Axial high-temperature superconducting-quantum-interference-device gradiometer composed of magnetometers with a monolithic feedback and compensation coil

A first-order axial gradiometer has been constructed using two high-critica l-temperature superconducting-quantum-interference-device magnetometers hav ing a double-pickup loop configuration on bicrystal substrates. The magneto meter, which had a slot-type structure but no weak links on the pickup loop s, can be operated steadily in an unshielded environment after in situ cool ing. The gradiometer was formed by connecting the feedback and the compensa tion coils of the two magnetometers in series using a normal conductive wir e. Sufficiently high noise reduction (31 dB at 1 Hz) was achieved without a ny need for adjustments and low-noise magnetocardiograms were obtained with the gradiometer in a moderately magnetically shielded room. (C) 2001 Ameri can Institute of Physics.