Electric field control of spin transport

Abstract

Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them 1-3 . This approach is illustrated by the operation of the most basic spintronic device, the spin valve 4-6 , which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction 7,8 . The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin-or energy-dependent 3 , the opposite can occur and the magnetoresistance is negative 9 . The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate 10,11 . In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.Early work on spin transport in multiwall carbon nanotubes (MWNTs) with Co contacts showed that spins could propagate coherently over distances as long as 250 nm (ref. 12). The tunnel magnetoresistance (TMR) = (R AP − R P )/R P , defined as the relative difference between the resistances R AP and R P in the antiparallel and parallel magnetization configuration, was found to be positive and amounted to +4% in agreement with Jullière's formula for tunnel junctions 4,13 . A negative TMR of about −30% was reported later for MWNTs contacted with similar Co contacts 14 . In these experiments, the nanotubes did not show quantum dot behaviour. It has been shown, however, that single-wall carbon nanotubes (SWNTs) and MWNTs contacted with nonferromagnetic metals could behave as quantum dots and FabryPérot resonators [15][16][17][18][19] , in which one can tune the position of discrete energy levels with a gate electrode. From this, one can expect to be able to tune the sign and the amplitude of the TMR in nanotubes, in a similar fashion as predicted originally for semiconductor heterostructures 11 .In this letter, we report on TMR measurements of MWNTs and SWNTs that are contacted with ferromagnetic electrodes and capacitively coupled to a back-gate 20 . A typical sample geometry is shown in the inset of Fig. 1. As a result of resonant tunnelling, we observe a striking oscillatory amplitude and sign modulation of the TMR as a function of the gate voltage. We have studied and observed the TMR on nine samples (seven MWNTs and two SWNTs) with various tube lengths L between the ferromagnetic electrodes (see the Methods section). We present here results for one MWNT device and one SWNT device.We first discu...

Electronic and Magnetic Properties of Partially Open Carbon Nanotubes

“…The response of CNTs to an external electric field (E ext ) is an interesting subject for future applications 12,13 . The three directions of E ext (+x, +y, −y), which were all perpendicular to the tube axis direction, z, were considered.…”

Signatures of spin-orbit interaction in transport properties of finite carbon nanotubes in a parallel magnetic field

“…1,2 Lately their application as building blocks in spintronics has also been addressed. [3][4][5][6][7][8] Nonetheless, the importance of spin-orbit coupling in these systems has long been underestimated. The spin-orbit interaction (SOI) was believed to be very weak, due to the low atomic number Z = 6 of carbon.…”

Gate-tunable split Kondo effect in a carbon nanotube quantum dot

“…Single-wall Carbon nanotubes can be used for the production of quantum dot devices, and allow for the study of the Kondo effect, as they usually show a strong confinement potential, a fact that leads to a large Kondo energy scale k B T K [4]. Carbon nanotubes can moreover be contacted by superconducting [5] and ferromagnetic [6,7] electrodes, and therefore allow the production of hybrid devices incorporating non-normal metal contacts. One aspect in which Carbon nanotube quantum dots are less versatile than e. g. lateral quantum dots, that are also widely used to study the Kondo effect, is the control over the contact transparencies and their asymmetry, two key parameters for the Kondo effect.…”