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...

We report electrical transport measurements through a semiconducting single-walled carbon nanotube (SWNT) with three additional top-gates. At low temperatures the system acts as a double quantum dot with large inter-dot tunnel coupling allowing for the observation of tunnel-coupled molecular states extending over the whole double-dot system. We precisely extract the tunnel coupling and identify the molecular states by the sequential-tunneling line shape of the resonances in differential conductance.Comment: 5 pages, 4 figure

We report the observation of thermally driven mechanical vibrations of suspended doubly clamped carbon nanotubes, grown by chemical vapor deposition (CVD). Several experimental procedures are used to suspend carbon nanotubes. The vibration is observed as a blurring in images taken with a scanning electron microscope. The measured vibration amplitudes are compared with a model based on linear continuum mechanics.

Using carbon nanotubes (CNTs) produced by chemical vapor deposition, we have explored different strategies for the preparation of carbon nanotube devices suited for electrical and mechanical measurements. Though the target device is a single small diameter CNT, there is compelling evidence for bundling, both for CNTs grown over structured slits and on rigid supports. Whereas the bundling is substantial in the former case, individual single-wall CNTs (SWNTs) can be found in the latter. Our evidence stems from mechanical and electrical measurements on contacted tubes. Furthermore, we report on the fabrication of low-ohmic contacts to SWNTs. We compare Au, Ti and Pd contacts and find that Pd yields the best results.

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