A device with two orthogonal low-Tc SQUID gradiometers - a comparison of series and parallel gradiometric SQUID´s

We have developed a planar gradiometric device with two orthogonal SQUID gradiometers integrated on a substrate. In developing this new device, both series- and parallel-type gradiometric SQUID´s (without pickup coils) suitable for the same pickup coils were fabricated. The parasitic inductance, parasitic areas, and cross talks of each SQUID were evaluated to compare their performances. Next, to evaluate the effects of changing field on the SQUID´s, the critical current reduction and noise increase under an applied magnetic field were measured. These results show that the parallel SQUID is superior to the series SQUID; thus the parallel SQUID was chosen for use in the new single-chip gradiometric device.