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 ${\chi }_1^{\prime }{(T,H}_0,f)$ and ${\chi }_1^{″}{(T,H}_0,f)$ and extract the temperature dependence of the critical current density $J_c\mathrm{}\left(T\right),$ as well as the temperature and field-dependent flux-creep exponent ${n(T,H}_0),$ is presented. With specific measurements at external ac fields $H_0$ in the range $7–100{\mathrm{Oe}}_{\mathrm{rms}}$ we determine the temperature dependence of the critical current density from a single temperature scan. The obtained temperature dependence, $J_c\mathrm{}\left(T\right),$ 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 ac fields and driving frequencies. The observed power law describing the frequency dependence of ${\chi }^{\prime }$ is consistent with a current-dependent effective activation energy of the form ${U\left(J\right)=U}_0\ln {(J}_c/J).\mathrm{}$ 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.