  Using a Hartman-Hahn protocol, we demonstrate spin polarization transfer from
a single, optically-polarized nitrogen-vacancy (NV) center to the ensemble of
paramagnetic defects hosted by an individual diamond nanocrystal. Owing to the
strong NV-bath coupling, the transfer takes place on a short, microsecond time
scale. Upon fast repetition of the pulse sequence we observe strong
polarization transfer blockade, which we interpret as an indication of spin
bath cooling. Numerical simulations indicate that the spin bath polarization is
non-uniform throughout the nanoparticle averaging approximately 5% over the
crystal volume, but reaching up to 25% in the immediate vicinity of the NV.
These observations may prove relevant to the planning of future bath-assisted
magnetometry tests.
