Magnetic And Related Properties Of Oxides And Small Organic Molecules By First Principles

During the past few years, great progress and development have been made on the computer technologies and first principles calculation methods based on density functional theory (DFT). It has become one of the most important areas of condensed matter physics and material science to study the properties of materials using first principle methods. In this dissertation, we studied the magnetic and related properties of oxides and small organic molecular materials using first principles calculations based on DFT as well as molecular orbital theory, the theories and knowledge of semiconductors and magnetism. These studies are related with magnetic semiconductors.Until recently, the spin of the electron was ignored in mainstream charge-based electronics. A technology has emerged called spintronics (spin transport electronics or spin base electronics), where it is not the electron charge but the electron spin that carries information, and this offers opportunities for a new generation of devices combining standard microelectronics with spin-dependent effects that arise from the interaction between spin of the carrier and the magnetic properties of the material. This approach can be followed to introduce magnetic elements into nonmagnetic semiconductors to make them magnetic. This category of semiconductors, called diluted magnetic semiconductors (DMSs), is alloy of nonmagnetic semiconductor and magnetic elements. Generally, the approach to make DMSs is by introducing a high concentration of magnetic ions into semiconductors. The introduction of magnetic semiconductors opens up the possibility of using a variety of magnetic phenomena not present in conventional nonmagnetic semiconductors in the optical and electrical devices already established. The major obstacle in making DMSs has been the low solubility of magnetic elements in the compands. Moreover, the magnetic dopants are easy to form clusters or secondary phases which are detrimental to intrinsic DMSs. Some experimental measurements revealed that the observed ferromagnetism in TM-doped DMSs was related with the native defects. Interestingly, unexpected room temperature ferromagnetism has also been observed in undoped wide band gap semiconducting or insulating even metallic thin films and nanoparticles. Though there is a consensus that the ferromagnetism is related with native defects, considerable controversy on the origin of the magnetism still exists. Thus, it is necessary to study the magnetic properties carefully using theoreotical methods.Traditionally, the research of spintronics has been focused on inorganic areas. Organic spintronics is a new and promising research field where organic materials are used to mediate or control a spin polarized signal. It is hence a fusion of organic electronics and spin electronics. Organic materials, on one hand, open the way to cheap, low-weight, mechanically flexible, chemically interactive, and bottom-up fabricated electronics. The application of the electron's spin (instead of or in addition to its charge), on the other hand, allows for non-volatile devices. Spintronics devices are also potentially faster and consume less electrical power, since the relevant energy scale for spin dynamics is considerably smaller than that for manipulating charges. The most attractive aspects for spintronics applications in organic semiconductors are the weakness of the spin-scattering mechanism and the very low spin-orbit coupling, implying that the spin polarization of the carriers can be maintained for a very long time. Nevertheless, the low spin injection efficiency, the presence of an "ill-defined layer" and the conductivity mismatch together have impeded further applications. Thus, a possible solution has been proposed by using organic-based magnetic semiconductors. As in the inorganic areas, this can be done by doping magnetic elements into organic semiconductors.In this dissertation, we studied the origin of the magnetism in undoped MgO and MoO2; the magnetic properties of Cr doped ZnO and the effect of native vacancies on the magnetic properties; the structure, electronic structure, electrical and optical properties of Zr doped ZnO; the structural and magnetic properties of Co-doped Alq3 by first principles calculations. The detailed contents and main results are given below:1. Magnetism in undoped MgOThe origin of magnetism in undoped MgO has been studied based on density functional theory. It is shown that Mg vacancies can induce local moments in MgO while O vacancies cannot irrespective of the concentration. The origin of the local magnetic moments comes from the partially occupied eg-and tlu- (or only eg) orbitals and the through-bond spin polarization mechanism mediates the ferromagnetism in MgO. For the ground state configuration of bulk MgO with two Mg vacancies, the energy difference between the antiferromagnetic and ferromagnetic states (28 meV) is smaller than the electron thermal energy at room temperature((?)kBT, more than 35 meV), which means that the ferromagnetic state is not stable in the bulk at room temperature. For MgO thin films, Mg vacancies tend to form at the surface region because of the lower formation energy at the surface site than at the subsurface site and in the bulk. Moreover, the formation energy of Mg vacancy in MgO quantum dot decreases much, allowing a larger concentration of Mg vacancies to appear. In conclusion, the magnetism can be sustained in the quantum dot when enough holes are introduced by the large concentration of Mg vacancies. This is consistent with the experimental result of magnetism observed in MgO nanocrystals.2. Origin of the magnetism in undoped MoO2The electronic and magnetic properties of undoped MoO2 have been studied using first-principles calculations within both the GGA and GGA+U method. The calculated results show that no magnetic moment forms in perfect MoO2. For MoO2 with Mo vacancies, the GGA results show some magnetic moments whereas the GGA+U (U=-1 eV for Mo) results indicate no magnetic moment forms. In the presence of type II O vacancies, both the GGA and GGA+U results show no magnetic moment can form irrespective of the vacancies concentration. Nevertheless, the type I O vacancies always lead to formation of magnetic moments which couple ferromagnetically and should be the main origin of the magnetism in undoped MoO2. The different structural properties and the corresponding charge density redistribution behaviors of the two inequivalent types of oxygen vacancies are the origin of the different magnetic behaviors. For the magnetic interaction, the RKKY interaction and the superexchange mechanism cooperatively underlie the magnetism.3. Magnetic and related properties of doped ZnOFirst-principles density-functional theory (DFT) calculations have been performed to study the magnetic properties of ZnO:Cr with and without vacancies. The results indicate that the doping of Cr in ZnO induces obvious spin polarization around the Fermi energy and a total magnetic moment of 3.77μB. The ferromagnetic (FM) exchange interaction between two Cr atoms is short-ranged and decreases with increasing Cr separation distance. It is suggested that the FM state is not stable with low concentration of Cr. The presence of O vacancies can make the half-metallic FM state of the system more stable, so that higher Curie temperature ferromagnetism can be expected. Nevertheless, Zn vacancies can result in the stability of the FM state decreasing slightly. The calculated formation energy shows that Vzn+CrZn complex forms spontaneously under O-rich conditions. However, under Zn-rich conditions, the complex of Vo+CrZn forms more easily. Thus, ZnO doped with Cr may exhibit a concentration of vacancies that influence the magnetic properties.The structural, electronic, optical and electrical properties of zirconium-doped zinc oxide have also been investigated by first principle calculations. Three possible structures including substitutional Zr for Zn (Zrzn), interstitial Zr (Zn) and substitutional Zr for O (Zro) are considered. The results show that the formation energy of Zrzn defect is the lowest, which indicates that ZrZn defect forms easier and its concentration may be the highest in the samples. It is also found that as the proportion of Zr increases, the lattice constants increase while the optical band gap first becomes larger and then smaller, which are consistent with our experimental results. The electronic structure calculations display that as Zrzn defect is introduced into ZnO, the Fermi energy shifts to the conduction band, and there are excess electrons in the conduction band, which leads to the n-type conductivity of Zr-doped ZnO and the enhancing conductivity. This is one of the reasons of the observed good conductivity of Zr doped ZnO films.4. Magnetism in Co-doped Alq3The electronic and magnetic properties of Co-doped tris-8-hydroxyquinoline aluminum (Alq3) have been studied by first-principles calculations. Our results indicate that local magnetic moments can form in Co-doped Alq3, and the local magnetic moments mainly originate from the localized d states of Co atom. After doping, Co atom tends to interact with Alq3 molecule and leads to electrons transferred from Co atom to Alq3 molecule. Co atom is in a positive charge state, which can act as electron-trap sites. The transferred electrons are mainly localized on the quinolate ligand, resulting in formation of bound magnetic polarons. The indirect ferromagnetic exchange interaction between two bound magnetic polarons antialigning with the same magnetic ion promotes the collective magnetism found in recent experiments.