Extension of the time window for investigation of relaxation effects in high- Tc superconductors

Abstract

Measurements of the dependence of the magnetic hysteresis loop amplitude in a superconductor on the magnetic field sweep rate d H e/d t are shown to provide essentially the same information as magnetic relaxation experiments (flux-creep) at fixed external field H e. For practical reasons, however, magnetic hysteresis loop measurements are better suited to investigate fast flux-line movements than conventional flux-creep measurements. From the general flux movement differential equation it is shown that each magnetic hysteresis loop recorded at a given d H e/d t can be labelled by an effective time t M 0 so that this experimental information can be used to extend the time window of flux-creep measurements to short times. This method is applied to the experimental data obtained on a YBa 2Cu 3O 7 single crystal. In this way the time window is extended to less than 10 -2 s which is valuable for a more precise check of existing theoretical models for the activation energy of flux motion. It is found that the experimental data are well described by using an activation energy U( j)=( U c / μ)[( j c / j) μ -1] with μ=0.74 at 16 K and 1.8 T.