YBa2Cu3O7 grain boundary junctions and low-noise superconducting quantum interference devices patterned by a focused ion beam down to 80 nm linewidth

Abstract

YBa 2 Cu 3 O 7 24 • (30 • ) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm) thick Au, were fabricated by focused ion beam milling with widths 80 nm ≤ w ≤ 7.8 µm. At 4.2 K we find critical current densities j c in the 10 5 A/cm 2 range (without a clear dependence on w) and an increase in resistance times junction area ρ with an approximate scaling ρ ∝ w 1/2 . For the narrowest GBJs j c ρ ≈ 100 µV, which is promising for the realization of sensitive nanoSQUIDs for the detection of small spin systems. We demonstrate that our fabrication process allows the realization of sensitive nanoscale dc SQUIDs; for a SQUID with w ≈ 100 nm wide GBJs we find an rms magnetic flux noise spectral density of S 1/2 Φ ≈ 4 µΦ 0 /Hz 1/2 in the white noise limit. We also derive an expression for the spin sensitivity S 1/2 µ , which depends on S 1/2 Φ , on the location and orientation of the magnetic moment of a magnetic particle to be detected by the SQUID, and on the SQUID geometry. For the not optimized SQUIDs presented here, we estimate S 1/2 µ = 390 µ B / √ Hz, which could be further improved by at least an order of magnitude.

Soft Nanostructuring of YBCO Josephson Junctions by Phase Separation

“…At present HTS nanostructures are realized by e-beam lithography in combination with a hard mask 5 and ion milling etching or by focus ion beam patterning. 6,7 In both cases the lateral damage of the structures is quite severe due to structural and chemical changes mostly related to oxygen out diffusion and/or Ga implantation. This fact puts severe limits to the smallest HTS Josephson tunnel junction size that can still retain its superconducting properties.…”