Impact of Tunnel-Barrier Strength on Magnetoresistance in Carbon Nanotubes

We investigate magnetoresistance in spin valves involving CoPd-contacted carbon nanotubes. Both the temperature and bias-voltage dependence clearly indicate tunneling magnetoresistance as the origin. We show that this effect is significantly affected by the tunnel-barrier strength, which appears to be one reason for the variation between devices previously detected in similar structures. Modeling the data by means of the scattering matrix approach, we find a nontrivial dependence of the magnetoresistance on the barrier strength. Furthermore, an analysis of the spin precession observed in a nonlocal Hanle measurement yields a spin lifetime of ${\tau }_s=1.1\mathrm{ns}$, a value comparable with those found in silicon- or graphene-based spin-valve devices.

Figure 1

Current-voltage characteristics of a typical device under consideration, which exhibits metallic behavior at room temperature (black circles), whereas the formation of a potential barrier at 4 K (red squares) is observed. The inset shows an image of a typical device taken by a scanning electron microscope. The position of the tube has been redrawn to enhance visibility.

Figure 2

The dependence of local magnetoresistance curves on temperature investigated in a temperature range of 3.4–75 K at $V_{\mathrm{bias}}=2\mathrm{mV}$. The left (right) panel shows results for a field sweep in the negative (positive) direction as indicated by the dashed arrows. The presented data set corresponds to device 2 (cf. Table 1). Solid arrows represent the magnetization direction of the two contacts.

Figure 3

Magnetoresistance measurements of the sample presented in Fig. 1 obtained at bias voltage: (a) $V_{\mathrm{bias}}=35\mathrm{mV}$ with $\mathrm{TMR}=15\%$ and (b) $V_{\mathrm{bias}}=15\mathrm{mV}$ with $\mathrm{TMR}=6\%$. Arrows indicate the sweep direction of the magnetic field. (c) The $\ln (I/V^2)-\ln (1/V)$ plot of the current-voltage characteristics at $T=4\mathrm{K}$ and with $B=0\mathrm{T}$ reveals that the different sizes of the MR effect can be attributed to different tunneling regimes. The gray shaded region marks the transition regime between direct tunneling (right) and Fowler-Nordheim tunneling (left) that is targeted for the following measurements.

Figure 4

Evolution of TMR as a function of the tunnel-barrier strength $Z$ at $T=4\mathrm{K}$. Experimental data (red squares) corresponding to different devices (see Table 1) are accompanied by theoretical calculations (solid lines) for several values of the magnetic polarization parameter $P$ of injected charge carriers at fixed gate voltage ($E_g=6\mathrm{meV}$) based on the model explained in the text. Error bars represent deviations in TMR between multiple measurements. The inset shows the magnitude of TMR with respect to the current through the different devices presented

Figure 5

(a) AFM image of the three-terminal device used in a nonlocal configuration. The CoPd terminals are shaded in green; the gold coarse lines are shaded yellow. The position of the tube has been redrawn for better visibility. The current flow and voltage measurement of the nonlocal signal are schematically indicated. The terminal on the left does not switch its magnetization up to fields as large as 2 T and is left floating. (b) The Hanle measurement (red dots connected by a line that serves merely as a guide to the eyes) is taken at a bias of $I_{\mathrm{bias}}=10\mathrm{nA}$. The fit (blue) with a Lorentz curve yields the spin lifetime of ${\tau }_s=1.1\mathrm{ns}$.