Protecting nickel with graphene spin-filtering membranes: A single layer is enough

We report on the demonstration of ferromagnetic spin injectors for spintronics which are protected against oxidation through passivation by a single layer of graphene. The graphene monolayer is directly grown by catalytic chemical vapor deposition on pre-patterned nickel electrodes. X-ray photoelectron spectroscopy reveals that even with its monoatomic thickness, monolayer graphene still efficiently protects spin sources against oxidation in ambient air. The resulting single layer passivated electrodes are integrated into spin valves and demonstrated to act as spin polarizers. Strikingly, the atom-thick graphene layer is shown to be sufficient to induce a characteristic spin filtering effect evidenced through the sign reversal of the measured magnetoresistance.
Oxidation has always been a key issue for spintronics. Indeed, any undesired surface oxidation of typical metallic ferromagnets (nickel, cobalt, iron, and their alloys) quenches their delicate spin polarization properties rendering them useless for spintronics. Hence, the fabrication of functional spin-valves, the basic building block of mainstream data-storage technologies,1 has up to now mainly relied on high vacuum physical deposition setups. However, recent developments in spintronics have highlighted the need to find new ways to circumvent this issue. Indeed, the integration of ambient/oxidative fabrication steps such as atomic layer deposition (ALD) of high quality dielectrics2,3 and liquid phase deposition of organic materials4 (e.g., self assembled monolayers of molecules5 and tunable conducting polymers6) would reduce costs and open up new opportunities (e.g., ultimate downscaling with single molecule magnets7 and chemically engineered functionalities with spinterfaces8,9). The identification of spin sources tolerant to oxidative fabrication conditions is thus central to the development of these novel applications of spintronics.
Graphene has shown a strong potential as a membrane preventing atomic diffusion. Bunch et al.10 demonstrated the impermeability even to helium of exfoliated pristine monolayer graphene flakes. Interestingly, since then, several studies have discussed the possibility of passivating metals with chemical vapor deposition (CVD) of graphene layers over large areas, however, with contrasting conclusions.11–17 For spintronics and in the case of multilayer CVD graphene on nickel, functional spin valves devices based on multilayer graphene protected ferromagnetic electrodes (GPFE) have demonstrated the feasibility of this approach.13 However, a controversy exists in the case of copper, where studies15,16 report on failure to passivate the metallic surface and even on the enhanced degradation of the Cu surface by the graphene layer in contrast to the previous studies.11 This failure of the passivation has been attributed by Prasai et al.14 to the defects of the CVD graphene sp2 structure while Kidambi et al.17 ascribed it to the weakly coupled graphene/copper interface which in turn allows further diffusion and accumulation of oxidative species. The sharp differences between these experiments question the possibility of passivating ferromagnetic electrodes with a single layer of graphene in spin-valve devices.
Here, we show that a single layer of graphene, derived by a direct CVD step with low enough temperatures (450 °C) to be compatible with complementary metal-oxide-semiconductor (CMOS) processes,18,19 is sufficient to protect the surface of a nickel electrode against oxidation and maintain a spin polarization. The downscaling of the graphene coating to an ultimate single atom thickness (Figure 1) still ensures that the metallic nature of the nickel electrode surface is preserved after ambient air exposure (Figure 2). The resulting ferromagnetic electrodes protected by a single layer graphene sheet are then shown to maintain a spin polarization through their integration in functional spin valves (Figure 3).