Low noise directly-coupled high Tc de superconducting quantum interference device magnetometers for magnetocardiogram

Directly-coupled de superconducting quantum interference device (SQUID) magnetometers have been fabricated on SrTiO3 bicrystal substrates with 30 degrees misorientation angle. YBa2Cu3O7-delta Single layers were deposited by laser ablation and patterned by standard photolithography and Ar ion milling. The magnetometer consists of a 9 mmx9 mm pickup loop coupled to a long slit. The magnetometers have been characterized at 77 K using conventional flux-locked loop electronics with a bias current reversal. The magnetometers exhibited high junction resistance of up to 10 Omega and large voltage modulation depth as high as 65 mu V with a flux-to-voltage transfer function of 253 mu V/Phi(0) at the working point. A magnetic field noise level of 74 f T/root Hz at 1 Mit and below 100 fT/root Hz down to 4 I-It, including electronics noise, was achieved inside a magnetically shielded room. The performance of the magnetometer was demonstrated by magnetocardiograms with high quality.