Development of a niobium nanosuperconducting quantum interference device for the detection of small spin populations

Electron-beam lithography and reactive ion etching have been used to fabricate thin-film Au/Nb bridges with widths ∼50 nm. The Au layer was used as both a mask for etching the Nb superconducting bridge and as a resistive shunt in the completed devices. Using these junctions, a dc superconducting quantum interference device (SQUID) design with a hole size of 200 nm×200 nm (nano-SQUID) has also been fabricated and characterized. A flux noise of approximately 7×10−6 Φ0/Hz1/2 at 4.2 K has been achieved, from which a calculated spin sensitivity of 250 spin/Hz1/2 is predicted.