Abstract
The magneto-transport properties of magnetic topological insulators MnBi2Te4 (MBT-124) and MnBi4Te7 (MBT-147) have been studied experimentally in this thesis. These include the surface-induced linear magnetoresistance (LMR), the magnetically tunable Shubnikov–de Haas (SdH) oscillation, the current-dependent magnetic states, and anomalous Hall effect (AHE).
Much progress has been made in antiferromagnetic topological insulator MBT-124, such as the observation of the quantum anomalous Hall effect and axion insulator states in few layer MBT-124. But in the bulk MBT-124, the transport signature of topological surface states has not been reported yet. In our study of the flake MBT-124, the existence of the high mobility surface states is demonstrated by two-dimensional (2D) LMR in high magnetic fields. It has been found that the nonsaturating LMR persists up to 14 T. The 2D transport of this LMR is revealed by rotating the magnetic field. The two-band analysis of the nonlinear Hall signal shows the close relation between this LMR and the topological surface state. Furthermore, whether the surface states of MBT-124 are gapped below Neel temperature still remains an open question. A suppression of this LMR occurs during the magnetic transition from paramagnetic (PM) phase to antiferromagnetic (AFM) phase at the Neel temperature, which may suggest a surface state gap opening in band structure due to this transition. In addition, the failure of both classical model and quantum LMR model in describing the LMR indicates that this LMR deserves further investigations. It is worth mentioning that a similar LMR is also found in MBT-147.
In previous studies, the close connection between the magnetic state of MBT-124 and its band structure or topological phase is reported. However, such a connection has not been directly investigated, especially in the quantum oscillation experiment. Here, we report on the discovery of the novel magnetic field-sensitive SdH oscillation in pristine MBT-124, demonstrating the effective manipulation of the band structure by external magnetic fields. Unlike what is reported in Sb-doped MBT-124 where the SdH oscillation has a single frequency at a fixed temperature, it is observed that the oscillation period is decreasing with increasing magnetic field, indicating the field-induced change in band structure. The period decreases gradually in the magnetic transition from canted antiferromagnetism (CAFM) to ferromagnetism (FM), and then saturates in high magnetic fields. Also, the theoretical studies reveal that the topological phase of MBT-124 will change to Weyl semimetal phase when the system is in FM state. By analyzing the high field oscillation, the nontrivial Berry phase and a small effective mass are extracted, which are consistent with the predicted Weyl semimetal phase in ferromagnetic MBT-124. Moreover, tilting the external magnetic field will induce a splitting of oscillation peaks, which suggests the enhanced asymmetry of the Weyl cones in tilted fields.
Due to the close connection between the magnetic state and band structure, it is crucial to search for new ways other than the magnetic field to effectively tune the magnetic state and the topological physics in the antiferromagnetic topological insulator. Our observation of the current-dependent magnetic states and anomalous Hall effect in MBT-124 and MBT-147 indicates that the current can be another option to manipulate the physical properties of MBT-124 and MBT-147. In MBT-124, the resistance peak position in resistance versus temperature (R(T)) curve is tuned to lower temperatures due to the large current, indicating the current influence on magnetic states. And the impact of the current can also be observed in the resistance versus magnetic field curve, where a clear decrease of critical magnetic fields is observed. Similar current-induced effects are also obtained in MBT-147. In addition, a current-induced AHE sign reversal is also found in MBT-147. For further analysis, the heating effect is ruled out as the dominant effect in these phenomena. And the magnetic state change may be associated with the current-induced spin-transfer torque (STT). Furthermore, we suggest that the current-induced Auger scattering can explain the sign reversal of AHE in MBT-147 due to its gapless Weyl semimetal phase in high fields.
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