Based on numerically accurate density functional theory calculations, we systematically investigate the ground-state structure and the spin and orbital magnetism including the magnetic anisotropy energy (MAE) of $3d$- and $4d$-transition-metal dimer-benzene complexes $\left({\mathrm{TM}}_2\text{Bz},\mathrm{TM}=\text{Fe},\text{Co},\text{Ni},\text{Ru},\text{Rh},\text{Pd};\text{Bz}={\text{C}}_6{\text{H}}_6\right)$. These systems are chosen to model $\mathrm{TM}$-dimer adsorption on graphene or on graphite. We find that ${\text{Fe}}_2$, ${\text{Co}}_2$, ${\text{Ni}}_2$, and ${\text{Ru}}_2$ prefer the upright adsorption mode above the center of the benzene molecule, while ${\text{Rh}}_2$ and ${\text{Pd}}_2$ are adsorbed parallel to the benzene plane. The ground state of ${\text{Co}}_2\text{Bz}$ (with a dimer adsorption energy of about 1 eV) is well separated from other possible structures and spin states. In conjunction with similar results obtained by ab initio quantum chemical calculations, this implies that a stable ${\text{Co}}_2\text{Bz}$ complex with $C_{6v}$ symmetry is likely to exist. Chemical bonding to the carbon ring does not destroy the magnetic state and the characteristic level scheme of the cobalt dimer. Calculations including spin-orbit coupling show that the huge MAE of the free Co dimer is preserved in the ${\text{Co}}_2\text{Bz}$ structure. The MAE predicted for this structure is much larger than the MAE of other magnetic molecules known hitherto, making it an interesting candidate for high-density magnetic recording. Among all the other investigated complexes, only ${\text{Ru}}_2\text{Bz}$ shows a potential for strong-MAE applications, but it is not as stable as ${\text{Co}}_2\text{Bz}$. The electronic structure of the complexes is analyzed and the magnitude of their MAE is explained by perturbation theory.

• Received 2 September 2010

DOI:https://doi.org/10.1103/PhysRevB.82.205125