Critical currents of $ex-situ$ YBCO thin films on ``RABiTS'' substrates: thickness, field and temperature dependencies

The critical current density $J_c$ flowing in thin $YBa_2Cu_3O_ {7-\delta}$ (YBCO) films of various thicknesses $d$ has been studied magnetometrically, both as a function of applied field $H$ and temperature $T$. The films, grown by a BaF$_2$ $ex- situ$ process and deposited on buffered ``RABiTS'' substrates of Ni-5$\%$W, have thicknesses $d$ ranging from 28 nm to 1.5 $\mu$m. Isothermal magnetization loops $M(H;T)$ and remanent magnetization $M_{rem}(T)$ in $H=0$ were measured with $H$ $\|$ c-axis (i.e., normal to film plane). The $J_c(d)$ values, which were obtained from a modified critical state model, increase with thickness $d$, peak near $d \sim$ 150 nm, and thereafter decrease as the films get thicker. For a range of temperatures and intermediate fields, we find $J_c \propto H^{-\alpha}$ with $\alpha \sim (0.56 - 0.69)$ for all materials. This feature can be attributed to pinning by large random defects. At higher fields approaching the irreversibility line, $J_c(H)$ decreases faster. The $J_c$ at self field varies as $J_c(T,sf) \sim (1-T/T_c)^n$ with $n \sim$ 1.2 - 1.4. This points to ``$\delta T_c$ pinning'' (pinning that suppresses $T_c$ locally) in these YBCO materials. Work at UTK was supported by AFOSR Grant F49620-02-1-0182. ORNL is managed by UT-Battelle, LLC for the USDOE.