| AVS 60th International Symposium and Exhibition | |
| Magnetic Interfaces and Nanostructures | Tuesday Sessions |
| Session MI+EM-TuM |
| Session: | Spintronics and Magnetoelectrics |
| Presenter: | B.T. Jonker, Naval Research Laboratory |
| Authors: | B.T. Jonker, Naval Research Laboratory E. Cobas, Naval Research Laboratory O.M.J. van 't Erve, Naval Research Laboratory C.H. Li, Naval Research Laboratory A.L. Friedman, Naval Research Laboratory J.T. Robinson, Naval Research Laboratory |
| Correspondent: | Click to Email |
Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. Tunnel barriers are the basis for many spintronic devices, and to date have relied upon oxides which often exhibit defects, trap states and interdiffusion which compromise performance and reliability. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We fabricate magnetic tunnel junctions, and demonstrate electrical spin injection/detection in silicon using graphene as a tunnel barrier.
The graphene was grown by chemical vapor deposition on copper foil and incorporated as the tunnel barrier by physical transfer and standard lithographic processes to form Co / graphene / NiFe magnetic tunnel junctions (MTJs) 20-40 um in diameter [1]. Non-linear I-V curves and weak temperature dependence of the zero-bias resistance provide clear evidence for tunneling. The magnetic field dependence exhibits the classic signature of MTJ behavior, and the structures exhibit tunneling magnetoresistance (TMR) to 425 K, in good agreement with theory [2]. The TMR decreases monotonically with both bias and temperature, typical of MTJ behavior.
Single-layer graphene also successfully circumvents the classic issue of conductivity mismatch between a metal and a semiconductor for electrical spin injection and detection, providing a highly uniform, chemically inert and thermally robust tunnel barrier. Hanle spin precession measurements demonstrate spin injection and provide quantitative values for spin lifetimes. Devices with NiFe / single layer graphene / Si contacts exhibit the classic Lorentzian lineshape due to spin injection and dephasing. We demonstrate electrical generation and detection of spin accumulation in silicon above room temperature, and show that (a) the corresponding spin lifetimes correlate with the silicon carrier concentration, and (b) the contact resistance–area products are two to three orders of magnitude lower than those achieved with oxide tunnel barriers on silicon substrates with identical doping levels [3]. This reduction of contact resistance enables spin injection and quantitative measurements of spin lifetimes in silicon nanowires, as well.
[1] Cobas, Friedman, van’t Erve, Robinson, Jonker, Nano Letters 12, 3000 (2012).
[2] Karpan et al, Phys. Rev. Lett. 99, 176602 (2007); Phys. Rev. B 78, 195419 (2008).
[3] van’t Erve, Friedman, Cobas, Li, Robinson, Jonker, Nature Nanotechnology 7, 737(2012).