The authors perform nonlocal four-probe spin-valve experiments on graphene contacted by ferromagnetic Permalloy electrodes. They observe sharp switching and often sign reversal of the nonlocal resistance at the coercive field of the electrodes, indicating the presence of a spin current between injector and detector. The nonlocal spin-valve signal changes magnitude and sign with back-gate voltage, and is observed up to T=300K. The gate voltage variation of the spin-valve signal may result from quantum-coherent transport, as evidenced by Fabry-Pérot-like oscillations of the current.

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