Magnetometers made from superconducting nanobridges

For the fabrication of nano-scale SQUIDs, e.g. having loop diameters of order 1 mu m or smaller, the conventional tri-layer window-type Josephson junctions are too large. Instead, we consider the use of nanobridge junctions with dimensions of order 100 nm. The problem of hysteresis in the junctions' current-voltage characteristic (IVC) is avoided by depositing a gold overlayer, which serves as both a thermal and electrical shunt. Noise measurement of dc SQUIDs which incorporate such Nb nanobridges as the Josephson junctions are reported. Devices having different IVCs were fabricated and characterized. The best device has a flux noise of 5 x 10(-6) Phi(0)/Hz(1/2) at 4.2 K. Techniques for increasing the voltage modulation depth, and hence improving the flux noise performance, are discussed.