Magneto-optical Raman Spectroscopy Study On Two-dimensional Magnetic Materials VX₃ And MnIn₂Se₄

The emergence of two-dimensional magnetic materials opens up a new field for condensed matter physics and provides an ideal platform for exploring the low dimensional magnetic theory.VX₃ and MnIn₂Se₄ materials are two types representative two-dimensional magnetic materials.Their potential surpris ing physical images will be laid a solid foundation for future electronic spin devices fields.As a non-destructive measurement technology with micron spatial resolution,Raman spectroscopy is an ideal tool to characterize the physical properties of two-dimensional materials,such as structure and phonon vibration.Due to its high spatial resolution and high sensitivity to lattice symmetry,Raman spectroscopy can reveal the magnetic order,phase transition and spin phonon coupling of two-dimensional magnets.Since the discovery of two-dimensional magnetic materials,the commonly used Raman spectra have only been used as indirect evidence for detecting magnetism,and the Raman methods for directly detecting magnetism has not been fully found.In recent years,the magneto-optical Raman effect observed in the two-dimensional magnetic material Cr I₃indicates that the magnetic order will greatly affect the scattering of phonons,which indicates that it is possible to directly detect magnetism using Raman spectroscopy.However,the mechanism of inelastic scattering light controlled by the magnetic order of two-dimensional magnetic materials is still unclear.On the other hand,many intrinsic two-dimensional magnetic materials need to be discovered,and their physical properties related to str ucture and magnetism need to be solved.These studies will provide a reference for the mechanism of the interaction between magnetic order and light,and reference for the discussion of the emerging two-dimensional magnetic theory,which affords a potential direction for the development and application of spin electronic devices in the future.In this thesis,the magneto-optical Raman effect of two-dimensional magnetic materials VX₃ and MnIn₂Se₄ have been studied systematically.High-quality VI₃,VBr₃ and MnIn₂Se₄ single crystals were successfully grown by chemical vapor transport method,and the air sensitive VI₃ and VBr₃ samples were encapsulated by hexagonal boron nitride thin layers which like sandwich structure using dry transfer technology for Raman measurement.The main research results are summarized as follows:（1）The structure,magnetic excitation and coupling of the two-dimensional magnetic material VI₃ were explored through the systematic Raman spectra.From the temperature dependent and polarized Raman spectroscopy,the results show that the structure of VI₃ changes from C2/m phase to R（?） phase with the decrease of temperature;We also measured the quasi-elastic scattering which representing the magnetic fluctuation.Interestingly,we found that it was strongly coupled with the adjacent phonon modes,emerging a Fano anti-resonance phenomenon.Besides the lattice excitations,single magnon and two magnon modes are measured at low temperature.Those modes are rare in two-dimensional magnetic materials.Then,the magnetic anisotropy and intralayer exchange coupling strength are estimated by combining the density function theory and the linear spin wave theory.More importantly,we measured the ferromagnetism of VI₃samples by helicity resolved Raman spectroscopy.It is proved that the magnetic order can greatly affect the Raman scattering characteristics of photons in VI ₃,which is consistent with the breaking of time reversal symmetry in the material.The selection rule of A_g mode of circularly polarized Raman is theoretically analyzed by group theory.The helicity resolved Raman spectra of the main A_gmodes with temperature-dependent and magnetic field dependent clearly show the ferromagnetism in VI₃.The helicity resolved Raman spectroscopy are directly used to detect the magnetic phase transition in two-dimensional ferromagnetism;（2）The structure and magnetism of two-dimensional magnetic material VBr₃were systematically studied.Single crystal X-ray diffraction（SC-XRD）and X-ray energy chromatography（EDS）proved that high quality VBr₃ samples were grown.The temperature-dependent XRD spectra revealed that the structural phase transition occurred at 90 K;Through systematic Raman spectra,it is found that Eg mode centered at 131 cm^-1 will split into A_g+B_g mode at low temperature.It is proved that the triple rotational symmetry is broken after structural phase transition,the VBr₃ structure changes from R（?） phase to C2/m phase with the decrease of temperature;The quasi-elastic scattering signal from the fluctuation of magnetic energy density was also measured by temperature-dependent Raman spectroscopy.The magnetism of the material was determined by the measure of magnetic susceptibility and specific heat.The measurements confirmed that VBr₃ undergoes a structural phase transition near 90 K and enters an antiferromagnetic ordered state below 26.5 K.Interestingly,when the magnetic field is parallel to the c-axis of the lattice and the applied magnetic field is less than 0.5 T,the susceptibility temperature dependent curve shows the ferromagnetic characteristics of multiple domains.In addition,a large ferromagnetic hysteresis is also observed in the isothermal magnetization curve,and the coercive field is about 1.5 T at 2 K.It is worth noting that there is an obvious antiferromagnetic background in the whole magnetization curve,so the ferromagnetic h ysteresis is also inclined.When the magnetic field is parallel to the ab plane,there is no obvious ferromagnetic hysteresis.Combined with in-plane and out of plane angle dependent susceptibility measurements,it can be considered that the magnetic ground state of VBr₃ should be canted anti-ferromagnetism.（3）Basis of the above researches on halogen binary magnetic materials,we further carried out the research on the structure and magnetic properties of ternary two-dimensional magnetic material MnIn₂Se₄.Firstly,the crystal quality and magnetism of the material were determined by X-ray diffraction,X-ray electron spectroscopy,magnetic susceptibility.It is proved that high-quality MnIn₂Se₄ material has spin glass state under antiferromagnetic interaction below 3.5 K.Secondly,the Raman spectra of MnIn₂Se₄ were systematically measured,and the Raman vibration mode of this material was determined by combining the first principal calculation.The results of temperature dependent Raman measurements show that there is a structural phase transition at 180 K.The polarization of the quasi-elastic scattering and the two magnetic continuous were studied by the temperature dependent polarized Raman measurement.It is found that both of them changes at 180 K,which further determines the structural phase transition temperature of the MnIn₂Se₄material.The quasi-elastic scattering and phonon mode are coupled to produce the Fano resonance phenomenon.T he layer dependent Raman spectrum proves that the quasi-elastic scattering increases with the increase of the interlayer interaction force.For the Raman spectrum below 10 layers,the phonon peak of 27 cm^-1 blue shifts with the increase of the layer number,and the phonon peak of 178 cm^-1 red shifts with the increase of the layer number.Therefore,the layer number change of the material can be determined by the Raman shift.