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 /spl times/ 3.0 mm devices on 10 mm /spl times/ 10 mm substrates and scaled up to the larger devices for use in cardiomagnetic measurements. A 17.6 mm /spl times/ 6 mm gradiometer with a 5.8 mm baseline showed an unshielded gradient noise of 0.84 pT/cm/sup 2//Hz/sup 1/2/ at 1 Hz, which corresponds to an equivalent field noise of 280 fT/Hz/sup 1/2/. A large balancing factor of 10/sup 3/ allowed us to record magnetocardiogram of a human subject.