Constructing a Vector Scanning SQUID System

Abstract

We succeeded in fabricating a vector scanning SQUID sensor, which was optimized to reduce the noise and to enhance the sensitivity by choosing the screening parameter as βL = 1. The sensors were fabricated by a superconductive foundry CRAVITY of Advanced Industrial Science and Technology (AIST) using AIST-STP2 standard process with the critical current density Jc of Josephson junctions (JJs) 320 A/cm2. Under this condition, the critical current Ic becomes 12.5 µA in the areas of 2.2 µm × 2.2 µm of our SQUID. The sensitivity and resolution of sensors were also improved by reducing inner diameter and increasing the number of the pick-up-coil windings with employing of multi-layer technique. We designed the structure of vector pickup coils, of which the same distance is kept between each coil and the sample surface. In addition, the distance between the coils were just set at an integer multiple of a scanning step so that it becomes convenient in reconstructing a magnetic-field vector at a certain position. The fabricated SQUID sensors were operated properly, and characteristics were in good agreement with our design parameters. In order to construct the vector scanning SQUID microscope, we succeeded in connecting our sensor with a commercial flux-locked loop (FLL) readout circuit (Magnicon Inc., Model XXF-1). The spatial resolution of vector SQUID microscopy can be improved by using an XYZ piezo-driven scanner for the precise control of the pick-up-coil positions over the three-dimensional range of 5 mm × 5 mm × 5 mm with a step size of 10 nm at 4K. A Gifford–McMahon cryocooler with an anti-vibration mechanism will be used to conduct long-time measurements.