Investigation of carbon nanotube quantum dots connected to ferromagnetic leads

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.