DEPENDENCE OF FLUX-CREEP ACTIVATION-ENERGY UPON CURRENT-DENSITY IN HGBA2CA2CU3OY

We have investigated the magnetic relaxation on a HgBa2Ca2Cu3Oy superc onductor 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 a nd the time-window constant t(0). A nonlinear dependence of U-eff on J , U-eff proportional to J(-mu), was found. mu is about 0.98 in the tem perature range 4.2-60 K, close to the value of 7/9 in the regime of fl ux creep in large vortex bundles predicted by the three-dimensional co llective-creep theory. The time scale t(0) depends exponentially on th e initial conditions. The relationship between U(J) and the magnetic r elaxation curve is discussed. With logarithmic accuracy one can constr uct the U(J) curve simply using several long-time relaxation curves at different temperatures. (C) 1997 Elsevier Science B.V.