Spin-dependent transport through a single-walled carbon nanotube coupled to a ferromagnetic and a nonmagnetic metal electrode

We report on transport measurements of an individual single-walled carbon nanotube (SWNT) coupled to a ferromagnetic and a nonmagnetic metal electrode. The low-temperature differential conductance shows a suppression of zero-bias conductance and Coulomb blockade oscillations, where the metallic SWNT behaves as a quantum dot. A marked hysteretic magnetoresistance is observed within the coercive region of the ferromagnetic metal film. The differential resistance has distinctly different values when the magnetization orientation is reversed. Our observed data suggest that the spin-split density of states lies within the SWNT quantum dot. We present an Anderson Hamiltonian to model the spin-polarized electron transport through the SWNT quantum dot with spin-split discrete levels, which supports the experimental observation.

Figure 1

SEM micrograph of the SWNT coupled to ferromagnetic Co electrodes represented by the numbers 1,2 and nonmagnetic metallic electrodes represented by the numbers 3,4. The electrode’s width is approximately $350\mathrm{nm}$ and the separation of adjacent contacts is approximately $370\mathrm{nm}$.

Figure 2

Temperature-dependent resistance $R_{2,3}$ measured in two-probe configuration with the bias voltage of $1\mathrm{mV}$ from 0.35 to $300\mathrm{K}$. The inset shows the corresponding temperature-dependent conductance from 1 to $100\mathrm{K}$.

Figure 3

(Color online) The differential conductance $dI∕dV$ as a function of the bias voltages at 0.35, 0.5, and $1.2\mathrm{K}$ between electrodes 2 and 3. The curve displays a pronounced suppression of zero-bias conductance below $1\mathrm{K}$.

Figure 4

(Color online) The differential conductance plotted against back-gate voltages measured at 0.5, 1.16, and $4.2\mathrm{K}$ between electrodes 2 and 3, displaying the periodic CB oscillations.

Figure 5

(Color online) The differential resistance $(dV∕dI)$ between electrodes 2 and 3 vs magnetic field with a bias voltage of $5\mathrm{mV}$ at $0.5\mathrm{K}$. A hysteretic switch appears and the $dV∕dI$ changes abruptly at the magnetic field of about $\pm 50\mathrm{mT}$ approximating to the coercive field of the Co film. Out of the hysteretic region, the $dV∕dI$ values of positive magnetic fields are larger than those of negative magnetic fields.

Figure 6

(Color online) The $\mathrm{\Delta }{R}_i∕R_A$ as a function of the external magnetic fields ranging from $-1$ to $1\mathrm{T}$ between electrodes 2 and 3 at different temperatures. The relative changes of the differential resistance are around 3.5%, 2.5%, and 2.3% at 0.5, 1.2, and $2\mathrm{K}$ at the coercive field, respectively.

Figure 7

Schematic diagram of a SWNT quantum dot coupled to a ferromagnetic metal (FM) and a nonmagnetic metal (NM) electrode. The central region denotes the SWNT quantum dot with the spin-split discrete levels, whose spin polarizations are indicated by arrows in the quantum dot region.

Figure 8

(Color online) Current-voltage characteristics for the cases that the ferromagnetic electrode is fully magnetized in opposite directions. The tunneling current of spin-down electrons represented by the downward triangle has a higher value than that of spin-up electrons represented by the downward triangle at the same bias voltage.