  Dynamic nuclear polarisation, which transfers the spin polarisation of
electrons to nuclei, is routinely applied to enhance the sensitivity of nuclear
magnetic resonance; it is also critical in spintronics, particularly when spin
hyperpolarisation can be produced and controlled optically or electrically.
Here we show the complete polarisation of nuclei located near the
optically-polarised nitrogen-vacancy (NV) centre in diamond. When approaching
the ground-state level anti-crossing condition of the NV electron spins, 13C
nuclei in the first-shell are polarised in a pattern that depends sensitively
and sharply upon the magnetic field. Based on the anisotropy of the hyperfine
coupling and of the optical polarisation mechanism, we predict and observe a
complete reversal of the nuclear spin polarisation with a few-mT change in the
magnetic field. The demonstrated sensitive magnetic control of nuclear
polarisation at room temperature will be useful for sensitivity-enhanced NMR,
nuclear-based spintronics, and quantum computation in diamond.
