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/cm
²
. 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 nΦ
₀
/Hz
^1/2
corresponding to a spin sensitivity of few Bohr magnetons for bandwidth unit.