  Key to future spintronics and spin-based information processing technologies
is the generation, manipulation, and detection of spin polarization in a solid
state platform. Here, we theoretically explore an alternative route to spin
injection via the use of dynamically polarized nitrogen-vacancy (NV) centers in
diamond. We focus on the geometry where carriers and NV centers are confined to
proximate, parallel layers and use a 'trap-and-release' model to calculate the
spin cross-relaxation probabilities between the charge carriers and neighboring
NV centers. We identify near-unity regimes of carrier polarization depending on
the NV spin state, applied magnetic field, and carrier g-factor. In particular,
we find that unlike holes, electron spins are distinctively robust against
spin-lattice relaxation by other, unpolarized paramagnetic centers. Further,
the polarization process is only weakly dependent on the carrier hopping
dynamics, which makes this approach potentially applicable over a broad range
of temperatures.
