Study On The First-principles Of Material Innovative Magnetic Mechanism And Electronic Structure

It needs efficient spin source for spintronics.For example,half-metal CrO₂ canown 96% spin polarization under the temperature of liquid He.The current based onspin Hall Effect with the scattering of impurities and the method of spin photoelectriceffect can also produce spin current.However these spin current must be kept undervery low temperature.The traditional ferromagnetic metal such as Fe can induce spincurrent at room temperature,but the current is scattered when it goes through the in-terface because of mismatch of energy band and impedance between metals and semi-conductors.Thus people consider so-called diluted magnetic semiconductors (DMS)as one kind of spin resource.Mn doped ZnO could have ferromagnetism at roomtemperature.The experimental results are however,full of contradictions.And allfirst-principles calculations show that (Zn,Mn)O is antiferromagnetic.Then what isthe mechanism of ferromagnetism of (Zn,Mn)O? As one of chemical compounds ownmagnetism,but do not contain transition metal ions,the mechanism of magnetism ofCaB6 puzzles people for a long time.Why could not people find it? Can people findmagmoments in bulk Si? What will happen when DMS under extreme condition,suchas high pressure? These problems are valuable to be studied.With development of computer science,the first-principles calculations in con-densed matter physics is one of the most important branches.This thesis studiessome new mechanism of producing magnetism and electronic structures of materialsbased on density functional theory.Chapter 1 of this paper is introduction.Chap-ter 2 is the method of calculations.Chapter 3 is origin of the co-dopant induced en-hancement of ferromagnetism in (Zn,Mn)O.Chapter 4 is what produces magnetism inCaB₆.Chapter 5 is about possible magnetic behaviors in amorphous silicon doped withhigh-concentration boron.Chapter 6 is on phase transition and electronic structure ofZn_1-xMn_xSe (x=0,0.25) under high pressure.Chapter 7 is about First-principles inves-tigation on the electronic structures of intercalated fullerenes M₃N@C₈₀(M=Sc,Y,andlanthanides).Chapter 8 is for conclusion.The detailed introductions for each chapterare listed below.Chapter 1 of this paper is introduction introduction to DMS and the magnetism ofmaterial containing no transition metal ions.Chapter 2 is the method of calculations.Chapter 3 is origin of the co-dopant induced enhancement of ferromagnetism in (Zn,Mn)O.Using the density functional calculations,we elucidate the mechanism ofco-dopant induced enhancement of ferromagnetism of (Zn,Mn)O.Li and Cu atomstends to segregate toward Mn atoms and strongly promote the ferromagnetic couplingvia either RKKY or superexchange interaction.The hole states introduced by eitherLi or Cu are rather delocalized and they are efficient in mediating magnetic ordering.These findings shed new light for the design of dilute magnetic semiconductors withco-dopants for spintronic applications.Chapter 4 is what produces magnetism in CaB₆.Through systematic density func-tional calculations,we revealed that the weak ferromagnetism in CaB₆ and relatedmaterials stems from the joint effect of boron vacancies and impurities.Each boronvacancy introduces strongly localized gap states around its first neighbor B atom rightabove the Fermi Level.The partial occupation of these gap states caused by either lat-tice distortion or addition charge triggers magnetic instability and the local magneticmoment can be as large as 0.8-1.2μB in various cases.Chapter 5 is magnetic moments on silicon atom of high-concentration boron inamorphous Silicon.We first find out magmoments on Si atom in bulk amorphous (a-)Si with high-concentration boron doping in by using first-principles molecular dynamic(MD) calculations.Fifteen a-Si₆₃B₁ (SiB) models have been calculated and six of themhave magnetism.The structure with magmoment of 0.993μ_B has the lowest energy.The magnetism of a-SiB is from one of the threefold coordinated Si atoms.Thesethreefold coordinated Si atoms all have high peaks in density of states (DOS) becauseof the single unsaturated bonds.And the unsaturated bond pins at Fermi level,it is tosay the hole from boron offers to the silicon.The partial DOS peaks then can be splitand have spin polarization based on Stoner criterion.Spin polarized energy of thesethreefold coordinated Si atoms are all around 50 meV and the magmoment can evenexit above room temperature.Chapter 6 is phase transition and electronic structure of Zn_1-xMn_xSe (x=0,0.25) under high pressure.High-pressure phase transition and electronic structure ofZn_1-xMn_xSe (x=0,0.25) are studied by using first-principles density functional cal-culations.Pressures at which phase transition from zinc blende to rock salt and bulkmoduli are obtained,both in good agreement with experimental results.The calculatedenthalpies along transition path can well explain the fact that transition pressure ofZn_0.75Mn_0.25Se is lower than that of undoped ZnSe.Obvious repulsion between Zn/Mn3d and Se 4p is found to be the driving force for the structural transition.The change of electronic structure from semiconductor to metal,accompanying the structural tran-sition,is ascribed to the decrease of constrain of Se atoms to s electrons.Chapter 7 is first-principles investigation on the electronic structures of interca-lated fullerenes M₃N@C₈₀(M=Sc,Y,and lanthanides).C₈₀ cages with encapsulatedgroupâ…¢and lanthanide nitrides are systematically studied through density functionalcalculation.Most intercalated fullerenes M₃N@C₈₀ exhibit similar spectroscopic fea-tures near Fermi level,in consistent with experimental results.We establish that theplane of Sc3N is perpendicular to the C₅ axis,rather than the C₃ axis as proposed be-fore.The charge transfer from metal atoms toward N and C atoms is quantitativelyanalyzed.The role of 4f states treated as valence states is explored through LSDA+Ucalculation finally.Chapter 8 is conclusion and prospect of the thesis.