  Since the celebrated discovery of graphene, the family of two-dimensional
(2D) materials has grown to encompass a broad range of electronic properties.
Recent additions include spin-valley coupled semiconductors, Ising
superconductors that can be tuned into a quantum metal, possible Mott
insulators with tunable charge-density waves, and topological semi-metals with
edge transport. Despite this progress, there is still no 2D crystal with
intrinsic magnetism, which would be useful for many technologies such as
sensing, information, and data storage. Theoretically, magnetic order is
prohibited in the 2D isotropic Heisenberg model at finite temperatures by the
Mermin-Wagner theorem. However, magnetic anisotropy removes this restriction
and enables, for instance, the occurrence of 2D Ising ferromagnetism. Here, we
use magneto-optical Kerr effect (MOKE) microscopy to demonstrate that monolayer
chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin
orientation. Its Curie temperature of 45 K is only slightly lower than the 61 K
of the bulk crystal, consistent with a weak interlayer coupling. Moreover, our
studies suggest a layer-dependent magnetic phase transition, showcasing the
hallmark thickness-dependent physical properties typical of van der Waals
crystals. Remarkably, bilayer CrI3 displays suppressed magnetization with a
metamagnetic effect, while in trilayer the interlayer ferromagnetism observed
in the bulk crystal is restored. Our work creates opportunities for studying
magnetism by harnessing the unique features of atomically-thin materials, such
as electrical control for realizing magnetoelectronics, and van der Waals
engineering for novel interface phenomena.
