Spin transport in topological insulator-based nanostructures

With the advent of spintronics, significant research effort is made to realize efficient injection, transport, manipulation, and detection of spin. The material platforms and device architectures suitable for this purpose 3D topological insulators (TIs) are a class of Dirac materials, which possess 2D spin-polarized helical surface states due to their strong SO coupling. This property offers research possibilities in several directions, specifically (i) to gain a deeper understanding of the electronic properties of massless Dirac fermions hosted by the surface states; (ii) to exploit the spin-momentum locking of the surface states towards efficient spin-charge conversion; and (iii) to investigate the interplay of the surface states with magnetic materials, superconductors, and 2D materials that could lead to the discovery of new surface and interface phenomena with exciting technological prospects. The main challenge in the field of 3D TIs is posed by the difficulty to selectively access the surface states in electrical transport experiments. This issue arises from the intrinsic bulk doping, which leads to a contribution of the bulk states to the conductivity. Furthermore, while they are prospective as efficient spin current generators, the efficiency of the electrical detection of spin currents is limited due to pronounced spin relaxation and dephasing. This motivates the realization of 3D TIs with improved properties and to engineer 3D TI-2D material heterostructures, whose interfaces would enable efficient spin-charge manipulation. The present thesis aims at the characterization of various Dirac materials by magnetotransport experiments at low temperatures, as well as the nanofabrication and investigation of spintronics devices based on 3D TIs. In the first experimental part, the electronic properties of four different 3D TIs are investigated - Sb2Te3 and Bi2Te2Se, which are grown in a vapor-solid process, ZrTe5, grown as single crystals in a Czochralski process, and finally the naturally occurring mineral Aleksite. In the second part, Bi2Te2Se is chosen for the fabrication of lateral spin valve devices, in which electrical detection of charge current-induced spin polarization due to the spin-momentum locking of the 2D TI surface states is demonstrated. Spin measurements are performed for devices with different coupling strength between the FM detector and the TI channel. Subsequently, the spin signal is investigated for the first time in the presence of a hBN tunnel barrier. An inversion of the spin signal is observed, which depends on the resistance of the hBN/FM/TI interface. The third experimental part addresses the spin generation properties of Bi2Te2Se in a van der Waals TI/graphene heterostructure, in which the TI acts as a spin injector, while the injected spin propagates within the graphene channel underneath and is detected non-locally by a ferromagnetic electrode on top of the graphene. This is a first-time demonstration of spin injection and detection in a heterostructure, which combines the best properties of two Dirac materials, determined by the presence of SO coupling in the TI and the lack thereof in graphene. These results are examples of emergent phenomena on 2D surfaces and interfaces, which could be further investigated in vertical and lateral heterostructures utilizing the spin Hall, Rashba, or Edelstein effect for efficient spin-charge conversion towards all-spin-based information technology