Applications du graphène pour la spintronique
Spin-based memory and logic devices hold the promise to overcome the power, performance and architectural bottlenecks of CMOS-based devices. Among potential material candidates, graphene carries great expectations due to its unique lateral electronic transport properties, together with predicted high spin filtering effects at the interface between few layers graphene and crystalline ferromagnetic electrode ((111) fcc Ni or Co mainly).
In order to exploit these properties, it is mandatory to develop strategies allowing epitaxial growth of ferromagnetic electrodes and tunneling barrier (MgO, AlOx..). Our aim is to explore the unique spin filtering and spin transport properties of graphene by developing nano and micro scale devices with high quality epitaxial magnetic electrodes.
This work is developed in close concert with D. Halley and Y. Henry (IPCMS/DMONS).
Example 1 : Spin filtering properties of hybridized graphene/Nickel interface
We fabricated and studied vertical spin-valve structures using a thick epitaxial MgO barrier as spacer layer and a graphene-passivated Ni film as bottom ferromagnetic electrode. The devices demonstrate robust and scalable tunnel magnetoresistance, with several changes of sign upon varying the applied bias voltage. These findings reveal the complexity of the spin-filtering effects at the (111) Ni|Gr hybrid interface. More generally, our work demonstrates that tailoring the tunneling spin polarization of well-known transition metals through hybridization with two-dimensional overlayers provides opportunities for realizing new spin injectors with unique bias-dependent properties.
Left Figure . (a-c) Bias dependence of the tunneling magnetoresistance ratio for three vertical Ni/Graphene/MgO/Co spin-valves with junction area of 1000 μm2 (c), 100 μm2 (a) and 1 μm2 (b). The insets show magnetoresistance loops measured at different bias voltages, confirming the sign reversal of TMR. (d) Sketch of the three conduction channels that contribute to transport in vertical Ni/Graphene/MgO/Co spin-valves with their respective tunneling spin polarization : Direct tunneling through the Γ point (A), inelastic tunneling at the M point (B), and inelastic tunneling at the K point (C).
More information :
Voltage-controlled inversion of tunnel magnetoresistance in epitaxial Nickel/Graphene/MgO/Cobalt junctions
F. Godel, M. Venkata Kamalakar, B. Doudin, Y. Henry, D. Halley and J.F. Dayen, Appl. Phys. Lett. 105, 152407 (2014)
Exemple 2 : Epitaxial growth of tunnel barrier and ferromagnetic electrodes on epitaxial graphene
Left Figure : (a) Scheme of deposited stack of MgO tunnel barrier and ferromagnetic electrode (iron or permalloy) over graphene. RHEED diagrams report the epitaxial growth of MgO tunnel barrier (b), iron (c) and permalloy ferromagnetic electrodes.
(work done in collaboration with E. Pichonnat and D. Vignaud, IEMN, Lille, France)
More information :
Epitaxy of MgO magnetic tunnel barriers on epitaxial graphene.
F. Godel, E. Pichonat, D.Vignaud, H. Majjad, D.Metten, Y. Henry, S.Berciaud, J.F.Dayen and D. Halley. Nanotechnology, 24, 475708 (2013)