  Size-calibrated commercial nanodiamonds synthesized by high-pressure
high-temperature (HPHT) technique were studied by high-frequency W and
conventional X band electron paramagnetic resonance (EPR) spectroscopy. The
numbers of spins in the studied samples were estimated. The core-shell model of
the HPHT nanodiamonds was proposed to explain the observed dependence of the
concentration of the N0 paramagnetic centers. Two other observed paramagnetic
centers are attributed to the two types of structures in the nanodiamond shell.
