formalism was tested and discussed. There is a close correspondence between the energy scaling parameter ${\mathit{U}}_{0}$ of the nonlinear model and the pinning energy of a pointlike pinning center. A comparison of the experimental results with predictions of collective-pinning theory reveals a consistent picture of vortex motion in this family of superconductors.