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

TL;DR

This study investigates how charge transfer and proximity-induced spin splitting in graphene influence magnetoresistance in vertical Co-graphene-NiFe junctions, revealing a layer-dependent and bias-sensitive spin transport mechanism.

Contribution

It demonstrates that graphene acts as a weak ferromagnetic electrode influenced by proximity effects, challenging the idea of graphene as a perfect spin filter in such devices.

Findings

Magnetoresistance sign depends on the number of graphene layers.

MR sign inverts at small bias voltages.

Proximity-induced effects dominate spin transport in these junctions.

Abstract

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.