A high-Tc SQUID gradiometer with integrated homogeneous field compensation

An integrated gradiometer system is presented, consisting of a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID) and two coils, realized on printed circuit boards. The coils compensate the homogeneous background field, thus allowing the gradiometer to be freely moved in the Earth's magnetic field. The gradiometer signal is not harmed by any additional noise introduced by the homogeneous field compensation. Therefore, a noise-limited magnetic field gradient resolution of 15 pT m-1 Hz-1/2 with a 1/f corner frequency of 2 Hz was achieved. This is preserved even after motion in the Earth's magnetic field. Because of the small dimensions of the compensation coils, the gradiometer system can also be used for non-destructive evaluation. Preliminary measurements were performed by moving the SQUID across a workpiece, without any shielding