First-principles Study On The Magnetism Of Oxides, Nitrides, Magnetic Tunnel Junction And DNA Base Radicals

Since the theoretical prediction of ferromagnetism (FM) in Mn doped semiconductors by Dietl et al. in 2000, many research works have been focused on the magnetism in semiconductors doped with transition metals (TMs). Many experimental works have reported unexpected room-temperature FM (RTFM) in thin films or nanograins of undoped oxides, claiming that the FM in these oxygen compounds stems from oxygen vacancies. In contrast, first-principles calculations demonstrate that cation vacancies are responsible for the magnetic moments in undoped oxides such as HfO2, TiO2, ZnO, SnO2 and ZrO2. Meanwhile, along with the increasing interest of FM in undoped oxides, the d0 FM is shaping up to be an important problem in magnetism to emerge so far this century. Ferromagnetism in materials that contains no d or f shells which could produce local magnetic moments challenges our conventional understanding of magnetism. MgO is a typical d0 oxide, it exhibits no magnetism in bulk when pure and defect-free. To our knowledge, however, there have been no systematic first-principles ab initio investigations about the influence of neutral Mg defects on the magnetism of undoped MgO. In the present dissertation, we perform GGA, LDA and LDA+U calculations on the magnetic properties of MgO with VMg0 vacancies in its lattice.Ferromagnetism could also be produced by doping nonmagnetic sp elements such as C, N, Mg and Al in oxides, nitrides and sulfides. These impurities could tansform nonmagnetic compounds to mangetic systems, and the magnetism originates from the spin-polarization of 2p electrons of the C, N, O, S atoms, etc. So far, however, there have been no systematic ab initio studies on magnetic properties of SrO with co-existence of neutral cation vacancies and anion substitution for O. Usually, the surface of nanograins is abundant of defects or vacancies. Therefore, detailed surface calculations on the magnetism of SrO surface with VSr0 vacancies and/or C/N substitutions are needed. Here, we perform LDA+U calculations on the magnetism of SrO (100) surface with VSr0 vacancies and/or C/N substitutions and the co-existence of certain concentration of VSr0 vacancies and C/N substitution for O.Recently, ZnO nanotubes have attracted much attention due to their applications in electronic and optoelectronic nano-devices. Density functional theory calculations have demonstrated that Zn vacancy could introduce magnetic moments in ZnO thin films and nanowires, and the magnetic moments came from the unpaired 2p electrons at O sites surrounding the Zn vacancy. It is also proved that Zn vacancies could lead to ferromagnetic state in ZnO thin films and nanowires. However, up until now, there has been no theoretical research on the magnetism of pure ZnO nanotubes. Therefore, we perform first-principles study on the magnetic properties of (5,0) ZnO single-walled nanotube (SWNT).GaN is a widely-used semiconductor material. Over the past years, it has attracted considerable attention because of its interesting physical properties and application as a basic material for optoelectronic devices working in the blue and ultraviolet spectral region. GaN has two different crystal structures:the wurtzite structure (space group P63mc) and the cubic structure (space group F4-3m). The observed ferromagnetism is due to the intrinsic defects confined to the surface of the nanoparticles, while the core of the particles remains diamagnetic. Ga vacancies in GaN give rise to a ferromagnetic ground state. GaN with cubic structure is usually in metastable state, which has very important application in laser pump. Here, we perform ab initio calculations on the magnetic properties of cubic structure GaN (100) and (101) surfaces containing VGa0 and VN0 vacancies.Recently, room temperature giant tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with (001) oriented MgO tunnel barriers has attracted much attention, due to its considerable applications in hard disk drive (HDD) read head and magnetoresistive random-access-memory (MRAM). The TMR originates from the dependence of tunnel current on the relative orientation of the magnetization of the two ferromagnetic (FM) electrodes. Its value for the Fe/MgO/Fe MTJ could reach as high as 1000%, theoretically, or 300%, experimentally. The interlayer exchange coupling (IEC) is considered to be an important measurement of the magnetic coupling between the two FM electrodes. In addition, as it is extremely difficult to obtain perfect MgO barrier grown on electrode layers, intrinsic point defects have been considered to play a very important role in the properties of the barrier (MgO) and consequently the IEC. In the present dissertation, we explore the Mg vacancy mediated interfacial magnetism, and how the neutral Mg vacancies at the interface of Fe/MgO/Fe(001) MTJ affect the IEC by first-principles calculations.Exposure to ionization radiation might cause direct or indirect damage to DNA. For the direct damage, the radiation energy is absorbed directly by DNA, yielding free radicals and their decay products. The indirect damage is from the attack on DNA by radicals formed from ionization of the immediate environment. These radicals are mainly derived from water, and hydroxyl radical (·OH) is the main species. The·OH radicals could react with DNA bases through adduction or H-abstraction reactions, which produce hydroxylated or dehydrogenated radicals, respectively. Many efforts have been made to investigate the initial stage of DNA-base damage induced by hydroxylation and dehydrogenation reactions. Most of the referred literatures are dedicated to the mechanism of the DNA-base damage, or the corresponding possible products of the hydroxylation and dehydrogenation reactions. The attack of·OH radical on the DNA bases due to the very strong ionization radiation might cause disease, if the damage is not repaired. An investigation on the spin-polarization or magnetic properties of the products from the attack of·OH on DNA bases is important for understanding the radiation damage to DNA. In the present paper, we perform ab initio calculations on the magnetism of DNA base radicals derived from hydroxylation and dehydrogenation reactions.The main outcomes of the present calculations are as follows:1. For the d0 ferromagnetism of MgO which contains different concentrations of neutral Mg and O vacancies, our calculations show that undoped MgO is nonmagnetic; and VO0 vacancies introduce no magnetism in the system. While, VMg0 vacancies could introduce strong FM. For certain concentration of VMg0 vacancies, the induced FM depends sensitively on the distribution of the Mg vacancies. Meanwhile, the magnetic moment enhances as the concentration of the VMg0 vacancies is increased. The magnetic moments of the system is found to originate from the spin-polarization of the 2p electrons of O atoms surrounding the Mg vacancies. Our LDA+U calculation indicates that a concentration of 6.25% VMg0 vacancies could transform MgO from insulator to half-metal, which could play an important role in the modern field of spintronics. Our MgO model (bulk with large concentration of VMg0 vacancies) and results may be approximately valid for the region near the surface of nanograins.2. For the magnetism of the SrO (100) surface containing neutral point defects or/and C/N substitution for oxygen, LDA+U calculations show that ideal SrO (100) surface is nonmagnetic. Neutral oxygen vacancies induce no ferromagnetism. Neutral Sr vacancies (VSr0) could bring about large local magnetic moments, locating mainly on the oxygen atoms surrounding the vacancy at the uppermost surface.6.25% VSr0 vacancies could lead to a ferromagnetic state of the surface. Carbon or nitrogen substitution for oxygen at the surface leads to holes in C/N 2p states, which could introduce large local magnetic moments. It is possible that co-existence of VSr0 vacancy and C/N substitution at the SrO (100) surface could weaken the ferromagnetism of the system, which is due to the charge compensation of C/N 2p electrons for O 2p holes.3. For the magnetism of single-walled (5,0) ZnO nanotube with the presence of VZn0 vacancies and C substitution for O atoms, the present results show that undoped (5,0) ZnO NT is nonmagnetic, and VO0 vacancies introduce zero magnetism. VZn0 vacancies could bring about large local magnetic moments in the tube, localized mainly on the neighboring O-loops. A concentration of 6.67% neutral zinc vacancies could lead to a stable state with ferromagnetically coupled magnetic moments. A single carbon substitution for oxygen also produces large local magnetic moments. Substituting a couple of carbon atoms for oxygen atoms might results in a nonmagnetic state with relatively low energy, or produce magnetic moments with ferromagnetic coupling for an excited state.4. For the magnetism of GaN (100) and (101) surfaces containing neutral point defects, LDA+U calculations indicate that ideal Ga-ended GaN (100) and (101) surfaces are found to be nonmagnetic. Ga-ended GaN (100) surface relaxed from the constructed N-ended GaN (100) surface exhibits ferromagnetism. Neutral gallium vacancies at Ga-ended GaN (100) surface could induce large magnetic moments, leading to a ferromagnetically coupled state of the system. The spin-polarization of 2p electrons of nitrogen atoms is responsible for the induced magnetic moments. Neutral nitrogen vacancies at the (101) surface bring about zero magnetic moment. Neutral gallium vacancies in the (101) surfaces might lead to antiferromagnetic state of the system. We conclude that the room-temperature ferromagnetism of GaN with cubic structure originates from the neutral gallium vacancies at the surface of nanograins.5. LDA+U calculations show that the IEC of Fe/MgO/Fe(001) tunnel junction with magnesium vacancy exhibits oscillating character with the increase of monolayers of MgO, which is different with the decreasing property of IEC in perfect junctions. The concentration of magnesium vacancies could obviously affect the IEC values, which affects directly the MR ratio of the magnetic tunnel junctions. Due to the charge compensation effect of Fe 3d electrons on the O 2p holes, natural Mg vacancy at the interface could not introduce large local magnetic moments on the surrounding O atoms.6. For the magnetism of radicals derived from the attack of OH on DNA bases, GGA calculations show that OH addition and H-abstraction result in considerable bond recombination in all the hydroxylated and dehydrogenated radicals, except for the H5-abstracted radical of cytosine. The hydroxyl radical adds predominantly to the C5 position in adenine, and to the C4 position in guanine when attacking the double bonds of C4 and C5 in adenine and guanine, leaving the purine rings in obviously distorted structures around the C4 and C5 atoms. Spin-polarization occurs in all the investigated hydroxylated and dehydrogenated base radicals. Each base radical exhibits a total magnetic moment of 1?B, and the induced magnetic moments mainly originate from the spin-polarization of C and N 2p electrons in the bases. The local magnetic moments on C, N and/or O atoms in each radical are provided.