  It is proposed that the observed small (100 ps) spin relaxation time in
graphene is due to resonant scattering by local magnetic moments. At
resonances, magnetic moments behave as spin hot spots: the spin-flip scattering
rates are as large as the spin-conserving ones, as long as the exchange
interaction is greater than the resonance width. Smearing of the resonance
peaks by the presence of electron-hole puddles gives quantitative agreement
with experiment, for about 1 ppm of local moments. While the local moments can
come from a variety of sources, we specifically focus on hydrogen adatoms. We
perform first-principles supercell calculations and introduce an effective
Hamiltonian to obtain realistic input parameters for our mechanism.
