  We investigate the photo-induced spin dynamics of single nitrogen-vacancy
(NV) centres in diamond near the electronic ground state level anti-crossing
(GSLAC), which occurs at an axial magnetic field around 1024 G. Using optically
detected magnetic resonance spectroscopy, we first find that the electron spin
transition frequency can be tuned down to 100 kHz for the \NV{14} centre, while
for the \NV{15} centre the transition strength vanishes for frequencies below
about 2 MHz owing to the GSLAC level structure. Using optical pulses to prepare
and readout the spin state, we observe coherent spin oscillations at 1024 G for
the \NV{14}, which originate from spin mixing induced by residual transverse
magnetic fields. This effect is responsible for limiting the smallest
observable transition frequency, which can span two orders of magnitude from
100 kHz to tens of MHz depending on the local magnetic noise. A similar feature
is observed for the \NV{15} centre at 1024 G. As an application of these
findings, we demonstrate all-optical detection and spectroscopy of
externally-generated fluctuating magnetic fields at frequencies from 8 MHz down
to 500 kHz, using a \NV{14} centre. Since the Larmor frequency of most nuclear
spin species lies within this frequency range near the GSLAC, these results
pave the way towards all-optical, nanoscale nuclear magnetic resonance
spectroscopy, using longitudinal spin cross-relaxation.
