Magnetocardiographic performance of single-layer second-order YBCO SQUID gradiometers

We have studied noise properties and magnetocardiographic performance of single-layer second-order YBCO SQUID gradiometers made on STO substrates. A three-loop pickup coil is directly coupled to a step-edge junction SQUID with the coupling polarity of the center loop opposite to that of two side loops. Optimized device parameters for balancing were obtained from 8.8 mm × 3.0 mm devices on 10 mm × 10 mm substrates and scaled up to the larger devices for use in cardiomagnetic measurements. A 17.6 mm × 6 mm gradiometer with a 5.8 mm baseline showed an unshielded gradient noise of 0.84 pT/cm²/Hz¹2/ at 1 Hz, which corresponds to an equivalent field noise of 280 fT/Hz¹2/. A large balancing factor of 10³ allowed us to record magnetocardiogram of a human subject.