NMR of room temperature samples with a flux-locked dc SQUID

Abstract

Superconducting quantum interference devices (SQUIDs) are the most sensitive detectors of magnetic fields. Since SQUIDs detect the magnetic flux rather than its rate of change, they can be used to great advantage to measure nuclear magnetic resonance (NMR) signals at low fields and frequencies. We have used a dc (direct-current) SQUID operated in flux-locked mode to significantly improve upon our previous low-field NMR results performed using an RF (radio-frequency) SQUID. The increase in sensitivity gained by using the dc SQUID has helped in reducing the signal acquisition time by a factor of more than 100 compared with our earlier measurements using an RP SQUID. We have also obtained a simple one-dimensional T/sub 1/-contrasted NMR image of a two-component sample consisting of mineral oil and tap water at room temperature. Our results highlight the sensitivity of the SQUID as an NMR detector and the promise of using SQUIDs in NMR imaging at low fields for both medical applications and for materials' nondestructive evaluation.