Fast-field-cycling, ultralow-field nuclear magnetic relaxation dispersion

TL;DR

This paper demonstrates the use of optically pumped magnetometers combined with fast-field-cycling hardware to measure nuclear magnetic relaxation dispersion, enabling insights into slow molecular motions and relaxation mechanisms in liquids.

Contribution

It introduces a novel application of OPMs with fast-field-cycling for quantifying relaxation phenomena across a wide magnetic field range, with high sensitivity and resolution.

Findings

Successfully measured relaxation rate dispersion from nT to mT fields.

Detected magnetic resonance signals from small liquid samples in porous materials.

Resolved inter-nucleus spin-spin couplings with high-resolution spectroscopy.

Abstract

Optically pumped magnetometers (OPMs) based on alkali-atom vapors are ultra-sensitive devices for dc and low-frequency ac magnetic measurements. Here, in combination with fast-field-cycling hardware and high-resolution spectroscopic detection, we demonstrate applicability of OPMs in quantifying nuclear magnetic relaxation phenomena. Relaxation rate dispersion across the nT to mT field range enables quantitative investigation of extremely slow molecular motion correlations in the liquid state, with time constants >1 ms, and insight into the corresponding relaxation mechanisms. The 10-20 fT/$\sqrt{\mathrm{Hz}}$ sensitivity of an OPM between 10 Hz and 5.5 kHz $^1$H Larmor frequency suffices to detect magnetic resonance signals from $\sim$0.1 mL liquid volumes imbibed in simple mesoporous materials, or inside metal tubing, following nuclear spin prepolarization adjacent to the OPM. High-resolution spectroscopic detection can resolve inter-nucleus spin-spin couplings, further widening the scope of application to chemical systems. Expected limits of the technique regarding measurement of relaxation rates above 100 s$^{-1}$ are discussed.