Flux-creep studies of vortex pinning in an aligned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductor with oxygen deficiencies δ≤0.2

Flux-creep measurements were carried out in magnetically aligned, sintered samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ with various oxygen deficiencies δ (in the range 0≤δ≤0.2), for temperatures T in the range from 5 to 75 K, with an applied magnetic field H∥c axis of 1 T. The normalized flux-creep rate S=d lnM/d lnt was determined as a function of T and δ. 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.