  High frequency electron spin resonance (ESR) spectroscopy is an invaluable
tool for identification and characterization of spin systems. Nanoscale ESR
using the nitrogen-vacancy (NV) center has been demonstrated down to the level
of a single spin. However, NV-detected ESR has exclusively been studied at low
magnetic fields, where spectral overlap prevents clear identification of
spectral features. Within this work, we demonstrate NV-detected ESR
measurements of single-substitutional nitrogen impurities in diamond at a NV
Larmor frequency of 115 GHz and the corresponding magnetic field of 4.2 Tesla.
The NV-ESR measurements utilize a double electron-electron resonance sequence
and are performed using both ensemble and single NV spin systems. In the single
NV experiment, chirp pulses are used to improve the population transfer and for
NV-ESR measurements. This work provides the basis for NV-based ESR measurements
of external spins at high magnetic fields.
