AC loss and thickness dependence of critical currents in coated conductors

A study has been conducted on the magnetic properties of a series of biaxially textured Ni1-xW x materials with compositions x = 0, 3, 5, 6, and 9 at.% W. These materials are important as substrates for "RABiTS"-type coated conductors that incorporate high temperature superconductors for current transport. The quasi-static do and ac hysteretic loss was determined to support estimates of the ferromagnetic contribution to the overall ac loss in potential ac applications. The hysteretic loss tended to be lower in materials that were recrystallized at higher temperatures. Some samples were progressively deformed to simulate winding operations; this increased the hysteretic loss, as did sample cutting operations that create localized damage.; Furthermore, we have made magnetometric studies of the critical current density Jc flowing in thin YBCO films of various thicknesses d, 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 mum. Isothermal magnetization loops M(H;T) and remanent magnetization Mrem(T) in zero applied field, were measured with H normal to film plane. The Jc(d) values, which were obtained from a modified critical state model, increase with thickness d, peak near a particular thickness, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find a power law falloff Jc ∝ H-beta with beta ∼ (0.56 - 0.69) for all materials. This feature compares well with the power-law exponent beta = 5/8 obtained theoretically by Ovchinnikov and Ivlev for pinning by large random defects, as are observed by TEM. Finally, we find Jc(T, sf) ∝ (1 - T/Tc)n with n ∼ 1.1 - 1.3. This points to "deltaTc pinning " (pinning that suppresses Tc locally) in all of these YBCO materials.