Magnetoresistance of vertical Co-graphene-NiFe junctions controlled by charge transfer and proximity-induced spin splitting in graphene

Abstract

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