NMR Relaxometry across ultra-wide range fields using atomic magnetometers

Nuclear magnetic relaxation dispersion (NMRD) measurements report on molecular dynamics via the field-dependence of longitudinal (R₁) and transverse (R₂) relaxation rates. Commercial fast-field-cycling (FFC) relaxometers cover the kHz to MHz regime using large electromagnets, but show reduced sensitivity at lower frequencies. Optically pumped magnetometer based ultra-low-field (ULF) methods are highly sensitive to low frequencies (Hz to kHz), making them sensitive to slow molecular dynamics, but are restricted to the sub-kilohertz band. In this work, we introduce a compact shuttle-based relaxometry system that combines high-field prepolarization with programmable relaxation fields from nanotesla to tesla, and uses a set of multi-channel optically pumped magnetometers for arrayed detection. To benchmark the system's sensitivity and reproducibility we performed initial experiments on water samples, and then studied the field-dependent relaxivity of Gd-DTPA. These results agree quantitatively with a commercial FFC relaxometer across the accessible frequency range of three orders of magnitude (10 kHz to 20 MHz), and our setup extended the measurement range to eight orders of magnitude (Hz to 100 MHz). We also performed measurements on aqueous metal-organic framework solutions, highlighting the ability of ULF detection to mitigate susceptibility-induced internal field gradients. This platform enables wide-range, quantitative relaxometry, naturally interfaces with low-field hyperpolarization and supports applications in biomedical sensing of contrast agents and metabolites, porous-media and materials studies, and on-site chemical screening.