Adsorption of Co and Ni on Graphene with a Double Hexagonal Symmetry: Electronic and Magnetic Properties

Inspired by Lie symmetries, we study the electronic and magnetic properties of cobalt (Co) and nickel (Ni) adatom adsorption on the graphene material using density functional theory calculations. The system we consider here consists of a static single layer of graphene interacting with transition-metal (TM) atoms. This system shows a nice geometrical shape having a double hexagonal structure appearing in the G(2) Lie algebra. This structure is associated 8 with 25% concentration corresponding to a coverage of 0.666 monolayers placed at H sites. This new symmetry forces the derived Co material to behave like a ferromagnetic metal with a strong spin polarization. However, the derived Ni material remains a nonmagnetic metal. For the Co case, we show that the magnetic mechanism responsible for such behavior is the interaction between the Co atoms. In fact, there are two interaction types. The first one is associated with the direct interaction between the Co atoms, while the second one corresponds to the indirect interaction via the carbon atoms. Using Monte Carlo simulation, the Curie temperature for the Co material is estimated to be around 438 K. This value could be explored in nanomagnetic applications.