Interaction Between Surface State And Magnetism In Magnetic Topological Heterostructures

The quantum anomalous Hall effect（QAHE）is a novel quantum phenomenon that can achieve a chiral edge state without dissipation in the absence of an external magnetic field.Hence,it will be a great significance in the design and application of new low-energy spintronics devices.QAHE was first observed in Cr-doped three-dimensional topological insulators（TI）with the about 30 m K observed temperature.Further studies found that the low observed temperature are mainly related to the non-uniform magnetic distribution caused by magnetic doping.In 2019,the intrinsic magnetic topological insulators Mn Bi₂Te₄ and Mn Bi₂Te₄/（Bi₂Te₃）_nsystems were successfully fabricated,which solved the problem of inhomogeneous distribution of magnetism.However,the system is an A-type interlayer antiferromagnetic sequence,the QAHE is not observed with high-precision quantization under zero magnetic field.Recent experiments have found that Sb doping can cause interlayer ferromagnetic coupling in the Mn Bi₄Te₇ system,which might be a candidate for achieving high-precision quantum anomalous Hall effect under zero magnetic field.But the conclusions on its magnetic phase transition mechanism from different experimental groups are not consistent.In addition,in heterostructures constituted by the topological insulator and the magnetic insulator（MI）are,the long-range ferromagnetism can be introduced into the topological insulator by using the magnetic proximity effect（MPE）without the problem of inhomogeneous distribution of magnetism.However,up to now,QAHE was only observed in the Zn_1-xCr_xTe/(Bi_ySb_1-y)/Zn_1-xCr_xTe system by MPE.In a heterojunction constituted by topological insulators and other materials,double-neighbor effect may affect the location of the topological surface state.This may be a key factor to unsuccessful observation of QAHE in many related experiments.If the general rule of the interaction between magnetism and surface state can be found,it will provide useful guide to the experiment to realize QAHE based on magnetic proximity effect.This paper focuses on the mechanism of magnetic phase transition in heterostructures constituted by magnetic insulators and topological insulators,and the general rules of interaction between magnetic and surface states in magnetic topological heterostructures.Firstly,the electronic structure and magnetic properties of Mn(Bi_1-xSb_x)₄Te₇system are studied in detail,and the intrinsic origin of the magnetic transformation in the researched system are provided,which provides a theoretical explanation for the experimental controversy on the transition from interlayer antiferromagnetic coupling to interlayer ferromagnetic coupling.In addtion,van der Waals（vd W）MI material and TI heterostructures were constructed,the interaction between magnetic and surface states in different heterojunction systems was studied,and the general rule on magnetic-surface interaction was revealed.And based on this law,some candidate systems for ferromagnetic topological insulators and semi-magnetic topological insulators were predicted.The detailed results will be presented in the following sections:1.The magnetic,electronic and topological properties of Mn(Bi_1-xSb_x)₄Te₇ systems doped with different Sb concentrations were systematically studied.In particular,the influence of Mn/Bi（Sb）anti-position defect on the magnetic coupling between the layers of the system is considered.Our results show that at 25%～40%Sb doping concentrations,the system undergoes a magnetic phase transition from interlayer antiferromagnetic coupling to ferromagnetic coupling,and then to antiferromagnetic coupling.In addition,at a Sb doping concentration of 30%,the carrier transition from n-type to p-type happens accompanied by a magnetic phase transition from antiferromagnetism to ferromagnetism.The above findings are consistent with some experimental observations.We further show that the super-exchange mechanism（30%Sb doping）and the direct exchange mechanism（40%Sb doping）are responsible for observed magnetic phase transitions of the studied system.The enhancement of the delocalization of Bi and Te element electrons caused by Sb doping is the microscopic origin of the magnetic phase transitions and carrier transition observed above.This work found the intrinsic origin of magnetic phase and carrier transition in Sb-doped Mn Bi₄Te₇ system,which provides a theoretical explanation of the experimental controversy on the magnetic phases transitions of this system.2.The electronic structure and magnetic properties of heterostructures constituted by topological insulator and vd W magnetic insulator（vd W FMI）related to five-layer Sb₂Te₃（ST）,Bi₂Te₃（BT）and Bi₂Se₃（BS）and single-layer Mn Bi₂Te₄（MBT）,VBi₂Te₄（VBT）,Ni Bi₂Te₄（NBT）,Mn Sb₂Te₄（MST）were systematically studied.We found that the surface state in the TI/vd W FMI heterojunction is suppressed into the topological insulator when TI is Bi₂Se₃ in the system,which will destroy the topology of the Bi₂Se₃ system.When TI is Sb₂Te₃ and Bi₂Te₃ in heterostructures,the surface state floats up to the magnetic insulator,and the Dirac point of the topological insulator is opened due to the interaction between magnetism and topological surface state.Among different heterostructures,MBT/ST and NBT/ST heterojunction are ideal semi-magnetic topological insulators.After then,the important physical parameters that affect the interaction between magnetism and surface states are provided.On the above findings,vd W FMI/TI/vd W FMI sandwich structures with up floating surface states were constructed,and the long-range ferromagnetism or ferrimagnetism in the NBT/ST/VBT and NBT/BT/MST systems are found with?E=-168.96me V and-7.79me V,respectively.And the topology of these system was maintained.It indicates that NBT/ST/VBT and NBT/BT/VBT good candidate systems to realize QAHE.In this study,the interaction relationship between magnetism and surface state was found,and the candidate system for realizing QAHE was finally designed by using the interaction relationship between magnetism and surface state,which will provide some reference for experiments using magnetic proximity effect to realize quantum anomalous Hall effect.