The Growth And Transport Properties Of Topological Insulator Thin Films In Proximity To A Ferromagnetic Insulator

Since the discovery of topological insulators?TIs?,it has attracted much attention in recent years due to their rich physics and promising applications in electronic and spintronic devices.Inducing magnetism in a topological insulator by exchange coupling with a ferromagnetic insulator?FMI?will break the time-reversal symmetry of topological surface states,and open up a gap in the Dirac surface states of TIs,offering possibilities to realize several predicted novel magneto-electric effects.Introducing ferromagnetism into topological insulators through proximity effect with a ferromagnetic insulator is an effective way to break the time-reversal symmetry,and it avoids bringing extra impurities compared with the doping method.The advantages of the heterostructure approach include better controllability of the electronic states,preservation of the TI's original crystalline structure and bulk properties,enhanced temperature of the magnetic ordering,and so on.However,experiments on insulating ferromagnetic substrates are limited to a few kinds of materials,since the existing FMIs are very scarce in nature.Seeking the suitable FMI materials is crucial to the coupling of heterojunctions,yet is challenging as well.The main challenge resides in finding better magnetic materials,which can form a high-quality interface with TI and,at the same time,provide a strong exchange coupling at the interface.In this thesis,we report the growth and magnetotransport properties of topological insulator Bi₂Se₃ thin films on a ferromagnetic insulating LaCoO₃ thin film on SrTiO₃ substrate for the first time.The epitaxial LaCoO₃ thin film on SrTiO₃substrate is shown to be a rare undoped high-symmetry insulating ferromagnet with a remarkably high T_C of up to 85 K.By systematically studied the growth conditions for topological insulator Bi₂Se₃ thin films on LaCoO₃/SrTiO₃,we obtained single-crystalline Bi₂Se₃ thin films with a well-organized morphology,fabricating a high-quality TI/FMI heterostructure.Magnetic and transport measurements manifest the emergence of a ferromagnetic phase in Bi₂Se₃ films.Moreover,a signal of anomalous Hall effect is observed and persists up to temperatures above 100 K,paving the way towards spintronic device applications.The main results of this thesis are summarized as follows:?1?We report that thin films of the prototype topological insulator,Bi₂Se₃,are successfully grown onto the?001?surface of LaCoO₃/SrTiO₃ by molecular beam epitaxy.By systematically studied the growth conditions,we find that the optimal condition for Bi₂Se₃ thin films to grow on La CoO₃/SrTiO₃ is at a much lower substrate temperature,around 150?,as compared with the optimal temperature on bare Sr TiO₃.The thus obtained Bi₂Se₃ thin films display a well-organized morphology and high carrier mobility,similar to those grown on bare SrTiO₃ substrates at higher temperature.While at higher growth temperatures,Bi₂Se₃ films on LaCoO₃ exhibit disordered film morphology and decreased carrier mobility.The magnetoresistance and characteristic weak antilocalization effect are also reduced at higher growth temperatures.These unusual behaviors may be attributed to the subtle strain stress and large thermal-expansion coefficient of LaCoO₃.?2?Magneto-transport properties and magnetization measurements of the Bi₂Se₃/LaCoO₃ heterostructure are reported for the first time.Transport measurements reveal that the mobility of Bi₂Se₃ films grown on LaCoO₃ at the optimal conditions reaches 659 cm²V^-1s^-1,which is comparable to its pristine counterpart on pure SrTiO₃substrate.The weak antilocalization effect of Bi₂Se₃ is much suppressed by the induced magnetism through proximity coupling with LaCoO₃,and the anomalous Hall resistance is observed to persist up to over 100 K.Besides,an inverse dependence of carrier mobility with the substrate temperature is found,and the magnetoresistance and coherence length of weak antilocalization also have a similar inverse dependence with the substrate temperature.Furthermore,magnetization data confirms that there are extra induced magnetic moments in the Bi₂Se₃ layer which can survive to a temperature higher than the Curie temperature of LaCoO₃/SrTiO₃.Such enhanced ferromagnetism in TI/FMI heterostructures might be attributed to the proximity effect resulted from the large spin-orbit coupling and topologically-protected robustness of TI nature,and will be promising for high-temperature device applications.?3?We broaden the above results to a tentary TI system,(Bi_1-xSb_x)₂Te₃ thin films.(Bi_1-xSb_x)₂Te₃?0<x<1?materials have more insulating bulk states,which is advantageous for the transport measurements.By improving the methods for thin film growth,we obtain high-quality(Bi_1-xSb_x)₂Te₃?x=0.5?thin films on SrTiO₃?111?subtrate with large terrace area and no impurty phases by MBE.Gating in the transport measurements allow the Feimi level to approach the Dirac point,where we obtain a type of carrier from the surface state with high mobility of more than 1700cm²V^-1s^-1.Then,we studied the growth and transport properties of(Bi_1-xSb_x)₂Te₃ thin film on LaCoO₃/SrTiO₃?001?substrate,and obtained samples with good quality by exploring the modified methods for growth conditions.The weak antilocalization of(Bi_1-xSb_x)₂Te₃ thin film is surpressed more significantly,and a larger abnormal Hall resistance is observed compared with the result in Bi₂Se₃/LaCoO₃ system.