Long baseline thin film SQUID gradiometers

We have developed a highly sensitive SQUID gradiometer for unshielded operation in the Earth's magnetic field. It is implemented in an all-refractory Nb-AlO/sub x/-Nb technology, and consists of gradiometer pick-up loops with an inductance of about 250 nH coupled to the SQUID by a flux transformer. The SQUID is designed as a highly balanced second order gradiometer consisting of four pairs of washers. It has an inductance of about 300 pH. The gradiometer chip has a size of 6 cm /spl times/2 cm. The baseline of the gradiometer is 4 cm and the effective volume is about 300 mm/sup 3/. We measured a white noise level of below 10 /spl mu//spl Phi//sub 0//Hz/sup 1/2 /, corresponding to a gradient field resolution smaller than 40 fT/m/spl middot/Hz/sup 1/2 /. The total flux noise in the frequency band from 0.01 Hz to 10 Hz is below 150 /spl mu//spl Phi//sub g/. The balance of the gradiometer was determined by moving the gradiometer in the Earth's magnetic field. Due to the high balance we can operate the gradiometer in field experiments.