Investigation of carbon nanotube quantum dots connected to ferromagnetic leads

Abstract

The implementation of ferromagnetic contacts in complex nanoelectronic devices, e.g. in spin-valves bears great potential for applications and fundamental investigations. Spin-valves are structures with two magnetic contacts and a non-magnetic medium (M) in-between, where a step-like change in magnetoresistance (MR) is observed when the relative orientation of the strip magnetization is changed by an external magnetic field. There is already a very successful use of the electron spin in electronic devices for magnetic field sensing for example in read-and-write heads of computer hard discs. Another upcoming application of spin valves are for example non-volatile random-access memories (MRAMs) for data storage. However, electronic devices which use the electron spin directly, like in a spintransistor or as quantum bits requires materials for the non-magnetic medium exhibiting long coherence times and electrical tunability. Carbon based materials like graphene or carbon nanotubes are due to their intrinsic large coherence times in principle ideal candidates for spintronic devices, as demonstrated in nonlocal spin-accumulation experiments on graphene or in electrically tunable spin valves on carbon nanotubes. Especially the observation of a gate dependent magneto-resistance in carbon nanotube quantum dots contacted with ferromagnetic leads in 2005 by S. Sahoo promises an electrical control over spin transport. These devices analog to field-effect transistors might pave the way for multi-functional spintronic devices. However, the implementation of ferromagnetic contacts in nanoelectronic devices has been a proven challenging task due to the complex nature of ferromagnets and interfaces, where oxidation, surface roughness and mesoscopic details may induce uncontrolled instabilities in transport measurements. Spintransport experiments on carbon nanotube quantum dots suffer mainly from irreproducibilities in the magneto resistance and from the low yield of electrical contacts to the nanotube. Therefore in this thesis carbon nanotube quantum dots connected to ferromagnetic leads are investigated, focusing on the fabrication of stable devices with higher contact yields and first experiments on stable devices. This allows for further investigations of the not well understood gate dependence of the magneto resistance in such devices. Moreover with such stable devices, even more complex experiments or applications like detectors for spin entanglement can become possible.