Studies On Magnetism Of Semiconductor And Ultiferroic Materials

In recent years, more and more semiconductor materials with ferromagnetic properties at room temperature receive much concern due to their wide range of applications. The experiments have confirmed that many oxide films or nanoparticles would have ferromagnetic properties at room temperature. However, the calculations of first-principle indicate that the vacancy of oxide semiconductor lead to the existence of their magnetism. The experiments have also confirm that GaN semiconductor nanoparticles also have ferromagnetic properties at room temperature while first-principle shows the cause of the magnetism of cubic structure and wurtzite structure is Ga vacancy. Ferromagnetic properties at room temperature are also detected in CdS nanometer particles in sulphide while the source of magnetism is uclear. Besides, Schwartz et al. find that after they induce Zn interstitial atom to Co2+: ZnO film which is annealed in Zn steam, the magnetism of the sample change. It is still unknown that whether the interstitial atoms have an influence on the magnetism of undoped semiconductor nanomaterials. For what have discussed above, we synthesize Bi2S3and ZnS in hydro-thermal method, which brings interstitial atoms in order to observe the magnetic variation. Then this essay researches the magnetism mechanism in the surface of Bi2S3(001) and ZnS(001) that contain vacancy and interstitial atoms.As recent20-year developed new functional materials, organic magnetic materials have a bright application prospect in many areas. Because of the incomplete occupancy of3d electronic state of transition metal atom, we believe that the coordination compound which consist Tris (8-hydroxy quinoline) and3d transition metals may have magnetism. As a result, we calculate the magnetism of TMq3the coordination compound of Tris (8-hydroxy quinoline) and3d transition metals within first-principles.Among all the semiconductor nanotube materials. SiC nanotube becomes the best material for the high power, high temperature electronic devices and biosensors etc. In addition, SiC nanotubes’high tensiometric property would the electronic property of theirselves after adsorbing H and N atoms. Most of reported researches main focus on the electronic characteristics of SiC nanotube. Compared with C and BN nanotube, the research on magnetism of SiC nanotube is a shortage. As a result, we discuss the magnetic variation of magnetism for the vacancy, B element, and transition metal TM (TM=V, Cr, Mn, Fe, Co, Ni, and Cu) doped single wall SiC nanotube.People pay much attention to SnO2nanotube because of it’s widely use in the areas such as transparent superconducting electrodes, gas transducer, and lithium ion battery. Due to the complexity of the SnO2nanotube’s model building, there is no relevant report about the magnetic properties of SnO2nanotube. With the first-principles, we quest the magnetic mechanism of SnO2nanotube within intrinsic vacancies, N element, and transition metal TM (TM=V, Cr, Mn, Fe, and Co) impuritis.For the past few years, multiferroic materials receive much concern due to the unique physical properties and potential applications. Many researches center on Bi-based perovskite compound, Tb-based manganese compounds, hexagonal rare earth manganese compounds and BaMF4(M stands for A divalent transition metal ions). As we all know that magnetism is always caused by not fully occupied d electron in the transitionmetal. However, this kind of d electron can inhibit the form of center of asymmetry in unit cell. Single-phase multiferroic materials are not so common and most of them are antiferromagnetic at room temperature such as BiFeO3or low-temperature ferromagnetic such as BiMnO3. In order to get the single phase multiferroic materials at room temperature, we introduce vacancy or impurity to ferroelectric materials aiming at making them ferromagnetic at room temperature.Traditional cubic phase SrTiO3is a kind of harbinger of the ferroelectric material with high permittivity and low dielectric loss and good thermal stability, so it is widely used in industrial circles such as electronics, ceramics. SrTiO3is harbinger of ferroelectrics, but it can appear ferroelectric state after stretching or compressing. It is what we are interested in the magnetism of SrTiO3. As a result, we study the magnetic properties of SrTiO3with vacancy in the method of first-principles. Besides, we prepare strontium titanate nanomaterials through sol-gel method and research their structure and magnetism.Na0.5Bi0.5TiO3(NBT) is perovskite ferroelectric compounds of ABO3including two different ions in the A position. It is famous for the widely application in maley brake and gas sensor, etc. NBT materials have unique ferroelectric properties and long-range ferroelectric ordering. And we research its magnetic mechanism with different vacancies in the method of first-principles.Except for vacancy, nonferromagnetic elements dopant can also make material be magnetic. As a practical and environmental-friendly lead-free ferroelectric material, LiNbO3have local magnetic moment when we instead Li and O atoms with the element of Ca/Sr/Ba and F/Cl/Br.From what has researched above, we get that ferroelectric materials with vacancy have magnetism, so these multiferroic materials should also have magnetoelectric effect which is ME effect (regulate of magnetic materials through electric field). With the help of experiments, we study the ferromagnetic properties of BaTiO3nanoparticals, and then research the magnetic variation of BaTiO3with adding electric field by first principle.Then we study the magnetism of BaTiO3ferroelectric nanotube. BaTiO3has properties in electrical, magnetic, optical areas. For a long time people research BaTiO3and its magnetic properties of nanoparticals and films. With the popularity of the research on nanotube materials, BaTiO3nanotube seems more and more important. In the method of first-principles, we research the magnetism of BaTiO3nanotube with different vacancy, and the reason of the generation of magnetism.This essay can get the following conclusions:1. Bi2S3and ZnS nanocrystalline powders prepared by hydro-thermal method could possibly exhibit room temperature ferromagnetism. Heat-treatment of Bi2S3and ZnS nanocrystalline could bring about an enhancement of ferromagnetism, which indicates that interstitial atoms could induce magnetic moments. The results of first-principles calculation show that the inducement of the magnetism of sulfide nanoparticles is cation vacancy. Additionly, interstitial atoms can make local magnetic moment appearing in the system. 2. For the coordination compound (TMq3, TM=Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) which consist Tris(8-hydroxy quinoline, Hq) and3d transition metals, coordination compound except Scq3and Coq3all have magnetism, which is different from what we have predicted. The reason is that inorganic compounds with Co impurity (such as oxide semiconductor like ZnO) are magnetic. The nomagnetic of Coq3is caused by the cancellation of spinning-up electrons and spinning-down electrons of the3d electron in Co atom. For Znq3composition, magnetic moment of Zn mainly comes from three Hq molecules around Zn while Zn has no magnetic moment itself. The magnetic moment of others TMq3is invited by TM atoms theslves, while Hq molecules also contribute some to the whole system.3. A Si vacancy could induce the magnetic moments in armchair (3,3),(4,4),(5,5),(6,6),(7,7), and (8,8) and zigzag (5,0),(6,0),(7.0),(8,0),(9,0), and (10,0) single-wall SiC nanotubes. However, C vacancy can only bring about the magnetic moments in armchair (3,3),(4,4), and (5,5) single-wall SiC nanotubes. The results imply that the magnetic properties of vacancies for single-wall nanotubes have a close relation with the radii, chiralities of the tubes, and the configuration of the vacancy. And we find one B atom substitution at the C site could induce magnetic moment, but the B atom substitution at Si site could not. In addition, our calculation results also show that such TM-doped SiC nanotubes could induce the magnetic moment.4. Stoichiometric single-wall (5,0) SnO2nanotube is non-magnetic, while O vacancy cannot bring about any local magnetic moment. Sn vacancy could bring about large local magnetic moments in the tube which due to the spin polarization of p electrons around Sn vacancy, Sn bi-vacancy has stable state with ferromagnetically coupled magnetic moments. A single nitrogen substitution for oxygen also produces large local magnetic moments. When substituting two nitrogen atoms, the ferromagnetic state is also stable only in special situation. In addition, our calculation results also show that such TM-doped SnO2nanotube could induce the magnetic moment, and the change of total magnetic moment follows Hund’s rule, which is not applies to (6,0) TiO2nanotube. 5. Stoichiometric SrTiO3is nonmagnetic and so is SrTiO3bulk because of the lack of high-concentration defect. The GGA calculated results indicate that titanium (Ti) or oxygen (O) vacancy could induce magnetism in SrTiO3while the LDA and LDA+U calculations show that only Ti vacancy could induce magnetism in SrTiO3. The ferromagnetic coupling between two Ti vacancies is energetically favorable. The results of the experiment confirm that SrTiO3nanocrystalline powders could exhibit room temperature ferromagnetism (FM). The vacuum heat treatment greatly reduces the FM of SrTiO3nanocrystalline powders, which implies that the observed magnetic moment of nanocrystalline SrTiO3results from cation vacancies.6. An ideal rhombohedral NBT is non-magnetic. Sodium and titanium vacancies could induce magnetism, but bismuth and oxygen vacancies could not. The magnetic moment induced by neutral Na or Tivacancies is connected the polarization of O atom p electrons. For rhombohedral NBT with Na or Ti vacancies, the ferromagnetic coupling is more stable.7. The substations for Li by Ca/Sr/Ba and for O by F/Cl/Br in LiNbO3could bring about magnetic moment, due to spin polarization of s electrons of Nb atoms surrounding the doping element. The ferromagnetic coupling is more favorable when two atom substitutions dopant.8. BaTiO3nanoparticals is room ferromagnetic material. We find the magnetism is caused by O vacancy after vacuum annealing. First-principles calculation shows that with the enhancement of the electricfield strength, the magnetism of BaTiO3(001)(including O vacancy) will enhance gradually. At the same time, material transforms to electrical conductor gradually, and the gaps of energy band become short and eventually disappear.9. Stoichiometric BaTiO3nanotube is nonmagnetic and different from the source of the magnetism of BaTiO3bulk. The results show that titanium (Ti) or oxygen (O) vacancy could induce magnetic moment. This is because spin polarization of p electron of O atoms surrounding the VBa0and VTi0and the spin d electrons of Ti atoms surrounding the VO0vacancy. The ferromagnetic coupling is more favorable when two atom substitutions dopant.