  The physical properties of two-dimensional van der Waals (2D vdW) crystals
depend sensitively on the interlayer coupling, which is intimately connected to
the stacking arrangement and the interlayer spacing. For example, simply
changing the twist angle between graphene layers can induce a variety of
correlated electronic phases, which can be controlled further in a continuous
manner by applying hydrostatic pressure to decrease the interlayer spacing. In
the recently discovered 2D magnets, theory suggests that the interlayer
exchange coupling strongly depends on layer separation, while the stacking
arrangement can even change the sign of the magnetic exchange, thus drastically
modifying the ground state. Here, we demonstrate pressure tuning of magnetic
order in the 2D magnet CrI3. We probe the magnetic states using tunneling and
scanning magnetic circular dichroism microscopy measurements. We find that the
interlayer magnetic coupling can be more than doubled by hydrostatic pressure.
In bilayer CrI3, pressure induces a transition from layered antiferromagnetic
to ferromagnetic phases. In trilayer CrI3, pressure can create coexisting
domains of three phases, one ferromagnetic and two distinct antiferromagnetic.
The observed changes in magnetic order can be explained by changes in the
stacking arrangement. Such coupling between stacking order and magnetism
provides ample opportunities for designer magnetic phases and functionalities.
