A stable, single-chip, two-dimensional gradiometric device based on two superconducting quantum interference devices

Abstract

A planar gradiometric device for estimating current vectors in specimens, was fabricated as two orthogonal magnetic field gradiometers integrated on a 14.5-mm-square substrate. The gradiometers consist of superconducting quantum interference devices (SQUIDs) and pickup coils made from a low-critical-temperature superconductor. The two gradiometers (one for the x direction and one for the y direction) have exactly the same structure, and their gradiometric centers coincide at the center of the substrate. The gradiometric balance of each gradiometer is 1/2000–1/3700, the cross-talk ratio between the two gradiometers is less than 1% below a few hundred hertz, and their intrinsic noise is 2–3 pT/(m√Hz). The gradiometric configurations of both the pickup coils and the SQUIDs provide good gradiometric balance. The parallel-type gradiometric SQUIDs produced a simple yet effective structure, although the superconducting loops of such SQUIDs have conventionally been thought to lead to adverse effects in the presence of a changing magnetic field. Magnetic fields up to 1 mT, i.e., 20 times the geomagnetic field, applied to a zero-field cooled SQUID reduced the critical current, but did not affect the intrinsic noise. This result shows that the configuration of the SQUIDs kept the circulating current in the superconducting loop away from the Josephson junctions.