Introducing Magnetic Ordering In Topological Insulator Bi₂Se₃

Topological insulator (TI) as a non-traditional insulator has a new quantum state. Topological insulators also has dual nature of conductor and insulator, namely in the bulk interior is insulating state energy gap, and the surface has a metallic state gapless. The surface states (SSs) are protected by time-reversal symmetry owing to strong spin-orbit coupling. Therefore, TI is not easily affected by the defects, non-magnetic impurities and other external environment in the system. TI has important potential application value in low energy consumption of electronic devices and fault tolerant quantum computation. Many exotic physical properties of 3D TIs confirmed by experimental and theoretical prediction, such as Quantum anomalous Hall effect, Majorana fermion, and magnetic monopole, leading to great potential for future application.In this work, we selected the typical 3D TI Bi2Se3 in the series of V2VI3 compound as the research object. Topological quantum states appear under low temperature and strong magnetic field in TIs. The TIs are not real TIs at all.In the real TI, strong magnetic field which is not dependent on external can also the quantum Hall effect can be observed independent of low temperature and strong magnetic field. Namely the anomalous quantum Holzer effect could stems from ferromagnetic of sample itself. Materials design is effective in getting this kind TIs. To introduce ferromagnetism in TI, two methods are proposed. Doping with transition metal elements can make the 3D TIs to become magnetically ordered, result in gap opening at the Dirac point, and lead to other exotic phenomena. There is a theoretical prediction that non-magnetic 2p light elements X (X=B, C and N) can induce ferromagnetism and manipulate SSs in TIs by substitutional doping at the host anions. In this thesis, we mainly studied the structure and properties of magnetic element (Co and Ni) and non-magnetic element C doping Bi2Se3 bulk crystal. Some groups’works demonstrate that the ferromagnetism at ambient temperature can be induced in heterojunction interface by the magnetic proximity effec. This result opens a new path to interface-controlled. Based on above considerations we studied the properties and modification effect of double membrane structure of Bi2Se3 and perovskite manganite oxide. At last, we explore two techniques for preparing Bi2Se3 thin film, vacuum evaporation and magnetron sputtering. The primary research work and achievements are as follows:The Bi2Se3 matrix is diamagnetic and doped samples CoxBi2-xSe3 is a superposition of ferromagnetism (FM) and paramagnetism (PM) behavior at low temperature. The values of Msmol, Hc, Mr increase as the Co concentration increasing. There are two possible explanations to origin of FM:one is the bulk ferromagnetic behaviors of Co-doped Bi2Se3 are due to nanoclusters of Co-Se compound in the crystals, and the other is localized Co ions diluted into host matrix. All samples exhibited weak metallic resistivity. The resistivity data could be fitted by different formulas below and above 30 K in all samples result from different mechanism corresponding to different temperature regions. The magnetoresistance (MR) of Bi2Se3 topological insulator showed a change dependence of concentration. The Hall mobility data indicated that incorporating Co in Bi2Se3 single crystal can reduce native defect of Bi2Se3.Transition metal doped topological insulators NixBi2-xSe3 were prepared by the self-flux method. The lattice constant c decreased with the increasing Ni concentration. All samples are highly the c-axis oriented and exhibit weak metallic resistivity. The resistivity increased with increasing applied magnetic field and Ni concentration. The magnetic changes monotonously as the Ni dopant concentrations increased, which implies the nickel entering the matrix structure. For the sample with small amount of Ni (x=0.03), a behavior in the curves of temperature dependent of magnetism (M (T)) closely resembling a paramagnet. The samples with (x≥0.05) showed clear hysteresis loops, which suggest the existence of ferromagnetism ordering. It was considered that there are two possible explanations to the origin of ferromagnetism:Ni-Se compound and magnetic contamination.C doped samples had the same lattice structure as pure Bi2Se3, the lattice parameter c firstly increased and then decreased. The surface of samples showed the typical terrace-and-step morphology. From the p-T curves, obvious M-I transformation in all samples were observed near 30K. p-T and MR-T characteristics around 30 K was observed which indicated that contribution of surface state had been significantly enhanced by C doping. After C doping, the magnetic properties change from diamagnetic to ferromagnetic.Bi2Se3/Si (100) thin films with good crystalline quality and highly c-axis orientation were synthesized by thermal evaporation deposition with post annealing treatment. The sintering parameters of holding time and post annealing temperature obviously affected the phase structure and electrical properties. The best sintering parameter was holding for 5 h and post annealing temperature 300 ℃. The annealing temperature affects the phase structures and electrical properties. The relative atomic ratio of Se/Bi decreases with increasing annealing temperature. WAL and resistivity upturn are both observed only in the film annealed at 300℃.Bi2Se3 thin films with good crystalline quality and highly c-axis orientation were synthesized by magnetron sputtering with post annealing treatment. The films have good crystalline quality, and their surfaces exhibit terracelike quintuple layers. Most good thin nanoplatelets exhibit hexagonal morphologies with planar dimensions about 400 nm, while some of them show truncated triangle morphology. The resistivity of films with different thicknesses exhibit different characteristics at different temperature regions. The weak antilocalization (WAL) cusp was observed in the magneto-transport measurements of topological insulators at low temperatures and low magnetic fields. We also find the linear-like magnetoresistance (MR) at high-field, which is associated with the gapless topological surface states and of quantum origin.LSMO thin films have been grown on LaAl03 (100) single crystal substrates by the polymer-assisted chemical solution deposition method. Then Bi2Se3 films were deposited on LSMO films. Bi2Se3/LSMO films have good crystalline quality with hexagonal structure. The p-T curves still showed different behavior in different thickness films, while the metal-insulator (M-I) transition in low temperature region was obvious in all samples. This may be due to long distance carriers in LSMO access to the Bi2Se3, resulting in strong coupling and competition between various types of carriers. Magnetic measurement has been confirmed the presence of ferromagnetic in Bi2Se3 thin films which may attribute to the magnetic proximity effect of the substrate introduce a ferromagnetic order.