Josephson behaviour for high critical current density YBCO+Ag thick films on YSZ substrates

Abstract

High critical current density thick films of yttrium barium copper oxide plus 10 wt.% silver have been fabricated on yttria stablised zirconia substrates. The films are approximately 10 μm thick, superconducting with T c( R = 0) in the range 90.5 to 91.0 K and are found to have j c's close to 3000 A cm -2 at 77 K in zero applied magnetic field. The thermal cycle for the production of these films requires the use of temperatures in excess of the peritectic and therefore involves melt processing. This has a number of advantages, including film texturing, which is significantly improved with the addition of Ag, and the formation of an inert BaZrO 3 barrier layer which has fine CuO needles dispersed throughout. This layer prevents atomic diffusion and film poisoning. At 77 K, the j c values decreased markedly for applied magnetic fields increasing from 0–20 Oe, and then decreased slowly for increasing fields, being almost constant for field values ∥#62; 150 Oe. The temperature variation, near T c, of the critical current i c was best described by i c £ (1 − T/ T c) 1.52, the Ginzburg-Landau 3 2 power law characteristic, and indicates that the granular films may be modelled as an array of Josephson-coupled grains which have a short coherence length. Variable thickness bridges patterned into the thick films showed very clear DC and AC characteristics that were very similar to those expected from a single Josephson junction.