  A nitrogen-vacancy (NV) center in diamond is a promising sensor for nanoscale
magnetic sensing. Here we report electron spin resonance (ESR) spectroscopy
using a single NV center in diamond. First, using a 230 GHz ESR spectrometer,
we performed ensemble ESR of a type-Ib sample crystal and identified a
substitutional single nitrogen impurity as a major paramagnetic center in the
sample crystal. Then, we carried out free-induction decay and spin echo
measurements of the single NV center to study static and dynamic properties of
nanoscale bath spins surrounding the NV center. We also measured ESR spectrum
of the bath spins using double electron-electron resonance spectroscopy with
the single NV center. The spectrum analysis of the NV-based ESR measurement
identified that the detected spins are the nitrogen impurity spins. The
experiment was also performed with several other single NV centers in the
diamond sample and demonstrated that the properties of the bath spins are
unique to the NV centers indicating the probe of spins in the microscopic
volume using NV-based ESR. Finally, we discussed the number of spins detected
by the NV-based ESR spectroscopy. By comparing the experimental result with
simulation, we estimated the number of the detected spins to be $\leq$ 50
spins.
