  The introduction and control of ferromagnetism in graphene opens up a range
of new directions for fundamental and applied studies. Several approaches have
been pursued so far, such as introduction of defects, functionalization with
adatoms, and shaping of graphene into nanoribbons with well-defined zigzag
edges. A more robust and less invasive method utilizes the introduction of an
exchange interaction by a ferromagnetic insulator in proximity with graphene.
  Here we present a direct measurement of the exchange interaction in room
temperature ferromagnetic graphene. We study the spin transport in exfoliated
graphene on a yttrium-iron-garnet substrate where the observed spin precession
clearly indicates the presence and strength of an exchange field that is an
unambiguous evidence of induced ferromagnetism. We describe the results with a
modified Bloch diffusion equation and extract an average exchange field of the
order of 0.2 T. Further, we demonstrate that a proximity induced 2D ferromagnet
can efficiently modulate a spin current by controlling the direction of the
exchange field. These results can create a building block for magnetic-gate
tuneable spin transport in one-atom-thick spintronic devices.
