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 co-planar edge structure of the low inductance. The junction capacitance was typically 15 approximately 40fF. The maximum voltage modulation was about 110-mu-V for the NbN nanobridge SQUID with an inductance of 0.18nH. The maximum value of the transfer function \partial-V/partial-PHI\ was approximately 1mV/PHI-0 without matching circuit. The minimum intrinsic energy sensitivity was about 20h at 50kHz. The SQUID was operated in a usual flux-locked loop. The flux noise spectrum for dc SQUID at 4.2K in a superconducting Nb shield was 4 x 10(-5)-PHI-o/square-root-Hz at 3kHz and one order larger than the intrinsic flux noise. The critical current and \partial-V/partial-PHI\ of these SQUIDs show no change after repeated thermal recycling and storage over 250 days.