Transport Studies Of Magnetic Order And Quantum Phase Transition In Magnetic Topological Insulators

This thesis presents transport studies of magnetic order and quantum phase transi-tion in magnetic topological insulators?TIs?.By performing electro-magnetic transport measurements on a series of magnetic TI thin films,we reveal a rich variety of exotic topological quantum phenomena originated from the interplay between topology and magnetism in magnetic TIs.In addition,in the quantum anomalous Hall?QAH?effect realized in magnetic TIs,we find novel behaviors of quantum phase transition.Bi₂Te₃ family three-dimensional TIs represent an interesting class of topological quantum materials that is characterized by the existence of metallic Dirac surface states with linear dispersion at the interface between bulk and vacuum.TI serves as an ideal platform for the study of exotic topological quantum phenomena and fabrication of new functional spintronic devices.Breaking the time reversal symmetry by magnetic doping is an efficient method to study and manipulate the properties of TI.We first study the transport behaviors of Mn and Cr co-doped Bi₂Te₃ TI films.We find the anomalous Hall effect?AHE?of samples with varied Mn and Cr ratios exhibit different chirality,which is sensitive to temperature,film thickness and the position of Fermi level.By analyzing the 3d-electrons occupation in different magnetic dopants,we propose the chirality of AHE in a magnetic TI is determined by the sign of exchange coupling between magnetic moments and itinerant electrons.Our results suggest the existence of competition between the magnetic orders mediated by the topological surface states and bulk states independently in magnetic TIs.By precisely tuning the film thickness of Mn-doped Bi₂Te₃ TIs,we realize the manipulation of the topological-surface-states-mediated magnetic order.We find that film with the thickness of 4 nm exhibits characteristic features of topological Hall effect that is distinct from the common AHE in magnetic TIs.Our theoretical calculations suggest that the hybridization between the top and bottom topological surface states stabilizes the formation of magnetic skyrmions in real space that are responsible for the THE.We have studied another important effect in magnetic TIs,i.e.the QAH effect.Through material engineering,we find that codoping Cr and V in TIs can improve the magnetic homogeneity significantly,giving rise to the highest temperature for observing the QAH effect up to date.Through analyzing the transport behaviors of a large number of QAH samples,we propose the existence of two distinct ground states in the QAH system:the QAH liquid and anomalous Hall?AH?insulator states.Most of the previous studies on the QAH effect focused on the QAH liquid state.Our scaling analysis of the quantum critical behavior indicates that the QAH phase transition is distinct from the quantum Hall?QH?phase transition in conventional two-dimensional electron gas,which suggests that the QAH effect is not merely a zero-magnetic-field version of the conventional QH effect.