High-gain DC SQUID magnetometers with NbN nanobridges

High-gain DC SQUIDs using NbN nanobridges have been fabricated for magnetometers with high sensitivity, and their device parameters and intrinsic energy sensitivity have been evaluated. The slit inductance of the square washer SQUID was reduced by using the coplanar edge structure of the low inductance. The junction capacitance is typically 15~40 fF. The maximum voltage modulation is about 110 μV for the NbN nanobridge SQUID with an inductance of 0.18 nH. The maximum value of the transfer function |∂V/∂Φ| was ~1 mV/Φ₀, without matching circuit. The minimum intrinsic energy sensitivity is about 20 h at 50 kHz. The SQUID was operated in a usual flux-locked loop. The flux noise spectrum for a DC SQUID at 4.2 K in a superconducting Nb shield was 4×10^-5Φ₀|√Hz at 3 kHz and one order larger than the intrinsic flux noise. The critical current and |∂V/∂Φ| of these SQUIDs show no change after repeated thermal recycling and storage over 250 days