  The magnetic properties of disordered graphene and irradiated graphite are
systematically studied using a combination of mean-field Hubbard model and
first-principles calculations. By considering large-scale disordered models of
graphene, I conclude that only single-atom defects can induce ferromagnetism in
graphene-based materials. The preserved stacking order of graphene layers is
shown to be another necessary condition for achieving a finite net magnetic
moment of irradiated graphite. Ab initio calculations of hydrogen binding and
diffusion and of interstitial-vacancy recombination further confirm the crucial
role of stacking order in $\pi$-electron ferromagnetism of proton-bombarded
graphite.
