First-Principles Caculations On Magnetism Of Nanostructured Materials

Nanometerial is a class of materials made up of basic units,at least one dimension of which are of size of1-100nm range.Their structures and properties are very different of those of bulk materials.The discovery of fullerenes and carbon nanotubes has aroused interest in the singular nanostructures. Searching of similar structural materials has be-come a focus for researchers. In addition, with the development of quantum mechan-ics,especially density functional theory (DFT), the computational simulation of nature of nanostructured materials has also become possible.TiO2is widely used in the fields of solar cells, lithium batteries, gas sensing materials, spin electronics and photocatalysis.The TiO2bulk has two common kinds of structures: Anatase and Rutile. Compared to the block, TiO2nanotubes are promising materials because of a larger surface-volume ratio. However, these structures generate defects more easily. We investigated the influence of vacancy and N doping defects to the structure and magnetic properties of single wall TiO2nanotubes.Due to the peculiar shape and electronic structure, the carbon nanocone probably can be used as a probe of the scanning tunneling microscope.Charlier found that nanocones with three pentagons is more suitable to the probe than other nanocones,because the peak of DOS(narrow gap) above the Fermi level is more acute and more obvious, which will help the transition and the tunneling of electrons. In this paper, we use first-principles to investigate the adsorption properties of carbon nanocones.The main research work and contents are listed as follows:1. For both rutile and anatase TiO2nanotubes, a concentration of2.78%VTi0can cause local magnetic moment. The magnitude of the moment is about4μB.For anatase nanotube, O vacancy of same positions can cause local moment(VO10(a) and VO30(a)). But VO20(a) cannot cause local moment,only cause the polarization of two adjacent Ti atoms.It is mainly due to cancellation of the polarization of the two adjacent Ti atoms.For rutile nanotube, all of the O vacancies cannot cause local moment because of the cancellation of the polarization.To these positions, VO1+can cause local moment of about1μB, while VO2+cannot cause local moment or polar-ization.2. For both rutile and anatase TiO2nanotubes, every kind of NO0can cause local mag-netic moment. Electron configuration of N is s2p5. When two O is substituted for N, the two NO0are ferromagnetism coupled.And with the increasing of distance, the coupling weakens. With the increasing of distance, the energy difference of FM and AFM lessens. So suitable N doping concentration can make the doping N ferromagnetism coupled, and then the system exhibits ferromagnetism.3. For the carbon nanocones with cone angle of39°,60°,85°å'Œ113°, We found that nanocones with cone angle of39°and60°have narrow band gaps.And high-value state occurs above the fermi level. Researchers indicated that these structures are suitable to be SEM probes. So we focused on the absorption properties of the60°nanocone. when absorbing N and O, The cone tip changed a lot:The C-C bond is broken and new C-O or N-O bond occurs. When absorbing N, The system exhibits local moment of about1μB. This indicates that the nanocone is oxidized. While when the cone absorbs transition elements, the structure of the system has no significant changes.The band gap disappears and except Ni,other kinds of absorptions cause local magnetic moment.4. The La0.7Sr0.3MnO3manganite prepared by sintering method exhibits a giant mag-netoimpedance. Our simulation based on the electrodynamics theory and mag-netic domain model shows that the magnetoimpedance is connected mainly with the change of permeability, via the penetration depth when a field is applied. In fact, both variations of permeability μand relaxation frequency ωrshould be con-sidered in simulation of GMI effect.