Multi-loop relaxation oscillation SQUID magnetometers with large flux-to-voltage transfer functions

A multi-loop Relaxation Oscillation SQUID (ROS) with direct voltage readout is fabricated and characterized. We designed and fabricated four-loop ROSs with the size of 5 mm x 5 mm. A maximum voltage modulation of 100 muV and a flux-to-voltage transfer function of more than 10 mV/Phi (0) were obtaine d. With a direct voltage readout flux locked loop (FLL), a magnetic field r esolution of 4 to 10 fT/root Hz was obtained in the frequency range from 0. 3 Hz up to the white region. Using this magnetometer, we demonstrated the m easurement of a brain response.