Tunneling Spin Injection into Single Layer Graphene

We achieve tunneling spin injection from Co into single layer graphene (SLG) using ${\mathrm{TiO}}_2$ seeded MgO barriers. A nonlocal magnetoresistance ($\Delta R_{\mathrm{NL}}$) of $130\Omega$ is observed at room temperature, which is the largest value observed in any material. Investigating $\Delta R_{\mathrm{NL}}$ vs SLG conductivity from the transparent to the tunneling contact regimes demonstrates the contrasting behaviors predicted by the drift-diffusion theory of spin transport. Furthermore, tunnel barriers reduce the contact-induced spin relaxation and are therefore important for future investigations of spin relaxation in graphene.

Figure 1

(a) Schematic diagram of the angle evaporation geometry. The gray layers are PMMA/MMA resist with undercut. The red and blue dashed lines show the 0° and 9° deposition of ${\mathrm{TiO}}_2$ and MgO. The black lines indicate the 7° evaporation of Co. (b) Schematic drawing of the $\mathrm{Co}/\mathrm{MgO}/{\mathrm{TiO}}_2/\mathrm{SLG}$ tunneling contacts. The arrow indicates the current flow through the MgO tunnel barrier. (c) Nonlocal MR scans of SLG spin valves measured at room temperature. The black (red) curve shows the nonlocal resistance as the magnetic field is swept up (down). The nonlocal MR ($\Delta R_{\mathrm{NL}}$) of $130\Omega$ is indicated by the arrow. Inset: The nonlocal spin transport measurement on this device with a spacing of $L=2.1\mu \mathrm{m}$ and SLG width of $W=2.2\mu \mathrm{m}$.

Figure 2

Predictions of the drift-diffusion theory of spin transport. The nonlocal MR as a function of gate voltage for three different types of contacts between Co and SLG: transparent, intermediate, and tunneling. The curves are normalized by their value at zero gate voltage.

Figure 3

(a)–(c) Nonlocal MR (black squares) and conductivity [gray (red) lines] as a function of gate voltage for SLG spin valves with transparent, pinhole, and tunneling contacts, respectively. Insets: The differential resistance of the contact, $(dV/dI{)}_C$, as a function of bias current.

Figure 4

(a) Hanle spin precession for SLG spin valves with tunneling contacts ($R_J=30–70\mathrm{k}\Omega$, nonlinear) for $L=2.1\mu \mathrm{m}$. (b) Hanle spin precession for tunneling contacts ($R_J=20–40\mathrm{k}\Omega$, nonlinear) with $L=5.5\mu \mathrm{m}$. (c) Hanle spin precession for pinhole contacts ($R_J=6\mathrm{k}\Omega$, linear) with $L=2.0\mu \mathrm{m}$. (d) Hanle spin precession for transparent contacts ($R_J<0.3\mathrm{k}\Omega$, linear) with $L=3.0\mu \mathrm{m}$. The top [gray (red)] and bottom (black) curves correspond to Hanle curves of the parallel and antiparallel states, respectively. The solid lines are best fit curves based on Eq. (3). The units for $D$ are ${\mathrm{m}}^2/\mathrm{s}$.