Zero field NMR is the acquisition of NMR spectra in an environment carefully screened from magnetic fields (Including from the Earth's field). It is useful for studying chemicals with magnetically active nuclei (spins 1/2 and greater), and for studying molecular dynamics.
Since the development of very sensitive magnetic sensors (SQUID, GMR, atomic magnetometer) it has become possible (in the 2000s) to detect NMR signals directly in zero-field environments. Previous zero-field NMR experiments relied on indirect detection where the sample had to be shuttled from the shielded zero-field environment into a high magnetic field for detection with a conventional inductive pick-up coil.
Most successful was the implementation of atomic magnetometers at zero-magnetic field working with Rubidium vapor cells to detect zero-field NMR.
It is sometimes but inaccurately referred to as nuclear quadrupole resonance(NQR).