  The pivotal role of magnetic anisotropy in stabilising two-dimensional (2D)
magnetism has been widely accepted, however, direct correlation between
magnetic anisotropy and long-range magnetic ordering in the 2D limit is yet to
be explored. Here, using angle- and temperature-dependent tunnelling
magnetoresistance, we report unprecedented metamagnetic phase transitions in
atomically-thin CrOCl, triggered by magnetic easy-axis flipping instead of the
conventional spin flop mechanism. Few-layer CrOCl tunnelling devices of various
thicknesses consistently show an in-plane antiferromagnetic (AFM) ground state
with the easy axis aligned along the Cr-O-Cr direction (b-axis). Strikingly,
with the presence of a magnetic field perpendicular to the easy-axis (H||c),
magnetization of CrOCl does not follow the prevalent spin rotation and
saturation pattern, but rather exhibits an easy-axis flipping from the in-plane
to out-of-plane directions. Such magnetic anisotropy controlled metamagnetic
phase transitions are manifested by a drastic upturn in tun- nelling current,
which shows anomalous shifts towards higher H when temperature increases. By 2D
mapping of tunnelling currents as a function of both temperature and H, we
determine a unique ferrimagnetic state with a superstructure periodicity of
five unit cells after the field-induced metam- agnetic transitions. The
feasibility to control 2D magnetism by manipulating magnetic anisotropy may
open enormous opportunities in spin-based device applications.
