A perpendicular graphene/ferromagnet electrode for spintronics

We report on the large-scale integration of graphene layers over a FePd perpendicular magnetic anisotropy (PMA) platform, targeting further downscaling of spin circuits. An L10 FePd ordered alloy showing both high magneto-crystalline anisotropy and a low magnetic damping constant, is deposited by magnetron sputtering. The graphene layer is then grown on top of it by large-scale chemical vapor deposition. A step-by-step study, including structural and magnetic analyses by x-ray diffraction and Kerr microscopy, shows that the measured FePd properties are preserved after the graphene deposition process. This scheme provides a graphene protected perpendicular spin electrode showing resistance to oxidation, atomic flatness, stable crystallinity, and perpendicular magnetic properties. This, in turn, opens the way to the generalization of hybrid 2D-materials on optimized PMA platforms, sustaining the development of spintronics circuits based on perpendicular spin-sources as required, for instance, for perpendicular-magnetic random-access memory schemes

WS2 2D Semiconductor Down to Monolayers by Pulsed-Laser Deposition for Large-Scale Integration in Electronics and Spintronics Circuits

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

Path to overcome material and fundamental obstacles in spin valves based on Mo S2 and other transition-metal dichalcogenides

Experimental studies on spin valves with exfoliated 2D materials face the main technological issue of ferromagnetic electrode oxidation during the 2Ds integration process. As a twofold outcome, magnetoresistance (MR) signals are very difficult to obtain and, when they finally are, they are often far from expectations. We propose a fabrication method to circumvent this key issue for 2D-based spintronics devices. We report on the fabrication of NiFe/MoS2/Co spin valves with mechanically exfoliated multilayer MoS2 using an in situ fabrication protocol that allows high-quality nonoxidized interfaces to be maintained between the ferromagnetic electrodes and the 2D layer. Devices display a large MR of 5%. Beyond interfaces and material quality, we suggest that an overlooked more fundamental physics issue related to spin-current depolarization could explain the limited MR observed so far in MoS2-based magnetic tunnel junctions. This points to a path towards the observation of larger spin signals in line with theoretical predictions above 100%. We envision the impact of our work to be beyond MoS2 and its broader transition-metal dichalcogenides family by opening the way to an accelerated screening of other 2D materials that are yet to be explored for spintronics

Atomic layer deposition of a MgO barrier for a passivated black phosphorus spintronics platform

We demonstrate a stabilized black phosphorus (BP) 2D platform thanks to an ultrathin MgO barrier, as required for spintronic device integration. The in-situ MgO layer deposition is achieved by using a large-scale atomic layer deposition process with high nucleation density. Raman spectroscopy studies show that this layer protects the BP from degradation in ambient conditions, unlocking in particular the possibility to carry out usual lithographic fabrication steps. The resulting MgO/BP stack is then integrated in a device and probed electrically, confirming the tunnel properties of the ultrathin MgO contacts. We believe that this demonstration of a BP material platform passivated with a functional MgO tunnel barrier provides a promising perspective for BP spin transport devices

Graphene as tunnel barrier : charge and spin injection properties

Mes travaux de thèse portent sur la fabrication et la caractérisation électrique et magnétique de jonctions tunnel à base de graphène. C’est autour de l’idée d’apporter une meilleur compréhension des mécanismes d’injection et de détection d’un courant de charge et de spin aux interfaces graphène/ferromagnétique que s’articule ce manuscrit. Après avoir démontré qu’il est possible de faire croître de manière épitaxiée une barrière tunnel de MgO sur graphène, nous avons étudié les mécanismes de transport dépendant en spin dans des jonctions verticales de Co/MgO/Gr/Ni. Nous avons mis en évidence l’interaction du graphène avec l’électrode de nickel à travers les inversions de signe de la magnétorésistance. Celles-ci peuvent être expliquées à l’aide d’un modèle de canaux de conduction assistés par phonons. Enfin du blocage de Coulomb reproductible a été mesuré dans des amas d’aluminium potentiellement mono disperses et auto assemblés sur graphène.My PhD thesis deals with the fabrication and the electric and magnetic characterizations of magnetic tunnel junctions based on graphene. The interaction of graphene with its close environment opens new possibilities for spintronics applications. The manuscript is focused on the improvement of the understanding of mechanisms involved in the injection and detection of a polarized spin current at the graphene/ferromagnetic interfaces. We show that it is possible to grow epitaxially MgO tunnel barrier on graphene. We study the spin transport mechanisms in vertical junctions of Co/MgO/Gr/Ni. The interaction of graphene with nickel electrode is probed through tunnel magnetoresistance inversions which can be explained by the activation of phonon assisted conduction channel. We also measure in vertical and lateral devices based on alumina barrier on graphene, reproducible Coulomb blockade processes linked to the presence of monodisperse aluminum clusters at the graphene edge

Le graphène comme barrière tunnel : propriétés d'injection de charges et de spin

My PhD thesis deals with the fabrication and the electric and magnetic characterizations of magnetic tunnel junctions based on graphene. The interaction of graphene with its close environment opens new possibilities for spintronics applications. The manuscript is focused on the improvement of the understanding of mechanisms involved in the injection and detection of a polarized spin current at the graphene/ferromagnetic interfaces. We show that it is possible to grow epitaxially MgO tunnel barrier on graphene. We study the spin transport mechanisms in vertical junctions of Co/MgO/Gr/Ni. The interaction of graphene with nickel electrode is probed through tunnel magnetoresistance inversions which can be explained by the activation of phonon assisted conduction channel. We also measure in vertical and lateral devices based on alumina barrier on graphene, reproducible Coulomb blockade processes linked to the presence of monodisperse aluminum clusters at the graphene edge.Mes travaux de thèse portent sur la fabrication et la caractérisation électrique et magnétique de jonctions tunnel à base de graphène. C’est autour de l’idée d’apporter une meilleur compréhension des mécanismes d’injection et de détection d’un courant de charge et de spin aux interfaces graphène/ferromagnétique que s’articule ce manuscrit. Après avoir démontré qu’il est possible de faire croître de manière épitaxiée une barrière tunnel de MgO sur graphène, nous avons étudié les mécanismes de transport dépendant en spin dans des jonctions verticales de Co/MgO/Gr/Ni. Nous avons mis en évidence l’interaction du graphène avec l’électrode de nickel à travers les inversions de signe de la magnétorésistance. Celles-ci peuvent être expliquées à l’aide d’un modèle de canaux de conduction assistés par phonons. Enfin du blocage de Coulomb reproductible a été mesuré dans des amas d’aluminium potentiellement mono disperses et auto assemblés sur graphène

Graphene as tunnel barrier : charge and spin injection properties

Mes travaux de thèse portent sur la fabrication et la caractérisation électrique et magnétique de jonctions tunnel à base de graphène. C’est autour de l’idée d’apporter une meilleur compréhension des mécanismes d’injection et de détection d’un courant de charge et de spin aux interfaces graphène/ferromagnétique que s’articule ce manuscrit. Après avoir démontré qu’il est possible de faire croître de manière épitaxiée une barrière tunnel de MgO sur graphène, nous avons étudié les mécanismes de transport dépendant en spin dans des jonctions verticales de Co/MgO/Gr/Ni. Nous avons mis en évidence l’interaction du graphène avec l’électrode de nickel à travers les inversions de signe de la magnétorésistance. Celles-ci peuvent être expliquées à l’aide d’un modèle de canaux de conduction assistés par phonons. Enfin du blocage de Coulomb reproductible a été mesuré dans des amas d’aluminium potentiellement mono disperses et auto assemblés sur graphène.My PhD thesis deals with the fabrication and the electric and magnetic characterizations of magnetic tunnel junctions based on graphene. The interaction of graphene with its close environment opens new possibilities for spintronics applications. The manuscript is focused on the improvement of the understanding of mechanisms involved in the injection and detection of a polarized spin current at the graphene/ferromagnetic interfaces. We show that it is possible to grow epitaxially MgO tunnel barrier on graphene. We study the spin transport mechanisms in vertical junctions of Co/MgO/Gr/Ni. The interaction of graphene with nickel electrode is probed through tunnel magnetoresistance inversions which can be explained by the activation of phonon assisted conduction channel. We also measure in vertical and lateral devices based on alumina barrier on graphene, reproducible Coulomb blockade processes linked to the presence of monodisperse aluminum clusters at the graphene edge

Three-dimensional skyrmionic cocoons in magnetic multilayers

Three-dimensional (3D) topological spin textures emerge as promising quasi-particles for encoding information in future spintronic devices. The third dimension provides more malleability regarding their magnetic properties as well as more flexibility for potential applications. However, the stabilization and characterization of such quasi-particles in easily implementable systems remain a work in progress. Here we observe a new type of 3D magnetic textures that we called skyrmionic cocoons that sits in the interior of magnetic thin films multilayers and possesses a characteristic ellipsoidal shape. Interestingly, these cocoons can coexist with more standard `tubular' skyrmions going through all the multilayer as evidenced by the existence of two very different contrasts in the MFM images recorded at room temperature. The presence of these novel skyrmionic textures as well as the understanding of their layer resolved chiral and topological properties have been investigated by micromagnetic simulations. In order to experimentally stabilize the combination of 3D skyrmion tubes and cocoons, we have elaborated metallic multilayers in which the magnetic properties, notably the anisotropy, of the magnetic films in the stacks is varied depending on their vertical position. Finally, in complement to the magnetic imaging, we also measure the magneto-resistive response of the multilayers as a function of the magnetic field, and succeed to fit its evolution using the 3D micromagnetic simulations as inputs for the magnetic configuration. The excellent agreement that is reached brings additional evidence of the presence of skyrmionic cocoons that hence can be electrically detected.Comment: 13 pages, 4 figure

Nonreciprocal Transport in a Rashba Ferromagnet, Delafossite PdCoO₂

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 Let

M -Plane GaN/InAlN Multiple Quantum Wells in Core–Shell Wire Structure for UV Emission

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