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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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Carelli et al. (1993) studied this question.
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