Carbon Nanotube Spinelectronics

Carbon nanotubes (CNT) represent a fascinating new class of materials. They consist of sheets of graphene rolled into a tube-like structure and closed by caps at the end. The CNTs come in different forms and sizes, such as single wall tubes (SWCNT), multiwall tubes (MWCNT), or combinations of these to "ropes” and fibers. These carbon nanotube systems exhibit many surprising and interesting properties, which make them perfect objects for research in nanoscale physics. The perhaps most intriguing aspect concerns their unique electrical transport properties. Due to the one-dimensional nature of the tubes, the electrical transport is dominated by strong electron-electron correlation effects leading, e.g., to a Luttinger liquid behavior. In particular, metallic single-wall carbon nanotubes have been found to behave as long ballistic quantum conductors with the charge carriers exhibiting a large phase-coherence length [1,2]. Other experiments observed quasiballistic transport properties also for multi-wall carbon nanotubes (MWCNTs) [3]. In addition to these chargedominated phenomena, however, CNTs also possess another peculiar property. Due to their low dimensionality, the tubes exhibit extremely large spin-flip scattering lengths, i.e., a spin polarization of the charge carriers entering the nanotube will be largely preserved during the transport. This gives rise to distinct spin-dependent transport effects, i.e., a magnetoresistance [4,5]. Thus, carbon nanotubes also hold a considerable promise for applications in the new field of spinelectronics. In order to investigate the spin-dependent transport properties of carbon nanotubes, the tubes must be contacted by ferromagnetic electrodes (Fig. 1a). The magnetization of one of these electrodes defines the spin direction of the charge carriers injected into the nanotube, the second electrode serves as a "spin detector”. Upon reversing the magnetization direction of one of these electrodes, the spin polarization of the charge carriers arriving at the detector electrode is changed, giving rise to a change of the device resistivity, i.e., a magnetoresistance. The preparation of our samples started from MWCNTs produced by an arcdischarge method. The tube diameters ranged from ~10 to ~60 nm. After ultrasonically agitating the nanotube material in ethanol, a droplet of the resulting suspension was left to dry on the surBin Zhao, Ingolf Mönch, Hartmut Vinzelberg, Thomas Mühl, Claus M. Schneider