THE INFLUENCE OF AMBIENT MAGNETIC ENVIRONMENTS ON HIGH-T-C SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE GRADIOMETERS

The performance of high-temperature superconducting gradiometers have been evaluated in fields of up to 70 mu T to determine their limitatio ns in unshielded environments. The sensors incorporate first-order gra diometric superconducting quantum interference devices (SQUIDs) couple d to first-order gradiometric flux transformers. A gradiometer was ope rated successfully at all angles with respect to an ambient field with less than 20% variation in critical current and voltage/flux ratio. M agnetic hysteresis at 77 K was measured for field excursions up to 70 mu T. The hysteresis factor, referred to the SQUID, averaged 10(-3) Ph i(0)/mu T. Flux movement in the SQUID washers rather than the flux tra nsformer was found to be the dominant cause of the hysteresis and the presence of the flux transformer reduced the hysteresis. This is inter preted as due to the coupling coil pinning the vortices against radial motion. The spectral noise density of the gradiometer was only weakly dependent on the orientation with respect to static fields of up to 7 0 mu T except for narrow angular ranges where dominating random telegr aph signal processes occurred. In contrast, field cooling the gradiome ter above 15 mu T was found to substantially increase the low-frequenc y noise. A gradiometer was successfully transported and operated out o f doors with no shielding. Only a relatively minor degradation of perf ormance was observed. (C) 1996 American Institute of Physics.