Direct Observation of Sublattice-Dependent Magnetic Ordering and Strong Electron-Magnon Coupling in Fe_{5}GeTe_{2}.

The van der Waals room-temperature ferromagnet Fe_{5}GeTe_{2} has garnered considerable research interest due to its intriguing magnetic and transport properties, as well as its potential applications, yet their microscopic origin remains unclear. Using angle-resolved photoemission spectroscopy and density functional theory calculations, we resolve an electronic structure corresponding to the up-up-up-type configuration. Pronounced Fermi surface reconstruction and a 25 meV dispersion kink referring to the strong electron-magnon coupling have been observed near T_{1}∼105  K, coincident with anomalies in resistivity and magnetic susceptibility. Our calculations identify the emergence of Fe(1) sublattice spin order as the driving force behind both phenomena. These findings reveal the decisive role of sublattice-dependent magnetism in governing the coupled electronic and magnetic properties of Fe_{5}GeTe_{2}, establishing sublattice engineering as a pathway for tuning correlated behavior in two-dimensional magnets.