Magnetism beyond the Monolayer: Long-Range Order in an Interfacial Phase of CoBr2

Transition metal dihalides are promising magnetic van der Waals materials because of their structural simplicity, chemical stability, and compatibility with nanofabrication. While magnetic order in these systems is typically associated with bulk-like monolayers, its relation to interfacial phases remains largely unexplored. Here we show that in CoBr2 grown on Au(111), long-range magnetic order already emerges in a structurally distinct interfacial precursor phase, prior to the formation of the bulk-like monolayer. During growth, this phase evolves into the monolayer, accompanied by a transition from molecular-like states to semiconducting behavior. Spin-resolved STM reveals an inhomogeneous in-plane magnetic response that is locally suppressed by point defects, whereas macroscopic XMCD demonstrates robust long-range order with pronounced in-plane anisotropy. These results establish that long-range magnetic order in CoBr2 does not require an ideal bulk-like monolayer but can emerge from interfacial phases with distinct electronic and structural properties, enabling magnetism beyond the monolayer limit.