Interaction of magnetic transition metal dimers with spin-polarized hydrogenated graphene.

The coadsorption of hydrogen and transition metal dimers Fe2, Co2, Ni2, and FeCo on graphene is investigated using density functional theory calculations. Our work is motivated by observations that the magnetic moments of these transition metal dimers are large and that hydrogen adsorption partitions the graphene lattice into magnetic subdomains. Thus, we expect the magnetic dimers to interact strongly with the lattice. Our results show that the majority-spin direction of the lattice electronic states depends upon the dimer identity, the lattice spin polarization being in the same direction as the dimer spin polarization for Fe2 and FeCo, but opposite for Co2 and Ni2. We can understand this by examining the electronic density of states of the dimer and the lattice. We also show that coadsorption significantly increases the adsorption energies of both dimer and hydrogen leading to a more strongly-adsorbed dimer, while the bond length and magnetic moment of the upper dimer atom, the latter important for potential magnetic storage applications, are negligibly changed. Our work shows that the coadsorbed hydrogen and metal dimer interact over a long-range, this interaction being mediated by the hydrogen-induced spin-polarization of the graphene lattice. We obtain general insight into how the elemental identity of these magnetic dimers determines the spin-polarized states on the hydrogenated graphene lattice. These results could be important for potential applications of magnetic properties of decorated graphene lattices.