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 spin-orbit interaction that facilitates lateral spin transport and tunability of its electronic properties [1-3], including a possibility to induce magnetism in graphene [4-9]. Another promising application of graphene is related to its use as a spacer separating ferromagnetic metals (FMs) in vertical magnetoresistive devices [10-20], the most prominent class of spintronic devices widely used as magnetic sensors. In particular, few-layer graphene was predicted [10-12] 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. Following the successful development of graphene-based lateral spintronic structures [1-8], the implementation of graphene as a spacer in vertical magnetic tunnel junctions (MTJ) has become a subject of intense interest [13-20]. Up to now, theoretical proposals [10-12] for graphene's role in MTJs focused on the so-called 'K-point spin filtering' expected in ideally lattice-matched single-crystalline ferromagnet-graphene-ferromagnet (FM-G-FM) structures and attributed to matching spin-polarized bands in the ferromagnet and the electronic states in the graphene treated as a tunnelling barrier. This mechanism was also used to interpret the MR sign inversion observed in conventional Ni-Al 2 O 3 -Co [13] and Ni-MgO-Co [14] tunnel junctions where the Ni electrode was passivated by CVD grown (epitaxial) mono-[13] or few-layer [14] graphene. However, despite several attempts,

2D-MTJs: introducing 2D materials in magnetic tunnel junctions

“…This is the largest reported value for wet-transferred CVD graphene, thus demonstrating the importance of preserving the quality of the interfaces. Similarly, in 2016 Asshoff et al [39] reported on the exfoliation and transfer of graphene on membranes with holes with a measured MR of about 1%. While the improvement in measured spin signals demonstrated in these studies points to the importance of preserving high-quality FM/2DM interfaces, the signal quality remained far from what was expected.…”

“…Another striking issue concerned the out-of-plane electrical conduction in devices encompassing only a graphene layer between the two ferromagnetic electrodes, with still no experimental consensus yet on the role of the graphene spacer. Some of the earlier mentioned studies reported on graphene acting mainly as a metallic spacer [20,21,24,32,39], whilst other studies reported on it acting as a tunnel barrier [14,24,28]. One study [25] even reported that its role varied with temperature.…”

“…Another original approach has been developed to integrate graphene in MTJs by two different teams [28,39]. The main idea was to deposit the graphene layer over a perforated membrane and to grow ferromagnets on both sides by flipping the sample.…”

“…In particular, it demonstrated the possibility of controlling the resistance of the tunnel barrier with the number of h-BN sheets deposited layer-by-layer through a conductive-tip atomic force microscopy (AFM) study, extracting a barrier height of φ = 0.85 eV. As alternatives to the CVD method, other promising h-BN integration pathways have been suggested, such as the MBE growth of h-BN on Co [73] or the in situ growth of FMs over both the faces of h-BN flakes suspended on etched membranes [39]. These methods also allowed the metallic state of the interfaces to be preserved.…”

Effective interactions in a graphene layer induced by the proximity to a ferromagnet

“…Theoretical estimates, mostly based on DFT calculations, for related systems give values in the 10 −1 − 10 1 meV range [10,[31][32][33][34][35][36]. Experimental values for these couplings also show a significant variety, although they tend to be smaller than the the theoretical predictions, 0.1 meV [2,3,6,[10][11][12][37][38][39][40]. The reason for this discrepancy is likely to be the uncertainty on the graphene-substrate distance, and the degree of commensurability between graphene and the substrate.…”