Application of magnetoresistive sensors to zero- and ultralow-field nuclear magnetic resonance

Zero- to ultralow-field nuclear magnetic resonance (ZULF NMR) is an alternative modality to conventional high-field (HF) NMR, where J-couplings dominate over the Zeeman interaction in the absence of large external magnetic fields. While HF NMR relies on induction detection, several alternative detection modalities have been demonstrated in the ZULF regime, with superconducting quantum interfering devices (SQUIDs) and optically-pumped magnetometers (OPMs) as leading options. In this work, we report the first NMR detection of J-couplings in molecules using magnetoresistive (MR) sensors, and discuss their advantages and limitations. To demonstrate the versatility of these sensors, we detected NMR signals from thermally polarized analytes (at 2 T) and analytes that were hyperpolarized by parahydrogen- induced polarization (PHIP). The key advantages of MR sensors include room-temperature operation, high dynamic range (from zero to geomagnetic fields), and optimal geometry for detection of minute (e.g., microfluidic) samples. To illustrate this, we performed microfluidic experiments using a commercial OPM as a benchmark. We quantified a trade-off between signal-to- noise ratio (SNR) and a detrimental effect of the magnetization of the MR sensor itself for stand-off distances below ∼5 mm. We discuss demagnetization protocols to circumvent this issue, which could be used in any scenario where traces of ferromagnetic material are unavoidable. Our work paves the way for MR sensors to become a standard choice for NMR experiments across a wide range of magnetic fields, offering benefits in cost-effectiveness, miniaturization, and large operational dynamic range.