  Certain two-dimensional (2D) materials exhibit intriguing properties such as
valley polarization, ferroelectricity, superconductivity and charge-density
waves. Many of these materials can be manually assembled into atomic-scale
multilayer devices under ambient conditions, owing to their exceptional
chemical stability. Efforts have been made to add a magnetic degree of freedom
to these 2D materials via defects, but only local magnetism has been achieved.
Only with the recent discoveries of 2D materials supporting intrinsic
ferromagnetism have stacked spintronic devices become realistic. Assembling 2D
multilayer devices with these ferromagnets under ambient conditions remains
challenging due to their sensitivity to environmental degradation, and magnetic
order at room temperature is rare in van der Waals materials. Here, we report
the growth of air-stable ultra-thin epsilon-phase iron oxide crystals that
exhibit magnetic order at room temperature. These crystals require no
passivation and can be prepared in large quantity by cost-effective chemical
vapor deposition (CVD). We find that the epsilon phase, which is energetically
unfavorable and does not form in bulk, can be easily made in 2D down to a seven
unit-cell thickness. Magneto-optical Kerr effect (MOKE) magnetometry of
individual crystals shows that even at this ultrathin limit the epsilon phase
exhibits robust magnetism with coercive fields of hundreds of mT. These
measurements highlight the advantages of ultrathin iron oxide as a promising
candidate towards air-stable 2D magnetism and integration into 2D spintronic
devices.
