Low Dimensional Nano Spintronic Materials:A First-principles Study

Spintronics,which uses the spin of electrons for information storage,transportation and processing,has attracted intensive interests from both science and industry in the past decades.In tradition,ferromagnetic crystals with large spin-polarization ratio,high Curie temperature,and large magnetocrystalline anisotropic energy(MAE)are fundamental to build practical spintronic devices that could work at room temperature.In addition,the research and application of nanomaterials is an important subject in the field of condensed matter physics and materials science,and many achievements have been made.To realize this concept at nanoscale,it is one key issue to develop low-dimensional ferromagnetic crystals that possess the above qualities.With the rapid development of grapheme and the unique and diverse electronic structure of low-dimensional materials,the researchers' interest in low-dimensional half metals has been growing exponentially.However,significant magnetism is rarely observed in experiment in carbon materials,Alternatively,2D inorganic graphene-like crystals generates significant interest in research.Metal-organic frameworks(MOFs)is a family of nanoporous crystalline compounds containing networks of metal ions connected by coordinating organic linkers,which are widely studied for that that have an enormous potential to form one(1D),two(2D)and three(3D)structures.The metal used,when designing MOFs materials,depending on the type of metal selected,the size and structure of the ligand or the steric hindrance will produce a variety of properties.In recent years,with the development of density functional theory and related numerical algorithms,the first-principles method has become an important research method to design and study new magnetic materials at the atomic scale.In the thesis,the electronic structure and magnetism of the new low-dimensional nanomaterials are investigated.This paper is divided into six chapters,and there are mainly three parts.The first part,that is,the first chapter mainly introduces the density functional theory.The second part includes the second,third,fourth and fifth chapters,mainly introduces the basic properties of low-dimensional nanomaterials.The third part introduces the ferroelectric and magnetic properties of the metal formate framework[NH4][M(HCOO)3].In the first chapter,we mainly introduce the development and application of density functional theory.Firstly,the adiabatic approximation of quantum mechanics and the Hartree-Fock approximation are introduced.Then the Hohenberg-Kohn theory,the Kohn-Sham equation,the exchange-related energy functional and the software packages that are commonly used are presented,respectively.Finally,we present a detailed description of the Berry Phase theory of polarization.In the second chapter,Based on first-principles calculations and ab-initio molecular dynamic(AIMD)simulation,we show that graphene-like 2D Fe2Si is a ferromagnetic crystal with high structural stability up to the temperature of 1200 K.2D Fe2Si crystal possesses both high Curie temperature of about 780 K and a sizeable MAE.These novel magnetic properties provide great opportunities for spintronics and electronics devices.In the third chapter,the electronic structure and magnetic properties in single-layer NbS2 are calculated by using HSE06.Differently from the results of PBE functional,HSE06 calculations show that 2D NbS2 is magnetic semiconductor with a small band gap.Besides,the effect on electronic properties by carrier doping and strain in 2D NbS2 are also investigated.In the fourth chapter,we propose a new structure of 2D CrOX(X=Cl,Br)by extracting one stoichiometric layer from CrOX bulk,and study the basic electronic properties and the stability of the structures.In the fifth chapter,we invetagate one-dimensional metal organic framework(1D-MOFs)of 1,2,4,5-Benzenetetramine and 1,2,4,5-tetrahydroxybenzene molecular nanowires.We perform the first-principles to investigate the stability and electronic structure of these nanowires.The results show that the structures of these nanowires is stable,and all of them are semiconductors.Furthermore,we studied the effect of carrier doping in the 1D-MOFs.In the last chapter,a comprehensive study on the ferroelectricity and magnetic properties of metal-formate frameworks[NH4][M(HCOO)3](M=3d transition metal atom)(metal-MFF)is performed based on first-principles calculations.We studied the origin of the paraelectric-ferroelectric phase transition in[NH4][Zn(HCOO)3],as well as the value of the polarization and the total energy in the transition.Next,the polarization values,the electronic structure properties and the magnetic properties of other structures are also computed.In addition to[NH4][Zn(HCOO)3],other structures are both magnetic and ferroelectric materials.