Submonolayer Eu superstructures—A class of 2D magnets

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.