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theses

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. Cr- and 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 textit{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 direct evidence of long-range ferromagnetic order eases the concern of the fragility of QAHE due to magnetic inhomogeneity, alleviating the need for ultra-low temperature to achieve full quantization. Our results encourage further exploration of QAHE and related phenomena in magnetically doped topological materials for disspationless conduction at elevated temperature.

In the second part, we will discuss topological Hall effect (THE), which is a real space Berry phase phenomenon originated from scalar spin chirality. The THE has been observed in static spin texture with integer Berry phase (skyrmions)in chiral magnets. Recently, this effect has been also observed in some ultra-thin film systems. The existence of static skyrmions in these systems is still unknown. While thermally driven spin chirality fluctuation in 2 dimensional chiral magnets has been predicted to exist, the resultant THE have not been observed. In this thesis, we report a surprising observation of substantial THE around the ferromagnetic transition Tc of ultra-thin films of SrRuO3, a uniaxial ferromagnet with very weak chiral interaction. The temperature, magnetic field, and thickness dependence of THE are in good agreement with our Monte-Carlo simulations, which unambiguously confirms the emergence of net topological charges driven by spin chirality fluctuation. Our discovery opens a new paradigm of exploring the chirality fluctuations with topological Hall transport in 2D ferromagnets. In addition to the chiral fluctuation driven THE (fTHE) in SRO films around Tc, the THE is also observed at low temperature in the ultra-thin SRO films, as they approach the critical thickness (3 u.c.). MC simulations indicate the magnetic disorder plays an important role in the low-temperature THE. This disorder-driven THE (dTHE) will be briefly discussed at the end of this thesis.

ETD_9220

Ph.D.

Includes bibliographical references

by Wenbo Wang

doi:10.7282/t3-n2w8-9898

ETD doctoral

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