Noise characteristics of double relaxation oscillation superconducting quantum interference devices with reference junction

We fabricated double relaxation oscillation SQUIDs (DROSs) with a reference junction and investigated their noise characteristics. Because of the large flux-to-voltage transfer of 1-3 mV/0, the room-temperature dc preamplifier could readout SQUID output voltage directly and the contribution of the preamplifier input noise to the total system noise was negligible. The flux noise of the DROS planar gradiometer is about 4 µ0 Hz-1/2 at 100 Hz, corresponding to a field gradient noise of 1 fT cm-1 Hz-1/2, when operated inside a moderately shielded room. Because of the large modulation voltage of about 100 µV, stable flux-locked-loop operation was possible against drift in the input offset voltage.