Scanning SQUID Microscopy for Sensing Vector Magnetic Field

We report a vector-scanning SQUID microscope for probing a vector magnetic field. We designed and fabricated a vector [three-dimensional (3-D)] SQUID sensor with the aid of Nb multilayer fabrication technology. Our vector SQUID sensor on a single chip is composed of a vector sensor with three orthogonal pick-up coils, which catch the X , Y, and Z magnetic field components. We improved both the sensitivity and the spatial resolution not only by reducing the inner diameter but also by increasing the number of pick-up-coil windings within the framework of the multilayer technique. Our SQUID sensors operated properly, and the characteristics showed a good agreement with our design parameters. Since the sensor is suitable for our SQUID microscope, we are constructing our scanning microscope platform by using a Gifford-McMahon cryocooler for allowing long-time measurements. We also succeeded in connecting our vector SQUID sensor to a commercial flux-locked loop readout circuit (Magnicon Inc., Model XXF-1) to generate the voltage outputs, which are proportional to the XYZ components of the external magnetic field. We also use an XYZ piezo-driven scanner for the precise control of the pick-up-coil positions over the 3-D range of 5 × 5 × 5 mm 3 with a step size of 10 nm at 4 K.