Resistively shunted YBa2Cu3O7 grain boundary junctions and low-noise SQUIDs patterned by a focused ion beam down to80 nm linewidth

Abstract

YBa2Cu3O7 24° (30°) bicrystal grain boundary junctions (GBJs), shunted with 60 nm (20 nm)thick Au, were fabricated by focused ion beam milling with widths80 nm ≤ w ≤ 7.8 µm. At 4.2 K we find criticalcurrent densities jc inthe 105 A cm − 2 range (without aclear dependence on w) and an increase in resistance times junction areaρn with an approximate scaling . 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 thedetection of small spin systems. We demonstrate that our fabrication processallows the realization of sensitive nanoscale dc SQUIDs; for a SQUID withw≈100 nm wide GBJs we find an rms magnetic flux noise spectral density ofSΦ1/2≈4 μΦ0 Hz − 1/2 in the white noise limit. We also derive an expression for the spin sensitivitySμ1/2, whichdepends on SΦ1/2, on the location and orientation of the magnetic moment of a magnetic particle to be detected bythe SQUID, and on the SQUID geometry. For the unoptimized SQUIDs presented here, we estimateSμ1/2 = 390 μB Hz − 1/2, which could be further improved by at least an order of magnitude.