Theoretical Design And Properties' Regulation Of Two-dimensional Spintronics Materials

Since the successful exfoliation of graphene in the experiment,two-dimensional(2D)materials with atomic thickness and novel properties,such as the two-dimensional magnetism and the Dirac half-metallicity,have attracted both academic and industrial widespread interest.In recent years,a variety of two-dimensional spintronic devices based on two-dimensional magnetic materials have been reported,including two-dimensional magnetic tunnel junctions and two-dimensional spin field-effect transistors,which have opened up new possibility of two-dimensional spintronics.However,these two-dimensional spintronics materials suffer from low critical temperature or degradation in ambient conditions,which not only hinders their further scientific investigation in the experiment and practical applications but also poses a challenge for the development of two-dimensional spintronics.Therefore,it is significant to explore and design two-dimensional spintronics materials with room-temperature magnetism,ambient stability,or unique physical properties for the fundamental research and practical application of spintronics.Density functional theory(DFT)developed from quantum mechanics is a powerful tool for the exploration of new materials and the study of material properties.Lots of successful predictions of the electronic structure and magnetic properties of two-dimensional spintronics materials have been made,showing the power of density functional theory toward providing guidance for experimental research and greatly shortening the development cycle of two-dimensional spintronics materials.Here,based on density functional theory,by constructing the combination of anions and cations with different filling states,two-dimensional spintronic materials with d state magnetism,d+p state magnetism and p state magnetism have been realized.These materials have excellent properties,including room temperature magnetism,ambient stability,or exotic anionic anionogenic Dirac half-metallicity.There are five chapters in the thesis.In the first chapter,the density functional theory and magnetic materials are briefly introduced.Firstly,we provide an overview of the density functional theory,including the Born-Oppenheimer approximation,one-electron approximation,Hohenberg-Kohn theorem,Kohn-Sham equations,exchange-correlation functionals,and several commonly used calculation software packages.Then,we briefly introduce the classification of magnetic materials,the exchange interaction in magnetic materials,and the research progress of two-dimensional spintronics materials.Finally,a brief introduction of our researches is presented.In the second chapter,we explore the two-dimensional transition metal borates with room temperature antiferromagnetism and ambient stability.Poor ambient stability is one of big obstacles for the research and application of two-dimensional spintronics materials.Two-dimensional magnetic materials with room-temperature magnetism and ambient stability are promising for practical high-performance spintronics nanodevices,yet have been mostly lacking.We notice that the three-dimensional chromium orthoborates(c-CrBO3)have excellent ambient stability and expect that its two-dimensional isomer CrBO3 has the ability to against the degradation in the atmosphere.The calculation results show that two-dimensional materials CrBO3 are antiferromagnetic semiconductors with a band gap of 5.09 eV and have excellent thermodynamic stability and high ambient stability.Interestingly,compared with the three-dimensional c-CrBO3,the bond angle of Cr-O-Cr in the two-dimensional CrBO3 is close to 90°,which is conducive to mediating the super-exchange interaction between adjacent Cr cations.The Neel temperature of two-dimensional CrBO3 is 397 K and can be significantly enhanced up to 672 K under compressive strain.We also explore the electronic and magnetic properties of two-dimensional TMBO3(TM=V,Mn,Fe).Our calculations show that these materials are antiferromagnetic semiconductors with a Neel temperature of 96-155 eV and a band gap of 2.92-3.39 eV.In the third chapter,we focus on the studies of the two-dimensional Janus Cr2X3S3(X=Br,I)semiconductor with intrinsic room-temperature magnetism.To date,several 2D magnetic semiconductors that have been experimentally synthesized and widely studied.However,the critical temperature of these materials is much lower than room temperature,hindering their further research.Exploring two-dimensional magnetic semiconductors with intrinsic room temperature magnetism is one of the current main tasks.Two dimensional Janus materials obtained by substituting one layer of surface atoms in pristine materials with other atoms usually process novel magnetic properties different from the pristine materials and are worthy of further study.Our research shows that the two-dimensional Janus Cr2X3S3(X=Br,I)materials derived from replacing a layer of halogen atoms in the two-dimensional CrX3 materials with S atoms are room-temperature magnets.The first-principles calculation results show that the two-dimensional Janus Cr2X3S3(X=Br,I)materials are stable ferrimagnetic direct bandgap semiconductors.The S anions with partially-filled porbitals are coupled with Cr cations through strong d-p direct exchange interaction,producing room temperature magnetism with a critical temperature(Tc)of 387-447 K,much larger than that of CrB r3 and CrI3.We also found that the room temperature magnetism inducing by strong Cr-S direct exchange is robust under in-plane biaxial strain.The band gaps of the two-dimensional Janus Cr2Br3S3 and Cr2I3S3 are 1.19 and 0.61 eV,respectively,and are sensitive to the in-plane strain.In the fourth chapter,we discuss the orbital design of two-dimensional transition metal peroxide kagome crystals h-TM2(O2)3(TM=Ti,Zr,Hf)with anionogenic Dirac half-metallicity.To realized 100%spin-polarized carriers with low decoherence in spintronics devices,the anionogenic half-metals(AHM)and Dirac half-metals(DHM)have been widely developed.With this progress,the possibility to combine AHM and DHM's behavior in a single material,named here as anionogenic Dirac half-metal(ADHM),is of particular interest to further enhance the spin-coherent transportation of carries in half-metallic materials,but few materials potentially possess this feature.In past decades,it is sporadically reported that few materials potentially possess this feature,but lacking a practical design strategy to realize this attractive idea.In this work,we present an orbital design of two dimensional anionogenic Dirac half-metal by patterning cations(Ti4+,Zr4+,Hf4+)with empty d orbitals and anions([O2]8/3-)with partially filledp-type orbitals into kagome lattice.The two-dimensional transition metal peroxides h-TM2(O2)3(TM=Ti,Zr,Hf)are stable two-dimensional anionogenic Dirac half-metals with a Curie temperature over 103 K.The partially filled*? and ?*orbitals in dioxygen anions are ferromagnetically coupled via the double exchange interaction,resulting in anionogenic half-metallicity.According to the results of the tight-binding model of the Kagome lattice,the 2/3-filled ?*orbitals of the dioxygen anion contribute to the spin-polarized Dirac point at the Fermi level.The last chapter is the summary and outlook of our research works.Based on density functional theory,we have studied several two-dimensional spintronics materials and explored their basic electronic,magnetic properties,and a variety of response behaviors under strain.These two-dimensional spintronics materials with interesting and unique properties can meet the requirements of spintronics in specific application scenarios,and their future is promising.