Overview of low-field NMR measurements using HTS rf-SQUIDs

High-resolution low-field nuclear magnetic resonance (LF-NMR) investigations of liquid samples, recorded using a HTS radio frequency (rf) superconducting quantum interference device (SQUID) are overviewed in this paper. The measurements were performed either in a magnetically shielded room (MSR) or in the Earth’s Magnetic Field (EMF). In MSR, measurements with Larmor frequencies ( f L) ranging from 2 Hz to 40 kHz were demonstrated. The natural spectral linewidth of water and the scalar coupling spectra of 2,2,2-trifluoroethanol and of fluorobenzene were determined. An additional nitrogen-cooled resonant LC input circuit was also introduced in higher f L (>10 kHz) region to enhance the signal-to-noise ratio up to an order of magnitude. The influence of the speed of decay of the polarizing magnetic field ( B P) on the free-induction-decay (FID) signal was analyzed quantitatively. In EMF, NMR spectra of different liquid samples were recorded. To compensate the line broadening due to EMF fluctuations we used frequency-adjusted averaging.