Magnetic And Magnetic Transport Properties In Two-dimensional Magnetic Crystals Cr_xTe_y

The magnetism of materials mainly originates from the orbit and the spin of electron,and its application in information storage,sensors,medical diagnosis and other important technologies has a great impact on the progress of science and technology.According to the Mermin-Wagner theorem,spontaneous magnetization does not exist in the two-dimensional（2D）isotropic Heisenberg model at a finite temperature,so the research about intrinsic two-dimensional magnetic materials has been missing for a long time since the discovery of 2D materials.Until 2017,the magnetism of Van der Waals materials Cr I₃ and Cr₂Ge₂Te₆was discovered,which experimentally confirmed the existence of intrinsic magnetism of 2D materials and quickly became a research hotspot.At present,the Curie temperature of most 2D ferromagnetic materials is lower than room temperature,which hiders their development and application.Therefore,looking for 2D materials with intrinsic magnetism at room temperature is still significant.Theoretical calculations show that the ferromagnetic material Cr_xTe_y has high Curie temperature,stable intrinsic magnetism and good metal conductivity which makes it a potential spintronic material.Various components of Cr_xTe_y have been gradually discovered and synthesized,including Cr Te,Cr₂Te₃,Cr₃Te₄ and Cr₅Te₈,etc.Cr content plays a crucial role in crystal structure and magnetic properties.In these compounds,Cr full layer and Cr deficient layer are stacked alternately along the c axis.Cr-Te compounds all have hexagonal crystal structure,among which each layer of Cr Te₂ has Three-layer Structure of Te-Cr-Te,and the adjacent two layers are combined by van der Waals force,which is a typical crystal with layered structure.Another Cr-Te compound,Cr₂Te₃,has many advantages and is an important spintronic material,while Cr₂Se₃ with similar structure has antiferromagnetic properties.The magnetic,magnetic anisotropy and magnetic transport properties of Cr₂Te₃ can be controlled by Se doping.In this thesis,the preparation,physical properties,magnetic properties and magnetic transport properties of Cr Te₂,Cr₂Te₃ and Se doped Cr₂Te₃ with different concentrations were studied in depth by combining experimental research and theoretical calculation.The main research contents of this thesis are as follows:1.KCr Te₂ crystal is grown by solid state reaction method,and 1T-Cr Te₂ single crystal is obtained by De-intercalation of the alkali（K）atoms.The structure of Cr Te₂is characterized by scanning electron microscopy（SEM）,X-ray energy dispersive spectroscopy（EDS）,X-ray diffraction（XRD）and Raman spectroscopy.Magnetic measurement at variable temperature is carried out with superconducting quantum interferometer（SQUID）.The result show that the Curie temperature of Cr Te₂ crystal is about317 K,the saturation field of the easy axis is about 2000 Oe and the saturation magnetic moment is about 30.53 emu/g at 5 K.The resistance of Cr Te₂ as a function of temperature,angle,and magnetic gield was measured by physical property measurement system（PPMS）.The results show that Cr Te₂ has good metal behavior,and the anisotropy magnetoresistance（AMR）value decreased with the increase of temperature,and the absolute value of AMR is 8.2%at 5 K.Combined with first-principles calculations,it is found that monolayer Cr Te₂ still maintains in-plane magnetic anisotropy and can transition from ferromagnetic to antiferromagnetic by applying stress.2.High quality Cr₂Te₃ single crystal and Se doped Cr₂(Te_1-xSe_x)₃ single crystal are prepared by chemical vapor transport method.The structure features characterized by EDS,XRD and Raman.The results show that both Cr₂Te₃ and Cr₂(Te_1-xSe_x)₃ belong to the hexagonal crystal system,the Se atom replaces the Te atom,and the crystal structure and symmetry are not damaged.The Cr₂(Te_0.87Se_0.13)₃ and Cr₂(Te_0.75Se_0.25)₃ crystals show ferromagnetism at low temperature,and the coercivity and magnetization of the crystals are improved compared with Cr₂Te₃,but the Curie temperature decreases with the increase of Se doping ratio.The magnetism changes with the increase of Se doping concentration in the system.The magnetism of Cr₂(Te_0.25Se_0.75)₃ is antiferromagnetic,similar to that of Cr₂Se₃.However,it is found that Cr₂(Te_0.25Se_0.75)₃ has semiconductor properties.The magnetic transport measurement results show that the AMR of Cr₂Te₃ crystal is 7.3%at 5 K.With the increase of Se doping concentration,the lattice constants,doping position,magnetism,density of states and energy band structures are investigated by first-principles calculation。...