A linearized Josephson-Fraunhofer magnetometer with high-temperature Josephson junction

We have fabricated a Josephson-Fraunhofer magnetometer based on the shrinking of the Fraunhofer pattern exhibited by high-T-c grain boundaries Josephson (GBJ) junctions. It is obtained using flux focuser and GBJ junction directly patterned on a single chip. The device is actively voltage biased and operates in a field locked loop as conventional superconducting quantum interference device systems. The amplitude of the auxiliary modulation field is adjusted to obtain the maximal sensitivity; the coupling between the feedback coil and the flux focuser has been studied to evaluate the magnetometer performances. The system bandwidth is 1 kHz and the measured noise level is 40 pT/root Hz above 10 Hz. The slew rate and the dynamic range of the magnetometer are 2 mT/s and 112 dB (1 Hz bandwidth), respectively. (C) 1996 American Institute of Physics.