The Stability, Energetics, and Magnetic States of Cobalt Adatoms on Graphene

We investigate the stability and electronic properties of single Co atoms on graphene with near-exact many-body calculations. A frozen-orbital embedding scheme was combined with auxiliary-field quantum Monte Carlo to increase the reach in system sizes. Several energy minima are found as a function of the distance $h$ between Co and graphene. Energetics only permit the Co atom to occupy the top site at $h=2.2$ \AA\ in a high-spin $3d^{8}4s^1$ state, and the van der Waals region at $h=3.3$ \AA\ in a high-spin $3d^{7}4s^2$ state. The findings provide an explanation for recent experimental results with Co on free-standing graphene.