  The recent discovery of magnetism in atomically thin layers of van der Waals
(vdW) crystals has created new opportunities for exploring magnetic phenomena
in the two-dimensional (2D) limit. In most 2D magnets studied to date the
c-axis is an easy axis, so that at zero applied field the polarization of each
layer is perpendicular to the plane. Here, we demonstrate that atomically thin
CrCl3 is a layered antiferromagnetic insulator with an easy-plane normal to the
c-axis, that is the polarization is in the plane of each layer and has no
preferred direction within it. Ligand field photoluminescence at 870 nm is
observed down to the monolayer limit, demonstrating its insulating properties.
We investigate the in-plane magnetic order using tunneling magnetoresistance in
graphene/CrCl3/graphene tunnel junctions, establishing that the interlayer
coupling is antiferromagnetic down to the bilayer. From the temperature
dependence of the magnetoresistance we obtain an effective magnetic phase
diagram for the bilayer. Our result shows that CrCl3 should be useful for
studying the physics of 2D phase transitions and for making new kinds of vdW
spintronic devices.
