Fully integrated dc SQUID magnetometers for ultra low frequency noise applications

Fully integrated dc SQUID magnetometers for ultra low frequency noise applications have been designed, fabricated and characterized. The basic design of the sensor is based on the Ketchen-type magnetometer. A moat surrounding the washer have been inserted to reduce 1/f noise due to hopping of flux vortices, trapped in the SQUID body during the cool-down process. To decrease flux motions in the pick-up coil structure a turn width as small as possible has been used. Further Josephson junctions with a high areas have been employed to reduce the 1/f arising from critical currant fluctuations. An additional positive feedback (APF) circuit and a feedback coil for Flux-Locked-Loop (FLL) have been integrated on the same chip to avoid any additional noise due to an external APF circuit. Details about design criteria and experimental results concerning voltage-flux characteristics and noise performances of the devices are presented. Due to a low magnetic field noise, such devices well meet the requirements of ultra-low frequency noise applications.