  Decorating two-dimensional materials with transition-metal adatoms is an
effective way to bring about new physical properties that are intriguing for
applications in electronics and spintronics devices. Here, we systematically
studied the coverage-dependent magnetic and electronic properties of graphene
decorated by Co adatoms, based on first-principles calculations. We found that
the if the Co coverage is larger than 1/3 ML, the Co atoms will aggregate to
form a Co monolayer and then a van der Waals bilayer system between the Co
monolayer and graphene forms. When the Co coverage is <= 1/3 ML, the Co adatom
is spin polarized with spin moment varying from 1.1 ~ 1.4 {\mu}B. The d(xz/yz)
and d(xy/x2-y2 ) orbitals of Co hybridize significantly with the {\pi} bands of
graphene, which generates a series of new bands in the energy range from -2 eV
to 1 eV with respect to Dirac point of graphene. In most cases, the new bands
near the Fermi level lead to topological states characterized by the quantum
anomalous Hall effect.
