  We use nominally forbidden electron-nuclear spin transitions in
nitrogen-vacancy (NV) centers in diamond to demonstrate coherent manipulation
of a nuclear spin ensemble using microwave fields at room temperature. We show
that employing an off-axis magnetic field with a modest amplitude($\approx$
0.01 T) at an angle with respect to the NV natural quantization axes is enough
to tilt the direction of the electronic spins, and enable efficient spin
exchange with the nitrogen nuclei of the NV center. We could then demonstrate
fast Rabi oscillations on electron-nuclear spin exchanging transitions,
coherent population trapping and polarization of nuclear spin ensembles in the
microwave regime. Coupling many electronic spins of NV centers to their
intrinsic nuclei offers full scalability with respect to the number of
controllable spins and provides prospects for transduction. In particular, the
technique could be applied to long-lived storage of microwave photons and to
the coupling of nuclear spins to mechanical oscillators in the resolved
sideband regime.
