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.