YBa2Cu3O7 nanoSQUIDs for detection of small spin systems

We report on the fabrication and characterization of nanopatterned YBa2Cu3O7 (YBCO) SQUIDs with grain boundary junctions (GBJs) for the investigation of small spin systems at 4 K and below. We use photolithography and Ar ion milling to prepattern bridges (several μm wide) in a YBCO/Au (50 nm/60 nm) bilayer across the grain boundary of a SrTiO3 bicrystal substrate. Subsequently the final SQUID geometry with junction widths down to 80 nm is patterned by focused ion beam (FIB) milling, without significant reduction of critical current density (jc > 105 A/cm2 at 4.2 K). The Au layer serves as a shunt resistor to provide overdamped GBJs and protects the YBCO during the FIB process. We obtained a flux noise down to SΦ1/2 ∼ 600 nΦ0/Hz1/2, corresponding to an estimated spin sensitivity Sμ1/2 ∼ 65 μB/Hz1/2.