Method to extract the critical current density and the flux-creep exponent in high-T-c thin films using ac susceptibility measurements

• High-precision ac susceptibility measurements have been made on high-quality Hg-1212 thin films. A method to analyze chi(1)'(T,H-0,f) and chi(1)" (T,H-0,f) and extract the temperature dependence of the critical current density J(c)(T), as well as the temperature and field-dependent flux-creep exponent n(T,H-0), is presented. With specific measurements at external ac fields Ho in the range 7-100 Oe(rms) we determine the temperature dependence of the critical current density from a single temperature scan. The obtained temperature dependence, J(c)(T), is found to be in good agreement with data obtained from measurements using the traditional "loss-maximum" approach. In addition we present a method to extract the temperature and ac field-dependent flux-creep exponent n(T,H-0) from a set of temperature scans taken at different cc fields and driving frequencies. The observed power law describing the frequency dependence of chi' is consistent with a current-dependent effective activation energy of the form U(J)= U(0)ln(J(c)/J). Furthermore, the flux creep is found to increase with ac field and with temperature except at about 20-30 K below T-c, where our data suggest a slowing down of the flux creep.

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