SQUID Magnetometric System and SQUID Parameters

This chapter introduces the concept of a Superconducting QUantum Interference Device (SQUID) magnetometric system. For a SQUID magnetometer, one strives for low magnetic field noise (i.e. high magnetic field sensitivity), which involves two aspects: a field-to-flux transformer circuit (converter) and an ordinary SQUID system with a flux locked loop. The chapter discusses the requirements of the converter and shows that the SQUID system is characterized by three dimensionless characteristic parameters, which determine the SQUID system noise power i.e. not only the SQUID intrinsic noise, but also the readout electronics noise. If a closed field-to-flux transformer circuit already exists, the SQUID magnetometric system is governed by the properties of the SQUID system. From the perspective of physics, SQUID operation is based on two effects: flux quantization and the Josephson effect. The estimated SQUID intrinsic noise is much higher than the pure SQUID thermal noise at 4.2 K caused by the junction's resistance.