  We investigate the electronic and magnetic properties of single Fe, Co, and
Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of
scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray
magnetic circular dichroism (XMCD), and ab initio calculations. STM reveals
different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms
to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with
theory. In contrast, we experimentally find a nonmagnetic ground state for Ni
monomers while an increasing cluster size leads to sizeable magnetic moments.
These observations are well reproduced by our calculations and reveal the
importance of hybridization effects and intra-atomic charge transfer for the
properties of adatoms and clusters on MLG.
