A high performance integrated DC SQUID magnetometer

The authors have carried out extensive mappings of the DC characteristics and flux noise of an integrated, DC superconducting quantum interference device (SQUID) magnetometer as functions of bias current and applied flux. The open-loop white flux noise at 1 kHz measured without flux modulation is Phi /sub n/<7*10/sup -7/ Phi /sub 0// square root Hz throughout a broad region of operation where the current-voltage and voltage-flux characteristics are smooth. The corresponding flux density noise B/sub n/<4 fT/ square root Hz. In addition, a recent model of the coupled DC SQUID was used to calculate the frequency-dependent impedance Z(f) seen by the Josephson junctions. The peaks observed in Z(f) are shown to be consistent with features in the current-voltage characteristics at high bias currents.<
>