Current-voltage characteristics and flux creep in melt-textured YBa₂Cu₃O_7-

We investigated the current-voltage (E - J) characteristics in melt-textured YBa₂Cu₃O_7-strips by measuring the magnetic-field sweep rate dependence of magnetization. We took account of the current density Jdistribution in the specimen using a previously developed method (Mawatari Y et al1997 Appl. Phys. Lett.702300). For a wide temperature and magnetic-field range (60-80 K, 0.2-5.0 T), the E - Jcurves in the electric-field window E= 10^-10 -10^-5V m^-1exhibited power-law behaviour E Jⁿ, and the power index ngenerally became smaller at higher magnetic fields and temperatures. In low magnetic fields (µ₀ H_a 0.5 T) the nvalues were large ( 20), and thus the Bean model becomes a good approximation. The E - Jcharacteristics in the lower Ewindow were also derived from the relaxation of magnetization, the flux creep, and we found that the wide-range E - Jcharacteristics exhibit near-power-law behaviour but that there exist slight downward curvatures in the log Eversus log Jplots. This downward curvature reveals that the dissipation approaches zero when the current is substantially reduced. The drastic decrease of the flux creep, which was observed when the sample temperature was decreased in a fixed magnetic field, is consistent with the observed E - Jcharacteristics.