Dependence of flux-creep activation energy upon current density in HgBa 2Ca 2Cu 3O y

Abstract

We have investigated the magnetic relaxation on a HgBa 2Ca 2Cu 3O y superconductor over the range 4.2–60 K. Using a linear relation between U( J) and kT ln(1 + t/ t 0), we extracted the effective activation energy and the time-window constant t 0. A nonlinear dependence of U eff on J, U eff∝ J − μ , was found. μ is about 0.98 in the temperature range 4.2–60 K, close to the value of 7 9 in the regime of flux creep in large vortex bundles predicted by the three-dimensional collective-creep theory. The time scale t 0 depends exponentially on the initial conditions. The relationship between U( J) and the magnetic relaxation curve is discussed. With logarithmic accuracy one can construct the U( J) curve simply using several long-time relaxation curves at different temperatures.