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First Principles Calculation of Diluted Magnetic Semiconductors and Rare Earth Permanent Magnets

Author: WuQingYun
Tutor: HuangZhiGao；LaiFaChun
School: Fujian Normal University
Course: Theoretical Physics
Keywords: diluted magnetic semiconductor Curie temperature non-magnetic dopant rare earth permanent magnet first principles calculation
CLC: O471
Type: Master's thesis
Year: 2007
Downloads: 391
Quote: 2
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Abstract


In this thesis, traditional and novel diluted magnetic semiconductors have beenstudied using first principles calculation based on density functional theory （DFT）. Theeffects of doping on magnetic properties of rare earth permanent magnet have also beenstudied. The thesis is arranged as follows:Firstly, some basic knowledge of Spintronics and diluted magnetic semiconductor（DMS）are introduced. A brief history of development of DMS is also presented. As thetheoretic foundation of this thesis, basic description of first principles calculation basedon density functional theory is also included in the first part of this thesis.Secondly, electronic structures of Co doped ZnO is studied based on DFT. It is foundthat the ferromagnetic ground state is stabilized by its half-metal band structure. Usingthe coupling strength from DFT, Monte Carlo simulation indicates Curie temperatureabove room temperature for higher doping concentration.Thirdly, Cu doped AlN, a novel DMS free of magnetic cation, is introduced. WhenCu is doped into AlN, it becomes spin-polarized and couple with each other ferromagneti-cally through p-d exchange mechanism. By comparison, we predict a Curie temperatureabove room temperature for this system.Fourthly, Ag doped GaN, another novel DMS with half-metallicity and strong ferro-magnetism, is also introduced. Ag dopants become spin polarized when substitutionallyincorporated into GaN and induce finite magnetic moments which render a ferromagneticground state in the host GaN semiconductor. Ag-doped GaN has a half metallic bandstructure with conduction holes in the minority spin channel. At the same concentration,Ag-doped GaN has a much stronger ferromagnetic coupling than that of 3d TM dopedone. Thus, a lower dopant concentration can induce room temperature ferromagnetismfor Ag, which allows to alleviate the clustering problem in traditional DMSs.Finally, the effects of doping on magnetic properties in YCo5-xFex and YCo5-xAgxsystems are investigated. We find: （1） The doping changes the volume and the shapeof the cell. （2） The magnetic moments of Co atoms show a transition from a high spinstate to a low spin state as a function of the volume of the cell in all YCo5, YCo3Fe2 andYCo3Ag2 alloys. （3） The change of c/a ratio also influences the magnetic moments andthe MAE drastically with constant lattice parameter a. （4） With the same geometrystructure, for YCo5-xFex, the total magnetic moment as a function of the number of doped magnetic atoms Fe appears a maximum peak at x=4. The magnetic moments ofCo atoms are little affected by Fe impurities. However, for Ag doping, the total magneticmoment declines linearly and Co atom shows the transition from a high spin state to alow spin state too.

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