Magnetoresistance in ferromagnetically contacted single-wall carbon nanotubes

We present two-terminal magnetotransport measurements on single-wall carbon nanotube devices, where one or two of the terminals are ferromagnetic. Both ferromagnetic semiconductor [(Ga,Mn)As] and metal (Fe) contact materials have been investigated. In both types of devices we have observed strong hysteretic magnetoresistance below 30 K. The magnetoresistance features develop into large peaks and dips at subkelvin temperatures and they are present even with only one ferromagnetic terminal.

Figure 1

Schematics of our devices and the measurement setup. The bias voltage $V$ is applied to the source, and $I$ is measured from drain to ground. $V_g$ is applied to the capacitatively coupled back gate. (a)–(c) A SWNT contacted by two ferromagnetic terminals (Fe-NT-Fe and FS-NT-FS). When the field is swept, the tube may be contacted by alternating parallel and antiparallel magnetized domains. (d) A SWNT contacted by a nonmagnetic metal source and a ferromagnetic semiconductor drain (M-NT-FS). (e) (Color online) Atomic force microscopy of the device Fe-NT-Fe2.

Figure 2

(Color online) Fe-NT-Fe devices. (a) Fe-NT-Fe1. $G\text{vs}{V}_g$ at $T=4.2\mathrm{K}$ for $B=1$ (bold), 0.5, and $-0.05\mathrm{T}$. (b) Fe-NT-Fe1. $I\text{vs}B$ measured at $T=4\mathrm{K}$, 1.6 K, and 350 mK. Sweep-up and sweep-down directions are indicated by the arrows. All curves of $I\text{vs}B$ are the average of 5–10 individual traces. (c) Fe-NT-Fe2 [see Fig. 1c]. $I\text{vs}B$ with applied source-drain voltage $V=10\mathrm{mV}$ at $T=4.2\mathrm{K}$. (d) Fe-NT-Fe2. The second cool down at $T=9\mathrm{K}$. $I\text{vs}B$ at $V=20\mathrm{mV}$, averaged over 20 traces. (e) The averaged magnitude of the MR plotted for the two devices Fe-NT-Fe 1 and 2 vs $T$. The points and error bars display the mean value and the variance of the MR measured for the devices Fe-NT-Fe 1 and 2.

Figure 3

(Color online). FS-NT-FS device. (a) $G$ or $I\text{vs}B$, measured at $T=14\mathrm{K}$, 2 K, and 310 mK. (b) $I\text{vs}B$ at $T=310\mathrm{mK}$ with applied source-drain voltages of $V=4$ and 2 mV. All $B$ curves are averaged over about eight individual measurements. (c) The hysteretic behavior vs $T$. The points and error bars display the mean value and the variance of the MR measured for the device FS-NT-FS.

Figure 4

(Color online). M-NT-FS device. (a) $T=4.2\mathrm{K}$. $G\text{vs}B$ with $V_g=0$ and 0.0025 V (average over ten individual measurements). (b) The mean value and the variance of the magnitude of the hysteresis vs $T$ for the device M-NT-FS.