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Magnetic relaxation, flux creep and the distribution of pinning potentials in ceramic Tl-1223 HTSC

The relaxation of the remanent magnetization of ceramic Tl-1223 HTSC has been studied in the temperature range 3.5-100 K after switching a field of 1 T on and immediately off. Because of the thermal activation of vortex lines the relaxation can be well described by a logarithmic time decay. The activation energies obtained in the single-barrier-height model show a strong temperature dependence with UO=40-50 meV at T=4 K and UO=200-400 meV at T=40-80 K. The distribution of pinning potentials has been evaluated from the relaxation data and exhibits a peak at UO=38 meV and can be well described by two log-normal distribution functions with Emax,1=36.6 meV and Emax,2=51.4 meV. In the framework of the distribution model it is possible to obtain the dependence of the activation energies on temperature in the single-barrier-height model. The small values of most of the pinning barrier heights lead to a drastic decrease of the remanent magnetization and therefore the critical current at T>or=20 K. The dependence of the critical current on temperature can be obtained by integrating the distribution function.