An integrated planar gradiometer based on a double relaxation oscillation SQUID

The design and performance of an integrated planar gradiometer based on a double relaxation oscillation SQUID (DROS) are presented. The DROS was made from hysteretic Nb/AlOx/Nb junctions and the devices were fabricated by a simple four-level process. The signal SQUID loop is a gradiometric type with two square holes connected in parallel and a reference junction is used instead of the reference SQUID. The high flux-to-voltage transfer coefficient of typically 3 mV Phi(o)(-1) enabled direct readout by a simple electronics with a modest voltage noise. The pickup coil integrated on the same wafer as the SQUID consists of two planar 10 x 10 mm(2) coils connected in series and has a baseline of 30 mm. The overall size of the device is 10 x 40 mm(2). The field gradient noise is 2.6 fT cm(-1) HZ(-1/2) in the white region and 4.4 fT cm(-1) HZ(-1/2) at 1 Hz.