Performance Optimization of a Three-Dimensional NanoSQUID Based on Niobium Tunnel Nanojunctions

We report results about an optimized three-dimensional nanoSQUID based on niobium tunnel nanojunctions having the loop suspended to reduce the parasitic capacitance. The SQUID loop has a size of 400×600 nm^2 while the dimension of the square tunnel nanojunctions is 150 × 150 nm^2 with a density of the critical current of about 35×10^3 A/cm2 . The nanodevice has been characterized at liquid helium temperature; it has shown nonhysteretic current-voltage characteristics and smooth voltage-magnetic flux characteristics resulting in a very stable operation in a wide range of bias points. The spectral density of the magnetic flux noise in the white region, measured with a two-stage noise measurement setup, was as low as 300 n?_0/Hz^1/2 corresponding to a spin sensitivity of few Bohr magnetons for bandwidth unit.