Gate-tunable graphene spin valve

Abstract

First we discuss whether R nl arises due to charge current or spin current flowing between F3 and F4. Ideally, charge current would flow only between F3 and F2, eliminating contributions to the R nl from magnetoresistance of the ferromagnetic electrodes (anisotropic magnetoresistance), the channel, or the electrode-channel interface. However, because R nl is ~3 orders of magnitude smaller than the device resistance, it is possible that some charge current flows through a tortuous path from F3to F4 and F5. We investigate this by measuring the gate voltage and temperature dependence of R nl . Figure 3a shows the gate voltage dependence of R nl in the parallel and antiparallel state, R nl,p and R nl,ap , as well as their average value. Figure 3b shows the non-local spinvalve signal ΔR. R avg , R nl,p and R nl,ap all show a peak near the CNP (10 V < V g < 30 V), while ΔR is near zero in this region. Well outside this region (V g < -20 or V g > 40 V), R nl,p and R nl,ap have nearly equal magnitude and opposite sign (R avg is near zero) and ΔR is larger and shows quasi-periodic oscillations with V g . The peak in R avg (V g ) near the CNP suggests that charge current does flow in the region between F3 and F4 for these gate voltages. However, R avg (V g ) is not simply proportional to ρ(V g ) but rather drops to near zero at large V g while ρ(V g ) remains finite. Thus the finite R avg (V g ) near the CNP is likely due to the inhomogenous nature of graphene near the CNP 21,22 ; here percolating electron and hole regions may cause a tortuous current path.Away from the CNP, R avg (V g ) drops to near zero, indicating small charge current.Yet R nl,p and R nl,ap remain finite, with near equal magnitude and opposite sign. This is as 4 expected for a pure spin current flowing from F3 to F4, and cannot be explained by a magnetoresistive signal arising from any charge current between F4 and F5. The Hall effect is another possible source of V nl , however, the Hall voltage would be expected to grow large and switch sign near the CNP, rather than showing a peak. Figure 4 shows the temperature dependence of R avg and ΔR for V g = 0. Here R avg is finite similar to Figure 3, but somewhat larger for this electrode configuration. The spin-valve signal ΔR is seen to drop with temperature approximately as ΔR ∝ T -1 , while R avg is much more weakly temperature dependent; again indicating a different origin for ΔR and R avg . The inset shows a measurement at 300 K performed at higher current; the spin-valve signal can still be observed, confirming expectations of reduced spin scattering in graphene even to high temperature.We now discuss the magnitude of the spin-valve signal ΔR. For an Ohmicallycontacted spin-valve device, the non-local signal may be estimated using Eqn. 22 of reference [17]; we estimate in this case the signal should be on order 10 -5 Ω. However, we observe finite contact resistance of order 10 kΩ per electrode as estimated from the difference between two-probe and four-probe resistance measurements. In the limit of...

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