  We present a promising method for creating high-density ensembles of
nitrogen-vacancy centers with narrow spin-resonances for high-sensitivity
magnetic imaging. Practically, narrow spin-resonance linewidths substantially
reduce the optical and RF power requirements for ensemble-based sensing. The
method combines isotope purified diamond growth, in situ nitrogen doping, and
helium ion implantation to realize a 100 nm-thick sensing surface. The obtained
10^(17) cm^(-3) nitrogen-vacancy density is only a factor of 10 less than the
highest densities reported to date, with an observed spin resonance linewidth
over 10 times more narrow. The 200 kHz linewidth is most likely limited by
dipolar broadening indicating even further reduction of the linewidth is
desirable and possible.
