  Quantum sensors have attracted broad interest in the quest towards
sub-micronscale NMR spectroscopy. Such sensors predominantly operate at low
magnetic fields. Instead, however, for high resolution spectroscopy, the
high-field regime is naturally advantageous because it allows high absolute
chemical shift discrimination. Here we propose and demonstrate a high-field
spin magnetometer constructed from an ensemble of hyperpolarized ${}^{13}C$
nuclear spins in diamond. The ${}^{13}C$ nuclei are initialized via Nitrogen
Vacancy (NV) centers and protected along a transverse Bloch sphere axis for
minute-long periods. When exposed to a time-varying (AC) magnetic field, they
undergo secondary precessions that carry an imprint of its frequency and
amplitude. The method harnesses long rotating frame ${}^{13}C$ sensor lifetimes
$T_2^{\prime}{>}$20s, and their ability to be continuously interrogated. For
quantum sensing at 7T and a single crystal sample, we demonstrate spectral
resolution better than 100 mHz (corresponding to a frequency precision
${<}$1ppm) and single-shot sensitivity better than 70pT. We discuss the
advantages of nuclear spin magnetometers over conventional NV center sensors,
including deployability in randomly-oriented diamond particles and in optically
scattering media. Since our technique employs densely-packed ${}^{13}C$ nuclei
as sensors, it demonstrates a new approach for magnetometry in the
"coupled-sensor" limit. This work points to interesting opportunities for
microscale NMR chemical sensors constructed from hyperpolarized nanodiamonds
and suggests applications of dynamic nuclear polarization (DNP) in quantum
sensing.
