Development and investigation of novel single-layer gradiometers using highly balanced gradiometric SQUIDs

The sensitivity of galvanically coupled single-layer gradiometers on bicrystal substrates for homogeneous magnetic fields can be minimized by an increase of the balance, which is the ratio of the effective area of the antennas and the effective area of the SQUID. In this paper, we describe possible realizations of new gradiometric SQUIDs (G-SQUIDs) on 10 × 10 mm² STO-bicrystal substrates, which minimize the effective area of the SQUID, and therefore the parasitic area of the gradiometer, to 34 μm². We integrate these gradiometric SQUIDs in well-defined gradiometer antennas. The G-SQUID layouts with coupling inductances between 20 pH and 100 pH do not show any dependence of the parasitic area from the SQUID inductance. With a gradient field resolution of 1.6 pT(cm√Hz)⁻¹ (white, unshielded) at 24 pH and 1.8 pT(cm√Hz)⁻¹ at 55 pH, these gradiometers possess outstanding noise characteristics. So the balance of comparable conventional gradiometers, which show values of about 100, can be increased by more than an order of magnitude to 1070 with the new layouts.

This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.