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 limitations in unshielded environments. The sensors incorporate first-order gradiometric superconducting quantum interference devices (SQUIDs) coupled to first-order gradiometric flux transformers. A gradiometer was operated successfully at all angles with respect to an ambient field with less than 20% variation in critical current and voltage/flux ratio. Magnetic hysteresis at 77 K was measured for field excursions up to 70 mu T. The hysteresis factor, referred to the SQUID, averaged 10(-3) Phi(0)/mu T. Flux movement in the SQUID washers rather than the flux transformer was found to be the dominant cause of the hysteresis and the presence of the flux transformer reduced the hysteresis. This is interpreted 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 70 mu T except for narrow angular ranges where dominating random telegraph signal processes occurred. In contrast, field cooling the gradiometer above 15 mu T was found to substantially increase the low-frequency noise. A gradiometer was successfully transported and operated out of doors with no shielding. Only a relatively minor degradation of performance was observed. (C) 1996 American Institute of Physics.