Low Critical Temperature dc-SQUIDs for High Spatial Resolution Applications

We present the design and the performances of LTc
SQUIDs with integrated micro pick-up coils to employ as magnetic
field sensors in high spatial resolution measurements. The circular
pick-up coils, having an inner diameter ranging from 5 to 50 m
and a line width of 2 m, are well separated (1.5 mm) from the
SQUID body to minimize magnetic interferences.
The micro pick-up coils are connected to the SQUID by both
magnetometer and gradiometric configurations. All SQUID types
contain an integrated modulation/feedback coil for flux locked loop
operations. At T = 4 2 K, the sensors have a measured white
magnetic flux noise spectral density as low as 3 8
0
Hz
1 2
. These
measurements were made in a flux locked loop, using a direct cou-
pled amplifier, without the use of additional positive feedback. The
low noise is attributed to the high intrinsic responsivity of our sen-
sors. Finally, flux gain measurements (the ratio between the mea-
sured magnetic flux and the flux applied to the micro pick-up coil),
are reported.