Large artificial ferromagnetic dot arrays for the critical current enhancement in superconducting YBa₂Cu₃O${}_{7-\delta }$ thin films

In order to enhance and/or control critical current density (J _c) in superconducting YBa₂Cu₃O${}_{7-\delta }$ (YBCO) thin films, different arrays of 3 µm large ferromagnetic (iron) dots with differing configurations and shapes have been deposited on top of high-quality YBCO thin films post-buffered with a layer of CeO₂. Some tremendous J _c enhancement of up to nearly 100% have been obtained at high temperatures and low fields. However, the J _c performance is strongly dependent on the array configurations, shape and amount of ferromagnetic iron involved. We show that it is possible to enhance J _c at high or low magnetic field ranges. The results are clearly different to similar non magnetic array structures used to previously manipulate J _c in YBCO films, which proves the magnetic origin of the changes in J _c we observed. The enhancement is likely due to the flux localization and magnetic pinning effects, rather than magnetic shielding alone, which is effective at relatively low fields only. The results also suggest that the observed J _c changes depend on rather minor variations in initial pinning and corresponding J _c levels of the films. At the same time a common trend for all of the various investigated magnetic arrays could not be established due to explained factors. (paper)