  We have synthesized unique colloidal nanoplatelets of the ferromagnetic
two-dimensional (2D) van der Waals material CrI3 and have characterized these
nanoplatelets structurally, magnetically, and by magnetic circular dichroism
spectroscopy. The isolated CrI3 nanoplatelets have lateral dimensions of ~25 nm
and ensemble thicknesses of only ~4 nm, corresponding to just a few CrI3
monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D
ferromagnetic ordering in these nanoplatelets with Curie temperatures similar
to those observed in bulk CrI3, despite the strong spatial confinement. These
data also show magnetization steps akin to those observed in micron-sized
few-layer 2D sheets and associated with concerted spin-reversal of individual
CrI3 layers within few-layer van der Waals stacks. Similar data have also been
obtained for CrBr3 and anion-alloyed Cr(I1-xBrx)3 nanoplatelets. These results
represent the first example of laterally confined 2D van der Waals ferromagnets
of any composition. The demonstration of robust ferromagnetism at nanometer
lateral dimensions opens new doors for miniaturization in spintronics devices
based on van der Waals ferromagnets.
