  We report hyperpolarization of the electronic spins associated with
substitutional nitrogen defects in bulk diamond crystal. Hyperpolarization is
achieved by optical pumping of nitrogen vacancy centers followed by rapid cross
relaxation at the energy level matching condition in a 51 mT bias field. The
maximum observed donor spin polarization is 0.9 \% corresponding to an
enhancement by 25 compared to the thermal Boltzmann polarization. A further
accumulation of polarization is impeded by an anomalous optical saturation
effect that we attribute to charge state conversion processes. Hyperpolarized
nitrogen donors may form a useful resource for increasing the efficiency of
diamond-based dynamic nuclear polarization devices.
