Theoretical Studies Of Magnetic-Electric-Valleytronic Coupling Effect In Two-Dimensional Magnetic Materials

Due to the quantum confinement effect,the micro-nanoelectronic technology based on the degree of freedom of charge has encountered the bottleneck predicted by Moore's Law.The spintronic device based on the degree of freedom of electron spin provides an opportunity for the realization of high-density,high-speed and low-power information transmission,processing and storage.In addition,some extreme points on the energy band dispersion curve are regarded as valley degree of freedom,which has unique electronic structures.Similar to the degrees of freedom of charge and spin,valley can also be used to encode and process information,which has broad application prospects of information storage.Since 2017,experiments have reported a series of two-dimensional magnetic materials.These magnetic materials exhibit interesting interactions between charge,spin and valley,which provides an ideal platform for studying the correlation of multiple degrees of freedom.In this regard,by first-principles calculations and theoretical model analysis,this thesis studies the characteristics and commonalities of different two-dimensional magnetic materials in electronic structure,magnetism,and valleytronics,constructs two-dimensional heterostructures with new physical effects,proposes the method of flexible manipulation of charge,spin and valley degrees of freedom to reveal the physical mechanism of magnetic-electric-valleytronic coupling effect.In Chapter 1,we briefly review the theoretical limitations in the process of exploring low-dimensional magnetism,and focus on the research progress of several two-dimensional intrinsic magnetic materials.Furthermore,we introduce the recent rise of valley that have largely been enabled by the isolation of two-dimensional materials.At the end of this chapter,the magnetoelectric coupling and magnetic-valley coupling in two-dimensional materials are introduced as well.In Chapter 2,we introduce the related approximation in the energy band theory of solid and the evolution of density functional theory.From Chapter 3 to Chapter 6,four studies carried out in the doctoral period are presented.The specific contents are as follows:In Chapter 3,we take the 2H-VSe₂ bilayer as an example,through first-principles calculations,to show that antiferromagnetic van der Waals bilayers can be made half metallic.Conductive electrons in half metallic have spin polarization in a specific direction,while spin-polarized electrons in the opposite direction are still insulated.Based on this finding,the spin field effect transistor with 100%spin polarization is proposed.In Chapter 4,we study the effect of electric field on the van der Waals Cr₂Ge₂Te₆bilayer with interlayer ferromagnetic coupling.The continuous enhancement of the electric field can induce spin-resolved charge transfer in the system,which in turn influences the magnetization of each constituent layer.Interestingly,the change of the magnetic properties in the system is related to the applied electric field.This kind of electric manipulation of magnetism generally exists in two dimensional magnetic systems.Band engineering provides a new idea for electric manipulation of magnetism in low-dimensional systems.In Chapter 5,we study the magnetic proximity effect induced valley splitting in the two dimensional magnetic heterostructures MX₂/Cr₂Ge₂Te₆.Combining a two band k·p model with first principles calculations,the dependence of valley splitting on the normal strain is revealed.Furthermore,the influencing factors of valley splitting are summarized by comparing the different MX₂/Cr₂Ge₂Te₆ heterostructures.The tunability of valley splitting in MX₂/Cr₂Ge₂Te₆ heterostructures is of great significance in the valleytronics.In Chapter 6,we study the dependence of the magnetocrystalline anisotropy in the antiferromagnetic monolayer MnPSe₃ on the charge injection.The magnetic order in the system can be switched from in-plane to out-of-plane by charge injection.Based on second-order perturbation Hamiltonian,the related physical mechanism is analyzed in detail.Moreover,the change of the magnetic properties in the system can be reflected as the absorption of circularly polarized light,which makes it possible to characterize the magnetic phase transition of the system through non-destructive and non-contact optical means.In the last Chapter,we summary the main research conclusions of this paper,and look forward to the research directions that can be deeply explored.