Design and performance of integrated DC SQUID gradiometers

Integration of thin film gradiometers with a dc superconducting quantum interference device (SQUID) is described. A parallel first-order gradiometer has been realized with an overall size of 17 mm × 12 mm and a sensitivity of typically 3 × 10^-12Tm^-1Hz^-1/2. An intrinsic balance of 300 ppm is easily achieved. Its usefulness for measuring biomagnetic signals is shown. A novel design for a second order gradiometer consisting of eight second order loops in parallel is presented. The design is based on a sensitivity of 3 × 10^-10Tm^-2Hz^-1/2with a size of 17mm × 17mm and a SQUID inductance of 1nH.