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2D materials offer the ability to expose their electronic structure to manipulations by a proximity effect. This could be harnessed to craft properties of 2D interfaces and van der Waals heterostructures in devices and quantum materials. We explore the possibility to create an artificial spin polarized electrode from graphene through proximity interaction with a ferromagnetic insulator to be used in a magnetic tunnel junction (MTJ). Ferromagnetic insulator/graphene artificial electrodes were fabricated and integrated in MTJs based on spin analyzers. Evidence of the emergence of spin polarization in proximitized graphene layers was observed through the occurrence of tunnel magnetoresistance. We deduced a spin dependent splitting of graphene's Dirac band structure (∼15 meV) induced by the proximity effect, potentially leading to full spin polarization and opening the way to gating. The extracted spin signals illustrate the potential of 2D quantum materials based on proximity effects to craft spintronics functionalities, from vertical MTJs memory cells to logic circuits., This work has received funding from the European Union’s H2020 Future and Emerging Technologies “Graphene Flagship” (Grant Core3 No. 881603), “SINFONIA” (No. 881603) projects and a Grant Agreement Marie Skłodowska Curie “ITN Spinograph” (No. 697904), as well as from EPSRC (EP/P005152/1, EP/K016636/1). This research is supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program Labex NanoSaclay (ANR-10-LABX-0035), as well as grants STEM2D (ANR-19-CE24-0015), MIXES (ANR-19-CE09-0028), and by the Flag-ERA JTC 2019 project “SOGraphMEM” (ANR-19-GRFI-0001-07). This work has also received fundings from the “Spanish ministry of Science and Innovation” through “Severo Ochoa” (CEX2019-000917-S) and “OXISOT” (PID2021-128410OB-I00)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

We report on large spin-filtering effects in epitaxial graphene-based spin valves, strongly enhanced in our specific multilayer case. Our results were obtained by the effective association of chemical vapor deposited (CVD) multilayer graphene with a high quality epitaxial Ni(111) ferromagnetic spin source. We highlight that the Ni(111) spin source electrode crystallinity and metallic state are preserved and stabilized by multilayer graphene CVD growth. Complete nanometric spin valve junctions are fabricated using a local probe indentation process, and spin properties are extracted from the graphene-protected ferromagnetic electrode through the use of a reference Al2O3/Co spin analyzer. Strikingly, spin-transport measurements in these structures give rise to large negative tunnel magneto-resistance TMR = −160%, pointing to a particularly large spin polarization for the Ni(111)/Gr interface PNi/Gr, evaluated up to −98%. We then discuss an emerging physical picture of graphene−ferromagnet systems, sustained both by experimental data and ab initio calculations, intimately combining efficient spin filtering effects arising (i) from the bulk band structure of the graphene layers purifying the extracted spin direction, (ii) from the hybridization effects modulating the amplitude of spin polarized scattering states over the first few graphene layers at the interface, and (iii) from the epitaxial interfacial matching of the graphene layers with the spin-polarized Ni surface selecting well-defined spin polarized channels. Importantly, these main spin selection effects are shown to be either cooperating or competing, explaining why our transport results were not observed before. Overall, this study unveils a path to harness the full potential of low Resitance. Area (RA) graphene interfaces in efficient spin-based devices.

Epitaxial clusters of chromium and chromium–vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr2O3-like phase, whereas usual Cr2O3 is known to be anti-ferromagnetic in the bulk. For mixed clusters, vanadium ions also contribute to the total magnetization and they are coupled to the chromium ion spins. By measuring the dichroic signal at different temperatures, we get insight into the possible spin configurations of vanadium and chromium ions: we propose that the magnetic dipoles observed in the clusters assembly could be related to ionic spins that couple at a very short range, as for instance in short one-dimensional spins chains.

International audience; We report on the achievement of a large-scale tungsten disulfide (WS2) 2D semiconducting platform derived by pulsed-laser deposition (PLD) on both insulating substrates (SrTiO3), as required for in-plane semiconductor circuit definition, and ferromagnetic spin sources (Ni), as required for spintronics applications. We show thickness and phase control, with highly homogeneous wafer-scale monolayers observed under certain conditions, as demonstrated by X-ray photoelectron spectroscopy and Raman spectroscopy mappings. Interestingly, growth appears to be dependent on the substrate selection, with a dramatically increased growth rate on Ni substrates. We show that this 2D-semiconductor integration protocol preserves the interface integrity. Illustratively, the WS2/Ni electrode is shown to be resistant to oxidation (even after extended exposure to ambient conditions) and to present tunneling characteristics once integrated into a complete vertical device. Overall, these experiments show that the presented PLD approach used here for WS2 growth is versatile and has a strong potential to accelerate the integration and evaluation of large-scale 2D-semiconductor platforms in electronics and spintronics circuits.

International audience; We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spec-troscopy. The WS2 layers are then integrated in complete Co/Al2O3 /WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of PCo/WS2 = 12%. Interestingly, for bi-and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.

MoS2-based vertical spintronic devices have attracted an increasing interest thanks to theoretical predictions of large magnetoresistance signals. However, experimental performances are still far from expectations. Here, we carry out the local electrical characterization of thin MoS2 flakes in a Co/Al2O3/MoS2 structure through conductive tip AFM measurements. We show that thin MoS2 presents a metallic behavior with a strong lateral transport contribution that hinders the direct tunnelling through thin layers. Indeed, no resistance dependence is observed with the flake thickness. These findings reveal a spin depolarization source in the MoS2-based spin valves, thus pointing to possible solutions to improve their spintronic properties.

International audience; We report on the integration of atomically thin 2D insulating hexagonal boron nitride (h-BN) tunnel barriers into magnetic tunnel junctions (2D-MTJs) by fabricating two illustrative systems (Co/h-BN/Co and Co/h-BN/Fe) and by discussing h-BN potential for metallic spin filtering. The h-BN is directly grown by chemical vapor deposition on prepatterned Co and Fe stripes. Spin-transport measurements reveal tunnel magneto-resistances in these h-BN-based MTJs as high as 12% for Co/h-BN/h-BN/Co and 50% for Co/h-BN/Fe. We analyze the spin polarizations of h-BN/Co and h-BN/Fe interfaces extracted from experimental spin signals in light of spin filtering at hybrid chemisorbed/ physisorbed h-BN, with support of ab initio calculations. These experiments illustrate the strong potential of h-BN for MTJs and are expected to ignite further investigations of 2D materials for large signal spin devices.

Atomically thin molybdenum diselenide (MoSe 2 ) is an emerging two-dimensional (2D) semiconductor with significant potential for electronic, optoelectronic, spintronic applications and a common platform for their possible integration. Tuning interface charge transport between such new 2D materials and metallic electrodes is a key issue in 2D device physics and engineering. Here, we report tunable interface charge transport in bilayer MoSe 2 field effect transistors with Ti/Au contacts showing high on/off ratio up to 10 7 at room temperature. Our experiments reveal a detailed map of transport mechanisms obtained by controlling the interface band bending profile via temperature, gate and source-drain bias voltages. This comprehensive investigation leads to demarcating regimes and tuning in transport mechanisms while controlling the interface barrier profile. The careful analysis allows us to identify thermally activated regime at low carrier density, and Schottky barrier driven mechanisms at higher carrier density demonstrating the transition from low-field direct tunneling/ thermionic emission to high-field Fowler–Nordheim tunneling. Furthermore, we show that the transition voltage V trans to Fowler–Nordheim correlates directly to the difference between the chemical potential of the metal electrode and the conduction band minimum in the 2D semiconductor, which opens up opportunities for new theoretical and experimental investigations. Our approach being generic can be extended to other 2D materials, and the possibility of tuning contact transport regimes is promising for designing MoSe 2 device applications.

Large assemblies of self-organized aluminum nanoclusters embedded in an oxide layer are formed on graphene templates and used to build tunnel-junction devices. Unexpectedly, single-electron-transport behavior with well-defined Coulomb oscillations is observed for a record junction area of up to 100 mu m(2) containing millions of metal islands. Such graphene-metal nanocluster hybrid materials offer new prospects for single-electron electronics.

International audience; Fabrication and spintronics properties of 2D–0D heterostructures are reported. Devices based on graphene (“Gr”)–aluminium nanoclusters heterostructures show robust and reproducible single‐electron transport features, in addition to spin‐dependent functionality when using a top magnetic electrode. The magnetic orientation of this single ferromagnetic electrode enables the modulation of the environmental charge experienced by the aluminium nanoclusters. This anisotropic magneto‐Coulomb effect, originating from spin–orbit coupling within the ferromagnetic electrode, provides tunable spin valve‐like magnetoresistance signatures without the requirement of spin coherent charge tunneling. These results extend the capability of Gr to act both as electrode and as a platform for the growth of 2D–0D mixed‐dimensional van der Waals heterostructures, providing magnetic functionalities in the Coulomb blockade regime on scalable spintronic devices. These heterostructures pave the way towards novel device architectures at the crossroads of 2D material physics and spin electronics.

Epitaxial growth of electrodes and tunnel barriers on graphene is one of the main technological bottlenecks for graphene spintronics. In this paper, we demonstrate that MgO(111) epitaxial tunnel barriers, one of the prime candidates for spintronic application, can be grown by molecular beam epitaxy on epitaxial graphene on SiC(0001). Ferromagnetic metals (Fe, Co, Fe20Ni80) were epitaxially grown on top of the MgO barrier, thus leading to monocrystalline electrodes on graphene. Structural and magnetic characterizations were performed on these ferromagnetic metals after annealing and dewetting: they form clusters with a 100 nm typical lateral width, which are mostly magnetic monodomains in the case of Fe. This epitaxial stack opens the way to graphene spintronic devices taking benefits from a coherent tunnelling current through the epitaxial MgO/graphene stack.

We observe, as a function of temperature, a second order magnetic phase transition in nanometric Cr2O3 clusters that are epitaxially embedded in an insulating MgO matrix. They are investigated through their tunnel magneto-resistance signature, the MgO layer being used as a tunnel barrier. We infer the small magnetic dipoles carried by the Cr2O3 clusters and provide evidence of a magnetic phase transition at low temperature in those clusters: they evolve from an anti ferromagnetic state, with zero net moment close to 0 K, to a weak ferromagnetic state that saturates above about 10 K. The influence of magneto-electric effects on the weak ferromagnetic phase is also striking: the second order transition temperature turns out to be linearly dependent on the applied electric field.

International audience; We present a growth process relying on pulsed laser deposition for the elaboration of complex van der Waals heterostructures on large scales, at a 400°C CMOS-compatible temperature. Illustratively, we define a multilayer quantum well geometry through successive in situ growths, leading to WSe2 being encapsulated into WS2 layers. The structural constitution of the quantum well geometry is confirmed by Raman spectroscopy combined with transmission electron microscopy. The large-scale high homogeneity of the resulting 2D van der Waals heterostructure is also validated by macro-and microscale Raman mappings. We illustrate the benefit of this integrative in situ approach by showing the structural preservation of even the most fragile 2D layers once encapsulated in a van der Waals heterostructure. Finally, we fabricate a vertical tunneling device based on these large-scale layers and discuss the clear signature of electronic transport controlled by the quantum well configuration with ab initio calculations in support. The flexibility of this direct growth approach, with multilayer stacks being built in a single run, allows for the definition of complex 2D heterostructures barely accessible with usual exfoliation or transfer techniques of 2D materials. Reminiscent of the III−V semiconductors' successful exploitation, our approach unlocks virtually infinite combinations of large 2D material families in any complex van der Waals heterostructure design.

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO$_2$, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 {\pm} 0.3 eV {\AA} and argue that our results qualify delafossites as a new family of oxides for nano-spintronics and spin-orbitronics, beyond perovskite materials., Comment: Accepted in Nano Lett