Resistively shunted YBa2Cu3O7 grain boundary junctions and low-noise SQUIDs patterned by a focused ion beam down to 80 nm linewidth

YBa2Cu3O7 240 (300) 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 jc in the 105 A cm-2 range (without a clear dependence on w) and an increase in resistance times junction area ρn with an approximate scaling ρn∝w1/2. For the narrowest GBJs jcρn = IcRn∼100 μV (with critical current Ic and junction resistance Rn), 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 SΦ1/2∼4 μΦ0 Hz-1/2 in the white noise limit. We also derive an expression for the spin sensitivity Sμ1/2, which depends on SΦ1/2, 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 unoptimized SQUIDs presented here, we estimate Sμ1/2 = 390 μB Hz-1/2, which could be further improved by at least an order of magnitude.