Noise and operational characteristics of magnetometers made from superconducting-normal-superconducting Josephson junctions

We have fabricated numerous magnetometers using our high temperature superconducting (HTS) Josephson junctions. These Josephson junctions are fabricated in a superconducting-normal-superconducting ramp edge configuration with silver doped YBa₂Cu₃O₇ (YBCO) for the superconducting electrodes and PrBa₂Cu₃O₇ (PBCO) for the normal layer. Small inductance quantum interference devices (SQUIDs) made from this junction technology have a transfer function exceeding 150 microvolts per flux quanta and a flux noise of 5×10^-6 flux quanta per root hertz. In addition, we have established that these junctions have identical electrical characteristics after either a year of storage or repeated thermal cycling. We have also examined the trade-off of 1/f noise versus thermal noise that is obtained as we vary the critical current of the devices. The SQUID magnetometers were made using galvanically coupled input coils. These devices exhibit excellent operational characteristics in the geomagnetic field. They functioned in an unshielded environment for more than 24 hours and operated in a moving dewar (without any feedback fields to compensate the changing applied field) - both without flux trapping. Noise characteristics under these conditions are presented.