Fabrication and magnetocardiography application of the second-order superconducting quantum interference device gradiometer made from a single-layer of $YBa2Cu3O7$ film Available to Purchase

We have fabricated single-layer, second-order high $Tc$ superconducting quantum interference device (SQUID) gradiometers on $SrTiO3$ substrates and investigated their noise properties and performance in magnetocardiography. The gradiometer consists of three parallel pickup loops that are directly coupled to a step-edge junction SQUID, in such a way that the coupling polarity of two side loops is opposite to that of the center loop. The overall size of the device is 17.6 mm×6 mm with a baseline of 5.8 mm. The measured gradient noises are 0.45 and $0.84 pT/cm2/√Hz$ at 1 Hz for the shielded and the unshielded cases, respectively, which correspond to equivalent field noises of 150 and $280 fT/√Hz,$ respectively. In spite of the short baseline of 5.8 mm, the high common-mode rejection ratio of the gradiometer, $103,$ allowed us to record a magnetocardiogram of a human subject, which demonstrates the feasibility of the design in biomagnetic studies.