  Rhombohedral stacked multilayer graphene displays the occurrence of a
magnetic surface state at low temperatures. Recent angular resolved
photoemission experiments demonstrate the robustness of the magnetic state in
long sequences of ABC graphene. Here, by using first-principles calculations,
we show that field-effect doping of these graphene multilayers induces a
perfect half-metallic behaviour with 100% of spin current polarization already
at dopings attainable in conventional field effect transistors with solid state
dielectrics. Our work demonstrates the realisability of a new kind of
spintronic devices where the transition between the low resistance and the high
resistance state is driven only by electric fields.
