High Temperature Ferromagnetism in Epitaxial Monolayers of Co-Doped Fe_{5}GeTe_{2}.

Magnetic van der Waals materials have mainly been investigated in their bulk form or as few-layer flakes. Because of the challenges in producing atomically thin films, only a few have been isolated as monolayers, which typically exhibit long-range magnetic order below 150 K. In this Letter, we use molecular beam epitaxy to synthesize Co-doped Fe_{5}GeTe_{2}, achieving precise control over both thickness and composition. We demonstrate ferromagnetism well above room temperature in multilayer samples and present clear evidence of ferromagnetic ordering in monolayers up to ∼200  K. The changes in Curie temperature and magnetic anisotropy as a function of composition exhibit similar trends in both monolayers and thicker films, indicating that the magnetic properties are primarily governed by intralayer magnetic interactions. Through element-specific x-ray magnetic circular dichroism and density-functional theory, we identify the substitution site of Co dopants and reveal the mechanism behind the Curie temperature enhancement induced by Co doping. Our findings suggest that, despite their weak magnetic moment, Co dopants strengthen the magnetic moments on neighboring Fe atoms and enhance the intralayer ferromagnetic exchange interactions.