3D nano-SQUIDs with nano-constriction junctions.

Nano-SQUIDs (superconducting quantum interference devices) significantly shrink the SQUID washer by replacing the traditional tunneling junctions with the nano-constriction junctions. In such a design, the spin sensitivity of nano-SQUIDs, which is proportional to the radius of SQUID washer, are greatly improved. Additionally, the nano-SQUID with nano-constriction junctions are also excel in a high working field range, a direct coupling from spins to the nano-constrictions. However, current planar nano-SQUIDs made of Nb and NbN showed relatively a shallow flux modulation depth. Here, we developed a fabrication method for nano-SQUIDs based on Nb and NbN by replacing the planar design with a 3D structure. We studied the main parameters limited the flux modulation depth of the Nb and NbN nanoSQUID. As a result, we made Nb and NbN nano-SQUIDs with a reversible current-voltage curve and flux modulation depth above 60{\%} and 35{\%} respectively. The working field range and flux noise of the Nb nanoSQUID is 0.5 T and 0.34 $\mu \Phi $0$\surd $Hz.