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 mu m and a line width of 2 mu 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 mu Phi(0)/Hz(1/2). These measurements were made in a flux locked loop, using a direct coupled amplifier, without the use of additional positive feedback. The low noise is attributed to the high intrinsic responsivity of our sensors. Finally, flux gain measurements (the ratio between the measured magnetic flux and the flux applied to the micro pick-up coil), are reported.