UC Riverside

The dissertation summarizes my work in past four years on the study of topological insulators and magnetic insulator, especially in the field of electrical properties and spin transport. There are mainly three parts in this dissertation. The first part is about our research on topological insulator compounds Bi2-xSbxTe3-ySey (BSTS). In our research, the successful growth and synthesis of BSTS single crystal is achieved. The bulk thin film resistance exhibits an insulating temperature behavior with a high resistivity up to 5 Ω·cm in low temperature region. A cusp-like low field magneto-resistance is observed in both bulk sample and nano device which is an indication of strong spin-orbit coupling. To further suppress the trivial bulk conductance, we develop an e-beam irradiation approach to localize bulk states by introducing disorder. The experiment results show the bulk conduction can be effectively reduced by introducing disorder and the transport of topological insulator device can be modified.

The second part of this dissertation is focused on the interface magnetism study of topological insulator and magnetic insulator yttrium iron garnet (Y3Fe5O12, YIG). We carefully optimize the conditions and obtain the layer-by-layer growth mode of single crystal YIG film. By growing topological insulator on a magnetic insulator YIG, we find that the topological insulator surface in contact with YIG becomes ferromagnetic via magnetic proximity effect which is revealed by anomalous Hall effect and anisotropy magneto-resistance. The Curie temperature of magnetized surface ranges from 20 K to 300 K (room temperature) and is uncorrelated with the doping level in the topological insulator. In contrast, as the Fermi level is tuned by effective doping or electric field effect, both the longitudinal and anomalous Hall resistance can be varied accordingly. In this heterostructure approach, we successfully decouple the electrical properties from ferromagnetism in topological insulator, so that each can be tuned independently. This discovery will shed light on the realization of quantum anomalous Hall effect at higher temperature.

The third part is the spin Seebeck effect study on the topological insulator-magnetic insulator heterostructure. We design and build a well-aligned current heating system for heat assisted magnetic study. Under a vertical temperature gradient, magnons in the magnetic insulator diffuse and carry a pure spin current which continues in the topological insulator layer via magnon-electron scattering at the interface. We observe a greatly enhanced spin Seebeck signal as the Fermi level is tuned near the Dirac point. Such a phenomenon indicates that the unique surface band structure of topological insulator is responsible for the highly efficient spin charge conversion.