  Quantum sensors based on solid-state spins provide tremendous opportunities
in a wide range of fields from basic physics and chemistry to biomedical
imaging. However, integrating them into a scanning probe microscope to enable
practical, nanoscale quantum imaging is a highly challenging task. Recently,
the use of single spins in diamond in conjunction with atomic force microscopy
techniques has allowed significant progress towards this goal, but
generalisation of this approach has so far been impeded by long acquisition
times or by the absence of simultaneous topographic information. Here we report
on a scanning quantum probe microscope which solves both issues, by employing a
nano-spin ensemble hosted in a nanodiamond. This approach provides up to an
order of magnitude gain in acquisition time, whilst preserving sub-100 nm
spatial resolution both for the quantum sensor and topographic images. We
demonstrate two applications of this microscope. We first image nanoscale
clusters of maghemite particles through both spin resonance spectroscopy and
spin relaxometry, under ambient conditions. Our images reveal fast magnetic
field fluctuations in addition to a static component, indicating the presence
of both superparamagnetic and ferromagnetic particles. We next demonstrate a
new imaging modality where the nano-spin ensemble is used as a thermometer. We
use this technique to map the photo-induced heating generated by laser
irradiation of a single gold nanoparticle in a fluid environment. This work
paves the way towards new applications of quantum probe microscopy such as
thermal/magnetic imaging of operating microelectronic devices and magnetic
detection of ion channels in cell membranes.
