  The recent isolation of two-dimensional van der Waals magnetic materials has
uncovered rich physics that often differs from the magnetic behaviour of their
bulk counterparts. However, the microscopic details of fundamental processes
such as the initial magnetization or domain reversal, which govern the magnetic
hysteresis, remain largely unknown in the ultrathin limit. Here we employ a
widefield nitrogen-vacancy (NV) microscope to directly image these processes in
few-layer flakes of magnetic semiconductor vanadium triiodide (VI$_3$). We
observe complete and abrupt switching of most flakes at fields
$H_c\approx0.5-1$ T (at 5 K) independent of thickness down to two atomic
layers, with no intermediate partially-reversed state. The coercive field
decreases as the temperature approaches the Curie temperature ($T_c\approx50$
K), however, the switching remains abrupt. We then image the initial
magnetization process, which reveals thickness-dependent domain wall depinning
fields well below $H_c$. These results point to ultrathin VI$_3$ being a
nucleation-type hard ferromagnet, where the coercive field is set by the
anisotropy-limited domain wall nucleation field. This work illustrates the
power of widefield NV microscopy to investigate magnetization processes in van
der Waals ferromagnets, which could be used to elucidate the origin of the hard
ferromagnetic properties of other materials and explore field- and
current-driven domain wall dynamics.
