  Controlling the crystal structure is a powerful approach for manipulating the
fundamental properties of solids. Unique to two-dimensional (2D) van der Waals
materials, the control can be achieved by modifying the stacking order through
rotation and translation between the layers. Here, we report the first
observation of stacking dependent interlayer magnetism in the 2D magnetic
semiconductor, chromium tribromide (CrBr3), enabled by the successful growth of
its monolayer and bilayer through molecular beam epitaxy. Using in situ
spin-polarized scanning tunneling microscopy and spectroscopy, we directly
correlated the atomic lattice structure with observed magnetic order. We
demonstrated that while individual CrBr3 monolayer is ferromagnetic, the
interlayer coupling in bilayer depends strongly on the stacking order and can
be either ferromagnetic or antiferromagnetic. Our observations provide direct
experimental evidence for exploring the stacking dependent layered magnetism,
and pave the way for manipulating 2D magnetism with unique layer twist angle
control.
