High-T/sub C/ SQUID magnetometers for low noise measurements of magnetocardiograms

YBCO dc superconducting quantum interference device (SQUID) magnetometers based on bicrystal junctions have been fabricated for magnetocardiograph measurements. The pickup coil of the device was designed employing 16 parallel loops with 50-/spl mu/m-wide lines. Noise performance was improved by applying a modification in the coupling line between pickup loop and SQUID washer. We could obtain optimized SQUID magnetometer design having magnetic field noise of about 38 fT/Hz/sup 1/2/ at 1 Hz and about 20 fT/Hz/sup 1/2/ at 100 Hz with an 1/f corner frequency of 2 Hz, measured inside a magnetically-shielded room. Demonstration of magnetocardiograph measurements showed quite promising results with high signal-to-noise ratio of 82 with a moderate magnetic shielding.