Reversible Phases and Magnetic Properties of Two-Dimensional Layered Iron Bromide Compounds

2D van der Waals (vdW) magnetic materials have gained significant attention in recent years due to their potential applications in spintronics. ^[1-3] Among them, binary transition metal halides (BTMHs) are considered as promising candidates with tunable insulating bandgap ^[2] and extremely high tunneling magnetoresistance ^[4]. Here, we used low-temperature scanning tunneling microscopy (LT-STM) to study the epitaxy growth of FeBrₓ on Au(111). By evaporating iron dibromide (FeBr₂) source onto the Au(111) surface at room-temperature (RT), four distinct FeBrₓ phases (x≤2) are observed within sub-monolayer thickness, including (√7×√7), (3×6), (1×1), and (3×3) superstructures. The formations of these superstructures are reversible, depending on the coverages and annealing process. Additionally, X-ray magnetic circular dichroism (XMCD) measurements indicate that the FeBr₂ monolayer exhibits ferromagnetic behavior, while the bilayer is antiferromagnetic. Combining with first-principles calculations, we find that the distinct FeBrₓ phases demonstrate diverse magnetic ordering. This study provides a valuable platform to understand the crystallization processes of 2D BTMHs.