  The sensitivity of the nitrogen-vacancy (NV) color centers in diamond-based
magnetometers strongly depends on the number of NV centers involved in the
measurement. Unfortunately, an increasing concentration of NV centers leads to
decreases of their dephasing and coherence time if the nitrogen content exceeds
a certain threshold level (approximately ${10^{17}}{\rm{ c}}{{\rm{m}}^{ -
3}}$). Here, we demonstrate that this increased dephasing can be efficiently
compensated by postprocessing procedures in the vicinity of the threshold
concertation, thus extending possible useful concentrations on NV centers with
the maximum possible decoherence time in diamonds with a natural carbon
content.
