Application of superconducting quantum interference devices to nuclear magnetic resonance

Nuclear magnetic resonance (NMR) provides information in low polarizing fields that is hard to obtain in high fields. A new generation of sensitive NMR detectors, the superconducting quantum interference device (SQUID), provides a fresh approach to low-field NMR studies. The SQUID is an ideal detector for low-field NMR, since its response does not depend on signal frequency as is the case of conventional NMR spectrometers. This review describes the experimental and theoretical studies in which SQUIDs have been used for the detection of NMR. Particular attention is paid to the calculation of the signal-to-noise ratio of SQUID NMR spectrometers with various input configurations as compared to that of conventional ones. The application of SQUIDs to nuclear thermometry and to absolute field measurements are also discussed. A SQUID directly measures the longitudinal nuclear magnetization $M_z$ and the review discusses extensively what we call $M_z$ spectroscopy.