  Spin ensembles of nitrogen vacancy (NV) centers in diamond are emerging as
powerful spin-based sensors for magnetic, electric and thermal field imaging
with high spatial and temporal resolution. Here we characterize the formation
of depth-confined NV center ensembles, activated by electron irradiation in
diamond layers grown by plasma enhanced chemical vapor deposition with nitrogen
co-doping. To do so, we exploit the high magnetic sensitivity of ensembles of
NV centers to probe their spin environment as a function of growth and
irradiation parameters. We engineer an NV ensemble whose magnetic sensitivity
is within a factor of two of the static NV-NV dipolar interaction limit, thus
demonstrating a powerful platform for quantum sensing.
