HTS SQUID gradiometer for application without shielding

An HTS SQUID gradiometer consisting of a series gradiometer pickup loop in flip-chip configuration with a small washer SQUID is presented. The series gradiometer configuration is advantageous for applications without shielding because of the avoidance of circulating shielding currents in the pickup loop. With a 20 mm × 10 mm substrate for the pickup loop a large effective area of 1.25 mm 2 and a gradiometer baseline of 9.6 mm are achieved. The balance of the gradiometer impaired by the washer read-out SQUID can significantly be enhanced by the weighted subtraction of a reference SQUID's signal. With this a balance of 5 × 10 4 is reached. The noise-limited magnetic field gradient resolution of the balanced SQUID gradiometer is 38 fT cm −1 Hz −1/2 . This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.