Concentration dependent magnetism induced by hydrogen adsorption on graphene and single walled carbon nanotubes

We performed first principles calculations to study magnetic properties of hydrogenated graphene and single-walled carbon nanotubes (SWNTs) with different hydrogen concentrations. The hydrogen adsorptions on graphene and SWNTs generate localized states and accordingly flat bands near the Fermi level, opening substantial gaps. The magnetic properties of the compounds depend on hydrogen concentration. At high hydrogen concentration, the flat band splits into spin-up and spin-down branches located above and below the Fermi level, respectively, making the systems to have spontaneous magnetization. However, the spin-up and spin-down branches of the flat band are energetically degenerated at low hydrogen concentration and the systems are therefore nonmagnetic. This result is understandable from the point of view of direct interaction between unpaired π electrons of adjacent hydrogen-adsorption sites.