NanoSQUIDs based on niobium constrictions

Superconducting quantum interference devices (SQUIDs) based on niobium nanobridges have been produced by means of focused ion beam milling. Typical critical currents of 4-25 mu A and flux sensitivities of 40-200 mu V/Phi(0) were measured for sensors based on 80 nm wide, 50 nm thick, and 150 nm long bridges. A white flux noise level of 1.5 mu Phi(0)/Hz(1/2) was measured for a device with an area of 900 mu m(2) and a critical current of 15 mu A. The effective area of the smallest produced SQUID was 3.6 x 10(-2) mu m(2). Possible applications for such miniature SQUIDs are in scanning SQUID microscopy and the study of magnetic nanoparticles.