This title appears in the Scientific Report : 2013 

Magnetoresistance and transport in carbon nanotube-based devices
Morgan, Caitlin (Corresponding author)
Elektronische Eigenschaften; PGI-6
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2013
VIII, 131 S.
Dissertation, Universität Duisburg, 2013
Dissertation / PhD Thesis
Spin-based and quantum information
Schriften des Forschungszentrums Jülich. Reihe Schlüsseltechnologien / key technologies 78
Please use the identifier: in citations.
In addition to exhibiting ballistic transport, the low atomic number and low abundance of $^{13}$C spin nuclei in CNTs lead to low spin orbit coupling [1, 2] and hyperfine interaction, indicating a long spin dephasing length. This makes CNTs a material of interest in spintronics, where injecting a spin-polarized current from a ferromagnetic lead into a nonmagnetic channel presents an ongoing challenge. As typical ferromagnetic materials form unreliable contact to CNTs [3, 4, 5], we investigate a novel contact material, the alloy CoPd. We thus combine the stable ohmic contact Pd forms to CNTs [6] and the high polarization of Co and Co-based alloys [7]. This work begins with a characterization of the material CoPd to find the optimal alloy composition. When grown on an SiO$_{2}$ surface, CoPd is shown to have both surface and interfacial roughness of less than 0.5 nm. Magnetically, extended films of CoPd exhibit a complicated behavior with a large out-of-plane component manifesting itself in bubble and stripe domains. However, arrays of fabricated nanostructures of CoPd show a clear in-plane easy axis with little or no out-of-plane component and a high saturation magnetization. Lastly, electrical measurements performed in CoPd-contacted CNTs indicate the formation of highly transparent ohmic contacts. The best performance was found with the alloy Co$_{50}$Pd$_{50}$. Local magnetoresistance (MR) measurements show a dependence on the contact geometry, temperature, and the electronic structure of the CNT. Devices with nanostructured contacts resulted in precise, reliable switching. The magnitude of local MR was shown to increase with lower temperatures and in devices where a stronger tunnel barrier was present. CNTs intrinsically form tunnel barriers at low temperatures, and the strength depends on the contact interface, although it may be suppressed and enhanced via tuning of the bias and gate voltages. Finally, nonlocal three-terminal measurements were performed. While the signal in local measurements may be enhanced by effects such as anisotropic magnetoresistance (AMR), Hall efects, and various local ohmic effects, nonlocal measurements probe only the pure spin current, and are proof that spin injection and detection occur in CNT-based devices with CoPd contacts. Furthermore, Hanle measurements showed a clear spin precession, with a spin lifetime $\tau_{s}$ = 1.1 ns. In conclusion, we have successfully demonstrated the occurrance of spin injection and detection in CNTs contacted by CoPd. The system has all the requirements of a spin valve device: highly polarized leads, intrinsic tunnel barriers, and transparent contact resulting in efficient injection, a nanotube channel that allows for a long spin lifetime, and reliable spin detection, and can therefore provide much useful information for the field of spintronics.