  Graphene is hailed as an ideal material for spintronics due to weak intrinsic
spin-orbit interaction that facilitates lateral spin transport and tunability
of its electronic properties, including a possibility to induce magnetism in
graphene. Another promising application of graphene is related to its use as a
spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive
devices, the most prominent class of spintronic devices widely used as magnetic
sensors. In particular, few-layer graphene was predicted to act as a perfect
spin filter. Here we show that the role of graphene in such devices (at least
in the absence of epitaxial alignment between graphene and the FMs) is
different and determined by proximity-induced spin splitting and charge
transfer with adjacent ferromagnetic metals, making graphene a weak FM
electrode rather than a spin filter. To this end, we report observations of
magnetoresistance (MR) in vertical Co-graphene-NiFe junctions with 1 to 4
graphene layers separating the ferromagnets, and demonstrate that the
dependence of the MR sign on the number of layers and its inversion at
relatively small bias voltages is consistent with spin transport between weakly
doped and differently spin-polarized layers of graphene. The proposed
interpretation is supported by the observation of an MR sign reversal in biased
Co-graphene-hBN-NiFe devices and by comprehensive structural characterization.
Our results suggest a new architecture for vertical devices with electrically
controlled MR.
