  The implementation of nuclear magnetic resonance (NMR) at the nanoscale is a
major challenge, as conventional systems require relatively large ensembles of
spins and limit resolution to mesoscopic scales. New approaches based on
quantum spin probes, such as the nitrogen-vacancy (NV) centre in diamond, have
recently achieved nano-NMR under ambient conditions. However, the measurement
protocols require application of complex microwave pulse sequences of high
precision and relatively high power, placing limitations on the design and
scalability of these techniques. Here we demonstrate a microwave-free method
for nanoscale NMR using the NV centre, which is a far less invasive, and vastly
simpler measurement protocol. By utilising a carefully tuned magnetic
cross-relaxation interaction between a subsurface NV spin and an external,
organic environment of proton spins, we demonstrate NMR spectroscopy of $^1$H
within a $\approx(10~{\rm nm})^3$ sensing volume. We also theoretically and
experimentally show that the sensitivity of our approach matches that of
existing microwave control-based techniques using the NV centre. Removing the
requirement for coherent manipulation of either the NV or the environmental
spin quantum states represents a significant step towards the development of
robust, non-invasive nanoscale NMR probes.
