Topological phenomena in magnetic thin films

OF THE DISSERTATION Topological phenomena in magnetic thin films by Wenbo Wang Dissertation Director: Weida Wu This thesis covers the study of various topological phenomena in magnetic thin films by utilizing magnetic force microscopy (MFM) at cryogenic temperature. In the first part, we will focus on the magnetic topological insulators (TI) which exhibit quantum anomalous Hall effect (QAHE). The QAHE is of great fundamental interest and potential application because of their dissipationless conduction without the need of external magnetic field. Up to now, this fascinating effect has been achieved in magnetic topological insulators, e.g. Crand V-doped (Bi,Sb)2Te3 thin films, at extremely low temperature (T < 50 mK). The magnetic inhomogeneity has been proposed as one of the main reasons that limit the temperature for realization of QAHE. In the first part, we will present direct visualization of long-range ferromagnetism in the Cr,V co-doped BST films, by using the MFM and in-situ transport measurement. The magnetization reversal process reveals typical ferromagnetic domain behavior, i.e. domain nucleation and domain wall propagation. The ferromagnetic behavior at charge neutral point is consistent with the Van Vleck exchange mechanism. Furthermore, this ferromagnetism is robust against significant change in bulk charge carriers, though exchange interaction is enhanced by hole doping. This indicates a significant contribution from the Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange coupling. The