High-T_c SQUIDs

Abstract

This chapter reviews the development of magnetometers based on high-T_c superconducting quantum interference devices (HTSC-SQUID) operated in liquid nitrogen. A HTSC-SQUID is usually made of a grain-boundary Josephson junction, and the SQUID inductance is directly connected to a pickup coil with a typical area of 10 mm x 10 mm. This type of magnetometer is called a direct-coupled magnetometer, and a field sensitivity around 50--100 fT/Hz^1/2 has been obtained at T = 77 K in the white noise region. In order to obtain such high performance SQUIDs, optimization of the SQUID parameters has been done, including the effects of the thermal noise at T = 77 K and the large dielectric constant of SrTiO₃ substrate. The main difficulty of the HTSC-SQUID was the strong 1/f noise. The 1/f noise arises both from the critical-current fluctuation of the junction and from the motion of vortices trapped in the superconducting thin film. The former noise has been solved by using readout electronics utilizing the so-called bias-reversal scheme. The latter noise has been avoided by optimizing the geometry of the pickup coil as well as by using the so-called flux dam. As a result, the 1/f noise of HTSC-SQUIDs has been much improved.