Superconducting thin-film gradiometer

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Abstract
We describe the design, fabrication, and performance of planar thin‐film dc SQUID’s and planar gradiometers in which a dc SQUID is incorporated as a null detector. Each gradiometer was fabricated on a planar substrate and measured an off‐diagonal component of changes in the magnetic field gradient. The gradiometer with the highest sensitivity had 127×33‐mm loops that could be connected in parallel or in series: The sensitivities were 2.1×10−13 and 3.7×10−13 T m−1 Hz−1/2, respectively. The intrinsic balance of the gradiometers was about 100 ppm for fields parallel to their plane, and a balance of about 1 ppm could be achieved for fields perpendicular to their plane. When the series‐loop gradiometer was rotated through 360° in the earth’s field, the output returned to its initial value to within an amount corresponding to a balance of 1 ppm. Possible improvements in sensitivity are discussed.