We have fabricated and characterized a superconducting planar gradiometer sensitive to the secondhyphen;order spatial derivative of the magnetic field. Our device has been patterned on a 12.8times;12.8 mm silicon chip on which the sensitive area of the gradiometer and the SQUID inductance are generated by the same four 0.5hyphen;mmhyphen;square holes. We measured the sensitivity both in fluxhyphen;lockedhyphen;loop (FLL) and in openhyphen;loop mode. With the second method we used another dchyphen;SQUID as amplifier and in this case we obtained the best noise performance. The gradiometer sensitivity was 18 fT/cm2sqrt;Hz. The magnetic isoflux line distribution generated by a dipolar source was measured by the gradiometer in FLL mode.
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