Critical currents of ex situ YBa_2Cu_3O_(7-δ) thin films on rolling assisted biaxially textured substrates: Thickness, field, and temperature dependencies

摘要

The critical current density J_c flowing in thin YBa_2Cu_3O_(7-δ) (YBCO) films of various thicknesses d has been studied magnetometrically, both as a function of applied field H and temperature T, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a BaF_2 ex situ process and deposited on buffered rolling assisted biaxially textured substrates ("RABiTS") substrates of Ni-5% W, have thicknesses d ranging from 28 ran to 1.5 μm. 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 resulting J_c(d) values (obtained from a modified critical state model) increase with thickness d, peak near d ～ 120 nm, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find J_c∝H~(-α) with α ～(0.56-0.69) for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent α=5/8. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find J_c(T,sf)～[1-(T/T_c)~2]~n with n ～ 1.2-1.4. This points to "δT_c pinning" (pinning that suppresses T_c locally) in these YBCO materials.