Measurements of pure spin current using highly conductive graphene/Co and graphene/Al/Co interfaces*

Experiments using graphene nonlocal spin valves have historically employed high resistance graphene/ferromagnet interfaces to confine spins to the graphene channel. In contrast, here we focus on how the pure spin current absorbed by a ferromagnetic contact increases as the conductance of the ferromagnet/graphene interface is improved. We show that the spin current density j_s, when reported for clarity as equivalent charge density j_e=j_s(4πe/h), increases from 10³ to 10⁹ A/m² as the interface conductance per unit area is increased from 10^-5 to 10^-2 S/μm². These conductive interfaces are developed by encapsulating exfoliated graphene flakes in AlO_x by atomic layer deposition, which allows resist residue to be removed prior to etching away the protective AlO_x layer and depositing the contacts. Furthermore, we show that inserting a thick conductive Al interlayer increases the spin injection/detection efficiency and spin lifetime without reducing the interface conductance. Finally, we comment on the challenge of switching by spin-transfer torque in graphene and suggest potential paths forward.