On narrowing coated conductor film: The emergence of granularity-induced field hysteresis of transport critical current

The critical current density J_c in polycrystalline or granular superconducting material is known to be hysteretic with applied field H due to the focusing of the field within the boundary between adjacent grains. This is of concern in the so-called coated conductors, wherein a superconducting film is grown on a granular, but textured, surface of a metal substrate. While previous work has mainly been on J_c determined using induced or magnetization currents, the present work utilizes transport current via an applied potential in strip geometry. It is observed that the effect is not as pronounced using transport current, probably due to a large difference in criterion voltage between the two types of measurements. However, when the films are narrowed by patterning into 200, 100, or 80 μm, the hysteresis is clearly seen, because of the forcing of percolation across higher-angle grain boundaries. This effect is manifested in ex situ films grown on ion-beam-assisted-deposited (IBAD) yttria stabilized zirconia (YSZ) substrate and those grown on rolling-assisted biaxially textured substrates (RABiTS) which have grains that are 100-1000 times larger. The hysteresis is more pronounced for the latter, which is more likely to have a weak grain boundary spanning the width of the microbridge. This is also of concern to applications in which coated conductors will be striated in order to reduce the AC losses.