First-principles Study Of Two-dimensional Magnetic Transition Metal Compounds

Nanomaterials are the basis for the research and development of nanoscale devices in the future.Among them,ultra-thin two-dimensional nanomaterials with a thickness of only a few or one atomic layer have attracted extensive attention from researchers for their excellent and unique properties.Thanks to the development of computer and computational materials science,people can predict,explain and control the properties of two-dimensional nanomaterials from the perspective of statistical mechanics and first principles,and design new two-dimensional nanomaterials with more powerful and ideal functions.Due to the atomic thickness and controllable electronic spin degree of freedom,as well as the required electronic functions,two-dimensional ferromagnetic materials provide the possibility for the manufacture of next-generation spintronic devices.Generally speaking,about data reading and writing,ferromagnetic is easy to read and difficult to write,ferroelectric is easy to write and difficult to read.Multiferroic materials combine both advantages,which can realize efficient electric writing+magnetic reading,providing an effective means for electromagnetism.Chapter one briefly introduces the development of computational materials science and its important role in the research of two-dimensional nanomaterials.Then we briefly introduce several types of related two-dimensional materials（such as graphene,group IV A,V A Xenes,BN and transition metal dichalcogenides,etc.）.Finally,the unique properties of ferromagnetism and ferroelectricity in the relevant two-dimensional material systems that this thesis focuses on are briefly introduced.Chapter two briefly introduces the main methods used in the computational materials science research,i.e.the statistic mechanics methods based on the classical mechanics and the quantum chemistry methods based on the quantum mechanics.Here we mainly introduce two widely used methods,namely density functional theory（DFT）method and the Monte-Carlo（MC）simulation method.The DFT method is used to study the ground state properties of materials at absolute zero kelvin,and the MC method is used to study the magnetic behavior of excited state at finite temperature.Then,we briefly introduce several software packages used in the calculation of this paper.Chapter three proposes a new kind of method to improve the Curie temperatures（T_C）of ferromagnetic materials by constructing heterostructures.The integration of ferromagnetic and semiconducting properties in a single two-dimensional（2D）material has been recognized as fertile ground for fundamental science as well as for practical applications in information processing and storage.The discovery of Cr I₃monolayer confirmed the existence of macroscopic spontaneous magnetization（two-dimensional ferromagnetism）in a two-dimensional system for the first time.However,its Curie temperature（T_C）is too low（～45 K）,far from satisfying the practical application of spintronics.Here,we show that the in-plane FM coupling of Cr I₃can be remarkably enhanced by constructing a 2D heterostructure where Cr I₃monolayer is supported on a non-magnetic normal semiconductor/insulator substrate.Choosing Mo Te₂monolayer as a substrate,we find that the Cr I₃/Mo Te₂2D heterostructure is an intrinsic semiconducting ferromagnet with T_Cof～60 K.The T_Ccan be further increased to～85 K by applying an out-of-plane pressure of～4.2 GPa.The doubling of the T_Cin this 2D heterostructure comes from the introduction of extra spin super-exchange（Cr-Te-Cr）paths.Our findings provide a promising pathway to improve ferromagnetism in 2D semiconductors,which can stimulate further theoretical and experimental interest.In chapter four,we turn to the two-dimensional ferromagnetic semiconductor Cr SBr monolayer,which is naturally vd W layered material and recently received much attention.Two-dimensional（2D）ferromagnetic（FM）semiconductors with direct electronic band gap have recently drawn much attention due to their promising potential for spintronic and magneto-optical applications.Here,we show that through isovalent alloying,one can increase the T_Cof a 2D FM semiconductor up to room temperature and simultaneously turn it from an indirect to a direct band gap semiconductor.Using first-principles calculations,we predict that the alloyed Cr Mo S₂Br₂monolayer is a direct band gap semiconductor with a T_Cof～360 K,whereas the pristine Cr SBr monolayer is an indirect band gap semiconductor with a T_Cof～180 K.These findings provide a promising pathway to realize2D direct band gap FM semiconductors with T_Cabove room temperature,which will greatly stimulate theoretical and experimental interest in future spintronic and magneto-optical applications.In chapter five,we study the metal doping induced multiferroic properties of two-dimensional transition metal halides.Intrinsic two-dimensional transition metal halides generally have a highly symmetrical structure(such as D_3dsymmetry)and therefore do not have multiferroic properties.In order to induce multiferroicity in transition metal halides,using density functional theory calculations,we systematically studied the structural stability,electronic properties of two-dimensional transition metal trihalides Rh X₃and Ir X₃（X=Cl,Br）by metal atom Li or Al doping.The calculated results show that Li or Al doping can cause Jahn-Teller distortion of the systems and reduce the crystal symmetry,resulting in in-plane polarization.Moreover,the doped electrons introduce a local magnetic moment on the d orbital of the transition metal,making the systems ferromagnetic and ferroelectric.Thus,our results provide a new research idea for the realization of two-dimensional ferromagnetic materials.In chapter six,we propose a new type of Cr O₃monolayer with a two-dimensional multiferroic structure.Two-dimensional（2D）multiferroic materials with the coexistence of electric and spin polarization offer a tantalizing potential for high-density multistate data storage.However,intrinsic 2D multiferroic semiconductors with high thermal stability are still rare to date.Here,we propose a new mechanism of single-phase multiferroicity.Based on the first-principles calculations,we predicted that in the Cr O₃monolayer,the unconventional distortion of the square antiprismatic crystal field on Cr-d orbitals will induce an in-plane electric polarization,making this material a single-phase multiferroic semiconductor.Importantly,the magnetic Curie temperature is estimated to be～220 K,which is quite high as comparison to the recently reported 2D ferromagnetic and multiferroic semiconductors.Moreover,both ferroelectric and antiferroelectric phases are observed,which provides opportunities for electrical control of magnetism and energy storage and conversion applications.These findings provide a comprehensive understanding of the magnetic and electric behavior in 2D multiferroics and will motivate further application researches on related 2D electromagnetics and spintronics.