Magnetic Proximity Effect Between Topological Insulsators And Magnetic Insulators

During the past couple of years,topological insulators?TIs?have attracted enormous attentions worldwide due to their unique electronic properties.Magnetic proximity effect between a topological insulator and a magnetic insulator?MI?has been considered as an effective method to break the time reversal symmetry of topological insulators without introducing additional impurities and defects.The magnetic proximity effect can in principle open an energy gap near the Dirac point of the surface states and is predicted to produce several novel magnetoelectric effects.However,experimental progress in exploring the TI/MI heterostructures has been hampered by scarcity of magnetic insulators with out-of-plane magnetic anisotropy and the complications in the interfacial interactions between TIs and MIs.In this thesis,we have studied two types of TI/MI heterostructures by combining topological insulator Bi₂Se₃ thin films with magnetic insulators with different magnetic anisotropies,BaFe₁₂O₁₉?BaM?and Y₃O₅Fe₁₂?YIG?.The main results are described as follows.?1?Bi₂Se₃/BaM heterostructures:Single crystalline Bi₂Se₃ thin films have been epitaxially grown on the magnetic insulator BaM substrates,in which the easy magnetization axis lies in the substrate plane.The magnetoresistance?MR?of the Bi₂Se₃/BaM heterostructures has a quadratic dependence on magnetic field,which shows a positive sign in perpendicular magnetic fields and a negative sign in parallel magnetic fields.Such parabola-shaped MRs are qualitatively different from the magnetotransport properties of similar Bi₂Se₃ thin films grown on non-magentic substrates.These results suggest strong magnetic interaction at the Bi₂Se₃/BaM interface.?2?Bi₂Se₃/YIG heterostructures:Bi₂Se₃/YIG heterostructures have been fabricated by epitaxial growth of Bi₂Se₃ thin films on high quality single-crystalline YIG films.The heterostructures with good interface quality exhibit positive MRs in perpendicular magnetic fields.However,the MRs in the magnetic fields below 60 G deviates from the Hikami-Larkin-Nagaoka?HLN?equation,which can satisfactorily describe the low temperature magnetotransport properties of non-magnetic TIs.Furthermore,the in-plane MRs of the Bi₂Se₃/YIG heterostructures are negative and have features correlated with the magnetization of YIG.As the parallel magnetic field becomes stronger,the longitudinal resistivity reaches maxima at the coercive fields of the YIG layer,which is close to zero magnetic field.This is followed by a sharp drop in the resistivity because of magnetization reversal,as well as a smooth change until it reaches magnetization saturation.The magnitudes of both perpendicular and parallel field MRs decrease very rapidly with increasing temperature.These results suggest that the strongly temperature-dependent MRs orginate from the phase coherent transport in Bi₂Se₃,which is influenced by the interfacial magnetic interactions.The interfacial proximity effect in TI/MI heterostructures may provide a new platform for observing the domain-wall induced electron dephasing effect predicted by Tatara and Fukuyama in 1997.Our work helps to deepen the understanding of magnetic interfacial interactions and their influences on the quantum transport properties in TI/MI heterostructures.The insights gained in this work will be valuable for further optimizing the TI/MI heteorostructures and realizing strong interface couplings,which are a basis for the observation of novel topological spintronic effects.