First-principles Study On Application Of Two-dimensional Materials In Spintronics

In the past decade,two-dimensional（2D）materials have been widely used in several technological fields,such as semiconductor,electronic information,and biomedical science,injecting new vitality into the development of these fields.Among them,it is one of the most popular research topics to use 2D materials for the low-power 2D spintronic devices.Different from traditional electronic devices,spintronic devices employ electron spin as the carrier,which greatly reduces the Joule heat generated in the process of signal transmission,and facilitates the realization of low-power,high-speed data processing,data transmission and data storage.However,the application of low-power 2D spintronic devices still faces a series of challenges.For instance,the dissipation path of magnon energy still is ambiguous,and there is very little magnetic materials whose Curie/Néel temperature is higher than room temperature.So there are a series of problems on physical mechanisms and material properties need to be solved for the practical application of low-power 2D spintronic devices.With the rapid development of computer hardware and the continuous improvement of theoretical calculation methods,the first-principles calculation based on density functional theory draws more and more attentions.Compared with the experimental characterization,the theoretical calculation is more conducive to investigate the quantum distribution in material,providing directional guidance and theoretical support for the implementation and result analysis of the experimental characterization from the perspective of basic physics.Thus,the unnecessary time and resource loss can be reduced significantly.In this dissertation,the first-principles calculations based on density functional theory will be used to systematically explore the physical properties,scattering mechanism and key parameters of2D nonmagnetic,ferromagnetic and antiferromagnetic materials for their application in spintronics.Our study can provide theoretical references for the design and application of low-power 2D electronic devices.The main contents and innovations of this dissertation are as follows.1.We took black phosphorene as an example because of its unique geometrical structure and excellent properties.Firstly,this dissertation investigates the influence of the substitutional doping with the fourth period main group elements on the band structure of2D nonmagnetic materials when the impurity content is 6.25%.It is obvious that the band gap of doped black phosphorene relies heavily on the number of valence electrons around impurity atoms.Then,the impurity content in the doped black phosphorene would be manipulated by changing the size of the supercell and increasing the number of impurity atoms.It is found that the doping of Si,S and Cl atoms may inject electron spin into black phosphorene.For instance,when the supercell is up to 4’4’1,the Si-and S-doped black phosphorene are ferromagnetic,while the doping of even Cl atoms also induces anti-ferromagnetic order into black phosphorene.These strategies make black phosphorene satisfy the basic requirements for application in spintronic devices.2.After the energy of the system with different spin configurations is obtained by the first-principles calculation,the theory of ferromagnetic spin-wave is derived,and the spin-wave spectrum of Cr₂Ge₂Te₆（CGT）at zero temperature is plotted by using isotropic Heisenberg model.On this basis,the Debye model is employed to study the scattering between random phonon modes and magnons caused by finite temperature.Subsequently,the magnon relaxation time governed by magnon-phonon scattering is calculated using the uncertain principle.It is found that with the increase of temperature,the strength of magnon-phonon scattering increases and the corresponding magnon relaxation time monotonously decreases.At the same time,the effects of gas adsorption on the Curie temperature,band structure and magnon-phonon scattering of CGT are also discussed.It can found that the gas adsorption can not only remarkably increase the Curie temperature of magnetic materials,but also enhance the strength of magnon-phonon scattering and shorten the magnon relaxation time.More importantly,it is found that the acoustic magnon relaxation time and the optical magnon relaxation time have completely different wave-vector dependence in the long-wavelength limit,and this wave-vector dependence owns excellent robustness to temperature change and gas adsorption.Those results and conclusions provide a meaningful theoretical support for the application of ferromagnetic materials in spintronic devices.3.When the contribution of magnon-magnon dynamic scattering at finite temperature is taken into consideration,the isotropic Heisenberg Hamiltonian is modified in this dissertation,and becomes temperature-dependent.Afterwards,the spin-wave spectrum of 2D ferromagnetic material CGT at finite temperature is obtained and plotted.Based on this spin-wave spectrum,the dependence of the strength of magnon-magnon scattering and the magnon relaxation time caused by magnon-magnon scattering on wave vector,magnon frequency,temperature and external magnetic field is investigated by spin autocorrelation functional.It is found that the magnon relaxation time determined by the magnon-magnon scattering increases with temperature T,but decreases with wave vector k,magnon frequencywand external magnetic field B.Results in this dissertation provide an insight into understanding the damping and energy dissipation of magnon in two-dimensional ferromagnetic materials.4.Compared with the ferromagnetic spintronic devices,the antiferromagnetic devices made of antiferromagnetic materials have advantages of the robustness to magnetic field perturbation,no stray field,and ultrahigh device operation speed,and can be one of the representative components for applications of spintronic devices.Taking Cr₂Ti C₂FCl in MXenes family as example,the antiferromagnetic spin-wave theory is derived,and the dispersion curve of spin wave including the nearest,next-nearest,and next-next-nearest neighbor couplings is plotted.Meanwhile,the phononic properties including vibration modes,Grüneissen parameters,and group velocities are obtained and discussed.Based on Grüneissen parameters and group velocity,the lattice thermal conductivity of Cr₂Ti C₂FCl is estimated through Debye-Callaway theory.Besides,the influence of intrinsic phonon modes with infrared activity on the band structure and spin-wave spectrum of two-dimensional antiferromagnetic materials is investigated,which not only improves the theoretical methods for the study of magnon-phonon scattering in 2D magnetic materials,but also lays a foundation for the study of magneto-optical effect and spin Seebeck effect in two-dimensional antiferromagnetic materials.