| First-principles calculation in condensed matter physics, which starts from the established principles of quantum mechanics, is a powerful tool to investigate electronic structure and other properties of materials. By directly solving the Schrodinger equation, the electronic structures of materials can be obtained.In principle, magnetism arises from spin and orbit motions of electons and other properties can be calculated based on the electronic structures. First-principles calculation is an extremely appropriate approach to explore the microscopic mechanisms of magnetism. In this dissertation, a number of selected magnetic materials have been studied based on density functional theory and are summarized as followings.(a) The local magnetic moment of Ti-doped ZnO (Ti:ZnO) is calculated from first-principles by using the corrected-band-gap scheme (CBGS). Results show that the system is magnetic with a magnetization of0.699μB per dopant. The origin of local magnetic moment is attributed to the impurity band partially occupied by the donor electrons from the conduction band. Further, the impacts of applying Hubbard U on Ti-d orbital on the magnetic moment have been investigated.(b) Magnetic anisotropy energy (MAE) of iron-gallium alloy in L12phase is calculated using density functional theory, by using GGA and GGA+U approach to investigate the effect of electon correlation (Hubbard U) on MAE. Results show that the MAE of iron-gallium alloy in L12phase sensitively depends on the band structure of minority spin near the Fermi energy. Depending on Hubbard U, the total MAE shows a complicated behavior, where most contribution is however from the Γ point instead of other high symmetric points in the Brillouin zone.(c) Since magnetic anisotropy has been considered as a singal of intrinsic ferromagnetism of diluted magnetic semiconductor (DMS), the MAE of anatase TiO2with oxygen vacancy (Vo:TiO2), a room-temperature ferromagnet by experiment, is studied using density functional calculation. It has been predicted that the system (Ti16O31) possesses a stronge magnetic anisotropy (2.24meV) with an easy axis parallel to the anatase c-axis. The local geometric distortion induced by Vo lowers the symmetry of the system and, consequently, leads to the further splitting of the degenerated d-orbital of Ti ions near the Vo. The MAE is attributed to the perturbation between these splitted d-orbitals, partially occupied by charge compensating electrons from Vo.(d) Based on first-principles calculations, it has been predicted that the MAE in Co-doped ZnO (Co:ZnO) depends on electron-filling. Results show that the charge neutral Co:ZnO presents an "easy plane" magnetic state. When the total number of electons is modified, the easy axis however rotates from in-plane to out-of-plane. The alternation of the MAE is attributed to the change of the ground state of Co ion due to the relocating of electrons on Co d-orbitals with electron-filling.(e) Based on ab initio calculations, we predict that the MAE of Co-doped TiO2sensitively depends on carrier accumulation. This magnetoelectric phenomenon provides a promising route to directly manipulate the magnetization direction of diluted magnetic semiconductor by external electric-field. We calculate the band structures to reveal the origin of carrier-dependent MAE in k-space. In fact, the carrier accumulation shifts the Fermi energy and regulates the competing contributions to MAE. This research suggests that it is possible to design spintronics materials with electrically controllable spin direction.In summary, five selected magnetic systems have been studied using density functional calculation. The magnetic properties, including local magnetic moment and magnetic anisotropy, are calculated from electronic structures of materials. First-principles calculations reveal the connections between magnetic properties and electronic structures, and offer a promising approach to design for new magnetic materials. |
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