  Thin layers of near-surface nitrogen-vacancy (NV) defects in diamond
substrates are the workhorse of NV-based widefield magnetic microscopy, which
has applications in physics, geology and biology. Several methods exist to
create such NV layers, which generally involve incorporating nitrogen atoms (N)
and vacancies (V) into the diamond through growth and/or irradiation. While
there have been detailed studies of individual methods, a direct side-by-side
experimental comparison of the resulting magnetic sensitivities is still
missing. Here we characterise, at room and cryogenic temperatures, $\approx100$
nm thick NV layers fabricated via three different methods: 1) low-energy carbon
irradiation of N-rich high-pressure high-temperature (HPHT) diamond, 2) carbon
irradiation of $\delta$-doped chemical vapour deposition (CVD) diamond, 3)
low-energy N$^+$ or CN$^-$ implantation into N-free CVD diamond. Despite
significant variability within each method, we find that the best HPHT samples
yield similar magnetic sensitivities (within a factor 2 on average) to our
$\delta$-doped samples, of $<2$~$\mu$T Hz$^{-1/2}$ for DC magnetic fields and
$<100$~nT Hz$^{-1/2}$ for AC fields (for a $400$~nm~$\times~400$~nm pixel),
while the N$^+$ and CN$^-$ implanted samples exhibit an inferior sensitivity by
a factor 2-5, at both room and low temperature. We also examine the crystal
lattice strain caused by the respective methods and discuss the implications
this has for widefield NV imaging. The pros and cons of each method, and
potential future improvements, are discussed. This study highlights that
low-energy irradiation of HPHT diamond, despite its relative simplicity and low
cost, is a competitive method to create thin NV layers for widefield magnetic
imaging.
