Abstract Submitted to the    NANOTUBE'06 Conference:

Gated Magnetoresistance in Carbon Nanotubes

Christian Schoenenberger, Sangeeta Sahoo, Takis Kontos, Jürg Furer, Matthias Gräber, Audrey Cottet

University of Basel

Christian.Schoenenberger@unibas.ch

XIV. Transport in Nanotubes

One of the basic building blocks of spintronic devices is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, the electrical resistance R depends on the orientation of the magnetization of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then defined to be positive. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realization of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Using carbon nanotubes with ferromagnetic contacts in a field-effect geometry, we demonstrate a spin field-effect transistor (Spin-FET), in which the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner.

We have studied both the MR in multi and single-wall carbon nanotubes using PdNi alloy to form ferromagnetic source and drain contacts. The two contacts have different shape rendering different magnetic switching fields. Two-terminal resistances are in the range of 5-100, k. The typical MR signal amounts to a 2-3 % in magnitude, in agreement with simple expectations from Julliere's model for tunneling MR, but the MR can also be enhanced to values up to 10 %. The most striking observations are sign changes in the MR. The MR oscillates in a regular fashion with gate voltage Vg
This oscillation is shown to be due to resonant tunneling.

We acknowledge fruitful discussions with R. Egger, C. Bruder and W. Belzig. We thank M. Calame, M. Gräber, S. Ifadir, and D. Keller for the preparation of SWNT devices, C. Sürgers (Karlsruhe) for magnetic measurements, and L. Forro (EPFL) for providing the MWNTs. This work has been supported by the EU RTN network DIENOW, the Swiss National Center (NCCR) on Nano-Scale Science,
and the Swiss National Science Foundation.

[1] S. Sahoo et al., Appl. Phys. Lett. 86, 112109 (2005)
[2] S. Sahoo et al. Nature Physics 1, 99 (2005)

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