Design And First-Principles Calculations Of Two Dimensional Magnetic Semiconductors

Along with the explosive development of semiconductor industry,Moore’s law has become unsustainable,and spintronics has emerged which incorporates spin into electronic devices and thus performs efficient information processing.As a member of spintronics materials,magnetic semiconductors harbor extensive application prospect,which combines the semiconductive logic and magnetic storage performances.In terms of faster information processing speed,lower power consumption and non-volatility,magnetic semiconductor behaves as a new information carrier,making spintronics technology a leading microelectronics technology in foreseeable future.In recent years,research of magnetic semiconductors has mainly focused on two aspects.On the one hand,people has pursued the integration of multiple functions in 2D magnetic semiconductor materials,which aims at improving material performance,realizing one material with diverse functions,and thus meeting the requirements of miniaturized device units.On the other hand,to truly realize industrial applications,tremendous effort has been devoted to synthesize 2D magnetic semiconductors available at room temperature.But so far,reports on 2D multifunctional magnetic semiconductors are still limited,and people have not yet fabricated 2D room-temperature magnetic semiconductor materials.Therefore,based on the first-principles calculation method,this thesis concentrates on designing various 2D multifunctional magnetic semiconductors and room-temperature multiferroic semiconductor materials.This dissertation is divided into six chapters.In chapter 1,after reviewing the origin and development of spintronics and summarizing the magnetic interactions in spintronics materials,we introduce the classifications of spintronics devices and materials.Then we briefly describe 2D spintronic materials,which belongs to the booming field for the past few years.Contributed by the swift development of computing methods and computer technology,computational quantum chemistry provides enormous theoretical support and guidance for designing original materials,thus forms a novel model of complementation between theory and experiment,which accelerates the synthesis of advanced materials with lower cost and shorter development cycle.In chapter 2,we introduce the theoretical knowledge of computational quantum chemistry and density functional theory that are widely implemented in various fields.In chapter 3,we acquire a 2D intrinsic ferromagnetic semiconductor with controllable magnetic phase transition by exfoliating from bulk crystal.Although 2D intrinsic ferromagnetic semiconductors harboring controllable magnetic phase transition are highly desirable for spintronics,reports on their successful experimental realization are still rare.Our proposed CrSbS3 monolayer is demonstrated to be an intrinsic ferromagnetic half semiconductor with a moderate bandgap of 1.90 eV.Furthermore,it features an intriguing magnetic phase transition from ferromagnetic to antiferromagnetic when applying a small compressive strain(～2%),making it ideal for fabricating strain-controlled magnetic switches or memories.In chapter 4,we predict a 2D multifunctional metal-organic framework material with both negative Poisson’s ratio and bipolar magnetic semiconductor properties.2D Cr(DCNQI)2 harbors an unusual in-plane negative Poisson’s ratio with a maximum absolute value of 0.85,which is fairly large and scarce in reported auxetic materials.Due to the strong d-p direct exchange interaction between Cr cations and DCNQI anoins,2D Cr(DCNQI)2 exhibits intrinsic ferrimagnetism with a relatively high Curie temperature of 217 K,which is higher than that of CrSbS3 in chapter 3.Moreover,2D Cr(DCNQI)2 is an intrinsic bipolar magnetic semiconductor,where gate voltage transforms it into a spin-polarized bipolar half-metal with revisible orientation.Therefore,this 2D metal-organic framework material has enormous potential for application in electronically controlled nanospintronics devices.In chapter 5,we present three 2D room-temperature metal-organic framework multiferroic semiconductor materials based on conjugate six-membered heterocycles.Herein,45 2D metal-organic frameworks are constructed by diverse conjugate sixmembered heterocycles and transition metal Cr.Then,according to the stability,magnetism,magnetic copuling strength,polarization,and polarization switching barrier,we screen aforementioned 45 materials and ultimately focus on three 2D room-temperature multiferroic metetials,namely Cr(1,2-oxazine)2,Cr(1,2,4-triazine)2,and Cr(1,2,3,4tetrazine)2.These mentioned systems possess higher Curie temperatures(290-470 K)than those of CrSbS3 and Cr(DCNQI)2.Meanwhile,three systems are not only bipolar magnetic semiconductors where gate voltage can induce half-metallic conduction with controllable spin-polarization direction,but also harbor polarization switching barriers(0.18-0.31 eV)which are sufficient to overcome thermal fluctuations and achieve switching by small external electric field.This study would provide novel perspective for research of 2D multiferroic semiconductor materials available at room temperature.In chapter 6,we summarize my research work during my doctoral period and give an outlook on future research directions.