  We present a study of dynamical spin injection from a three-dimensional
ferromagnet into two-dimensional single-layer graphene. Comparative
ferromagnetic resonance (FMR) studies of ferromagnet/graphene strips buried
underneath the central line of a coplanar waveguide show that the FMR linewidth
broadening is the largest when the graphene layer protrudes laterally away from
the ferromagnetic strip, indicating that the spin current is injected into the
graphene areas away from the area directly underneath the ferromagnet being
excited. Our results confirm that the observed damping is indeed a signature of
dynamical spin injection, wherein a pure spin current is pumped into the
single-layer graphene from the precessing magnetization of the ferromagnet. The
observed spin pumping efficiency is difficult to reconcile with the expected
backflow of spins according to the standard spin pumping theory and the
characteristics of graphene, and constitutes an enigma for spin pumping in
two-dimensional structures.
