Ferromagnetically contacted Carbon nanotube quantum dots

This thesis presents research into spin-transport in Carbon nanotube quantum dots. Sputtered Permalloy electrodes designed with shape anisotropy were used to contact Carbon nanotubes grown by chemicalvapour deposition in lateral spin-valve structures. The magnetoresistance of these spin-valves were measured at low-temperatures and as a function of the charge state of the quantum dots.

Two conductance regimes were measured in a Carbon nanotube spinvalve with Permalloy nucleation pads. At high bias outside of the coulomb blockade regime a ~ 10% magnetoresistance was measured that is analogous to giant-magnetoresistance, in that it is due to spindependent
scattering at the ferromagnet-Carbon nanotube interfaces.
At lower bias the device enters the coulomb blockade regime and the magnetoresistance observed develops a different structure, over a larger field range, together with the development of an offset in conductance between saturations. The maximum value of this MR was MR ~ 245% and it was attributed to changes in the induced charge on the quantum dot.

By modifying the design of the Permalloy electrodes, a single domain state at the point of contact of the Carbon nanotube was achieved. A well defined anti-parallel state of the Permalloy electrodes, with associated changes in the conduction of the devices was observed, yet the conductance offset remained, with a maximum MR of ~ 60%.
The positions of the coulomb peaks were measured during magnetic reversal of the electrodes, showing the change in induced charge on the quantum dot, with a maximum MR ~ 350%. Predictions of device transport based on the magneto-coulomb effect and spin-dependent interfacial phase shifts were compared to experimental results and found to not fit the observed behaviour. This led to the conclusion
that changes in the charge state of the quantum dots must be due to a fixed spin-quantisation axis intrinsic to the Carbon nanotube.