Single-molecule magnet Mn12 on graphene

We study energetics, electronic and magnetic structures, and magnetic anisotropy barriers of a monolayer of single-molecule magnets (SMMs), [Mn₁₂O₁₂(COOR)16](H2O)4 (abbreviated as Mn₁₂, with R=H, CH₃, C₆H₅, and CHCl₂), on a graphene surface using spin-polarized density-functional theory with generalized gradient corrections and the inclusion of van der Waals interactions. We find that Mn₁₂ molecules with ligands -H, -CH3, and -C₆H₅ are physically adsorbed on graphene through weak van der Waals interactions, and a much stronger ionic interaction occurs using a -CHCl₂ ligand. The strength of bonding is closely related to the charge transfer between the molecule and the graphene sheet and can be manipulated by strain in the graphene; specifically, tension enhances n doping of graphene, and compression encourages p doping. The magnetic anisotropy barrier is computed by including the spin-orbit interaction within density-functional theory. The barriers for the Mn₁₂ molecules with ligands -H, -CH₃ and -C₆H₅ on graphene surfaces remain unchanged (within 1K) from those of isolated molecules because of their weak interaction, and a much larger reduction (10K) is observed when using the -CHCl2 ligand on graphene due to a substantial structural deformation as a consequence of the much stronger interaction. Neither strain in graphene nor charge transfer affects the magnetic anisotropy barrier significantly. Finally, we discuss the effect of strong correlation in the high-spin state of a Mn₁₂ SMM and the consequence of SMM-surface adsorption.