  Based on numerically accurate density functional theory (DFT) 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 (TM2Bz, TM = Fe, Co, Ni, Ru, Rh,
Pd; Bz = C6H6). These systems are chosen to model TM-dimer adsorption on
graphene or on graphite. We find that Fe2, Co2, Ni2, and Ru2 prefer the upright
adsorption mode above the center of the benzene molecule, while Rh2 and Pd2 are
adsorbed parallel to the benzene plane. The ground state of Co2Bz (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 Co2Bz complex
with C6v 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 Co2Bz 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 Ru2Bz shows a
potential for strong-MAE applications, but it is not as stable as Co2Bz. The
electronic structure of the complexes is analyzed and the magnitude of their
MAE is explained by perturbation theory.
