  Near-surface nitrogen-vacancy (NV) centers have been created in diamond
through low energy implantation of 15N to sense electron spins that are
external to the diamond. By performing double resonance experiments, we have
verified the presence of g=2 spins on a diamond crystal that was subjected to
various surface treatments, including coating with a polymer film containing
the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH). Subsequent acid cleaning
eliminated the spin signal without otherwise disrupting the NV center,
providing strong evidence that the spins were at the surface. A clear
correlation was observed between the size of the detected spin signal and the
relaxation time T2 for the six NV centers studied. We have developed a model
that takes into account the finite correlation time of the fluctuating magnetic
fields generated by the external spins, and used it to infer the signal
strength and correlation time of the magnetic fields from these spins. This
model also highlights the sensitivity advantage of active manipulation of the
longitudinal spin component via double resonance over passive detection schemes
that measure the transverse component of spin.
