Low field nuclear magnetic resonance

Low field NMR spans a range of different nuclear magnetic resonance (NMR) modalities, going from NMR conducted in permanent magnets, supporting magnetic fields of a few T, all the way down to zero field NMR, where the Earth's field is carefully shielded such that magnetic fields of nT are achieved where nuclear spin precession is close to zero. In a broad sense, "Low-field NMR" is the branch of nuclear magnetic resonance that is NOT conducted in superconducting high-field magnets. Low field NMR also includes Earth's field NMR where simply the Earth's field is exploited to cause nuclear spin-precession which is detected. With magnetic fields on the order of μT and below magnetometers such as SQUIDs or atomic magnetometers (among others) are used as detectors. "Normal" high field NMR relies on the detection of spin-precession with inductive detection with a simple coil. 
However, this detection modality becomes less sensitive as the magnetic field and the associated frequencies decrease. Hence the push toward alternative detection methods at very low fields.