A thin-film second-order gradiometer with integrated dc-SQUID

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Abstract
We have fabricated and characterized a superconducting planar gradiometer sensitive to the second‐order spatial derivative of the magnetic field. Our device has been patterned on a 12.8×12.8 mm silicon chip on which the sensitive area of the gradiometer and the SQUID inductance are generated by the same four 0.5‐mm‐square holes. We measured the sensitivity both in flux‐locked‐loop (FLL) and in open‐loop mode. With the second method we used another dc‐SQUID as amplifier and in this case we obtained the best noise performance. The gradiometer sensitivity was 18 fT/cm2 √Hz. The magnetic isoflux line distribution generated by a dipolar source was measured by the gradiometer in FLL mode.

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