Mn. Keene et al., THE INFLUENCE OF AMBIENT MAGNETIC ENVIRONMENTS ON HIGH-T-C SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE GRADIOMETERS, Journal of applied physics, 79(11), 1996, pp. 8783-8791
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.