  Since their first observation in 2017, atomically thin van der Waals (vdW)
magnets have attracted significant fundamental, and application-driven
attention. However, their low ordering temperatures, $T_c$, sensitivity to
atmospheric conditions and difficulties in preparing clean large-area samples
still present major limitations to further progress. The remarkably stable
high-$T_c$ vdW magnet CrSBr has the potential to overcome these key
shortcomings, but its nanoscale properties and rich magnetic phase diagram
remain poorly understood. Here we use single spin magnetometry to
quantitatively characterise saturation magnetization, magnetic anisotropy
constants, and magnetic phase transitions in few-layer CrSBr by direct magnetic
imaging. We show pristine magnetic phases, devoid of defects on micron
length-scales, and demonstrate remarkable air-stability down the monolayer
limit. We address the spin-flip transition in bilayer CrSBr by direct imaging
of the emerging antiferromagnetic (AFM) to ferromagnetic (FM) phase wall and
elucidate the magnetic properties of CrSBr around its ordering temperature. Our
work will enable the engineering of exotic electronic and magnetic phases in
CrSBr and the realisation of novel nanomagnetic devices based on this highly
promising vdW magnet.
