Two-dimensional (2D) magnetic materials promise unconventional properties and quantum phases as well as advances in ultra-compact spintronics. Miniaturization of 2D magnets often reaches a single monolayer but in general can go beyond this limit, as demonstrated by 2D magnetism of submonolayer Eu superstructures coupled with Si. The question is whether the submonolayer magnetism constitutes a general phenomenon. Herein, we demonstrate that regular Eu lattices form a class of 2D magnets displaying various structures, stoichiometries, and chemical bonding. We synthesized and studied a set of Eu superstructures on Ge(001). Their magnetic properties are consistent with the emergence of a magnetic order such as ferro- or ferrimagnetism. In particular, control over the magnetic transition temperature by weak magnetic fields indicates the 2D nature of the magnetism. Taken together, Eu/Ge and Eu/Si superstructures seed a nucleus of the research area addressing the emergence of magnetism in submonolayer chemical species.

Recent discoveries of intrinsic two-dimensional (2D) magnets open up vast opportunities to address fundamental problems in condensed matter physics, giving rise to applications from ultra-compact spintronics to quantum computing. The ever-growing material landscape of 2D magnets lacks, however, carbon-based systems, prominent in other areas of 2D research. Magnetization measurements of the Eu/graphene compound—a monolayer of the EuC₆ stoichiometry—reveal the emergence of 2D ferromagnetism but detailed studies of competing magnetic states are still missing. Here, we employ element-selective X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) to establish the magnetic structure of monolayer EuC₆. The system exhibits the anomalous Hall effect, negative magnetoresistance, and magnetization consistent with a ferromagnetic state but the saturation magnetic moment (about 2.5 µ_B/Eu) is way too low for the half-filled f-shells of Eu²⁺ ions. Combined XAS/XMCD studies at the Eu L₃ absorption edge probe the EuC₆ magnetism in high fields and reveal the nature of the missing magnetic moments. The results are set against XMCD studies in Eu/silicene and Eu/germanene to establish monolayer EuC₆ as a prominent member of the family of Eu-based 2D magnets combining the celebrated graphene properties with a strong magnetism of europium.