Improving the sensitivity of a high-T_c SQUID magnetometer with a serial array

To pursue a high-reproducibility and high-quality SQUID for high-T _c superconducting applications, we have designed and fabricated a flip-chip magnetometer as a sensor for measuring weak magnetic signals. The magnetometer consists of a serial SQUID magnetometer coupled to a flux focuser. The SQUIDs are connected in series, thereby increasing the peak-to-peak value of the voltage-flux transfer function, V_φ. A flux focuser is employed to increase the effective area of the SQUID. The serial SQUID array has high-quality device characteristics. High-T_c SQUID magnetometers exhibiting a magnetic field sensitivity of ∼45 fT/Hz^1/2 in the white region and 200 fT/Hz^1/2 at 1 Hz have been demonstrated in this work. The flip-chip SQUID magnetometer shows a sufficiently high sensitivity to record magnetocardiograms.