Cheung, Wai Ming Edwin. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 107-112). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts also in Chinese.

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Physics. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / by William J. Gallagher. / Ph.D.

本論文主要研究了磁性雜質在三種不同的石墨烯體系中的性質。這三個體系分別是：伯爾納堆垛（Bernal stacked）結構的雙層石墨烯(bilayer graphene)，包含空位的雙層石墨烯和zigzag 型石墨烯納米帶(graphene nanoribbon)。本文中主要運用的數值方法為Hirsch-Fye 量子蒙特卡羅方法 （quantum Monte Carlo method）和貝葉斯最大熵方法（Bayesian Maximum Entropy method）。 / 在論文的第二章會詳細介紹這兩種方法。 我們用量子蒙特卡羅方法得到的結果是準確的，因為原則上我們可以計算無窮大的體系， 並且不需要采用任何近似去處理多體問題。 / 第三章，我們討論磁性雜質在伯爾納堆垛結構的雙層石墨烯中的性質。我們主要考慮兩種情況：磁性雜質分別位於A 亞晶格和B 亞晶格上。我們發現類似于單層石墨烯中的情況，隨著化學勢的變化，雜質原子的磁矩在一定的能量範圍內可調。但是由於雙層石墨烯中的兩套亞晶格不等價，雜質的性質很大程度上取決於雜質的位置，並且其區別隨著s-d 混合強度的增加和d 電子的關聯能的減少而變得更加明顯。我們也討論了雜質原子的譜密度和s-d 電子之間的關聯函數。我們的所有結果都體現雜質性質對空間位置的相同依賴關係。 / 在第四章，我們研究在伯爾納堆垛結構的雙層石墨烯中磁性原子在空位附近的性質。在雙層石墨烯系統中，空位引起的局域態的性質依舊取決於空位所屬的亞晶格。這也是由於兩套晶格的不等價性。我們討論雜質在空位周圍時雜質性質對空位所屬亞晶格的依賴關係，以及兩種缺陷之間的相對位移對雜質磁性的影響。當磁性原子附著在空位的最近鄰格點上時，A 亞晶格上的空位會對雜質原子的磁矩有更強的抑制。而隨著磁性原子和空位之間的距離增加，局域態對磁性原子的影響迅速衰減，並且B 亞晶格上的空位對雜質的影響相對長程一些。 / 在第五章，我們討論兩個磁性雜質在zigzag 型石墨烯納米帶邊緣上的間接互作用隨化學式的變化。由於在zigzag 型石墨烯納米帶邊緣有局域的零能態，雜質的磁矩被嚴重抑制，兩個雜質原子之間的自旋-自旋關聯函數也與石墨烯中的行為有很大區別。有趣的是，當兩個雜質附著在兩個最近鄰的碳原子上時，隨著化學式的降低，雜質之間的反鐵磁關聯會有一個顯著增強再降低的過程。我們也討論了自旋關聯隨著距離的變化，并發現關聯強度隨著距離的變化迅速衰減。 / 最後再第六章，我們會本論文中的內容進行總結和討論。 / In this research thesis, we study the magnetic properties of Anderson impurities in three different graphene based systems: pure Bernal stacked bilayer graphene, Bernal stacked bilayer graphene with a vacancy and graphene ribbon with zigzag edges. Quantum Monte Carlo method based on the Hirsch-Fye algorithms are used to obtain the basic thermodynamic quantities, and the Bayesian maximum entropy method is used to obtain the spectral densities of the impurity sites. We discuss the numerical methods in chapter 2. / In chapter 3, we investigate the local moment formation of a magnetic impurity in Bernal stacked bilayer graphene. In the two cases we study, impurity is placed on top of the two different sublattices in bilayer system. We find that similar to the monolayer case, magnet moment of the impurity could still be tuned in a wide range through changing the chemical potential. However, the property of the impurity depends strongly on its location due to the broken symmetry between the two sublattices. This difference becomes more apparent with the increase in the hybridization and decrease in the on-site Coulomb repulsion. Additionally, we calculate the impurity spectral densities and the correlation functions between the impurity and the conduction-band electrons. All the computational results show the same spatial dependence on the location of the impurity. / In chapter 4, we address the issue of single magnetic adatom located in the vicinity of a vacancy in bilayer graphene with Bernal stacking. In bilayer system, the property of vacancy induced localized states depends on whether the vacancy belongs to A or B sublattice. The magnetic impurity is placed in the vicinity of the vacancy, and the dependence of its magnetic property on the location of the vacancy is discussed. We switch the values of the chemical potential and study the basic thermodynamic quantities and the correlation functions between the magnetic adatom and the carbon sites. When the magnetic adatom is located on the nearest site of the vacancy, the local moment is more strongly suppressed if the vacancy belongs to the sublattice A. The influence of the zero-energy localized states decays fast as the displacement between the two defects increases, and the effect of B vacancy on the local moment of magnetic adatom is relatively long ranged. / In chapter 5, we examine the behavior of the indirect magnetic interactions of two magnetic impurities on the zigzag edge of graphene ribbon as a function of chemical potential. We find that the spin-spin correlation between two adatoms located on the nearest sites are drastically suppressed at the Dirac energy, and as we lower the chemical potential, the antiferromagnetic correlation is first enhanced and then decreased in values. If the two adatoms are adsorbed on the sites belong to the same sublattice, we find similar behavior of spin-spin correlation except for a cross over from ferromagnetic to antiferromagentic correlation in the vicinity of zero-energy. We also calculated the weight of the wave functions and basic thermodynamic quantities for various values of parameters, and compare the results with their counterpart in bulk graphene. / Finally in chapter 6, we summarize our results. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Sun, Jinhua = 石墨烯體系中磁性雜質的量子蒙特卡羅研究 / 孫金華. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 92-[100]). / Abstract also in Chinese. / Sun, Jinhua = Shi mo xi ti xi zhong ci xing za zhi de liang zi Mengte Kaluo yan jiu / Sun Jinhua. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Electronic properties of graphene --- p.1 / Chapter 1.1.1 --- Band structure --- p.1 / Chapter 1.1.2 --- Dirac fermions --- p.4 / Chapter 1.1.3 --- Mesoscopic effects --- p.4 / Chapter 1.1.4 --- Superconductivity in graphene --- p.6 / Chapter 1.2 --- Potential applications --- p.7 / Chapter 1.2.1 --- In semiconductors --- p.8 / Chapter 1.2.2 --- In spintronics --- p.10 / Chapter 1.3 --- Magnetic impurities in graphene based systems --- p.11 / Chapter 1.4 --- Outline of this thesis --- p.13 / Chapter 2 --- Numerical method --- p.15 / Chapter 2.1 --- Quantum Monte Carlo method based on Hirsch-Fye algorithm . --- p.15 / Chapter 2.1.1 --- Anderson impurity model Hamiltonian --- p.16 / Chapter 2.1.2 --- Key steps of Hirsch-Fye algorithm --- p.16 / Chapter 2.1.3 --- Basic thermodynamic quantities --- p.23 / Chapter 2.1.4 --- Extended Hirsch-Fye algorithm --- p.24 / Chapter 2.1.5 --- Two-impurity case --- p.25 / Chapter 2.1.6 --- Relation to the determinant quantum Monte Carlo method based on BSS algorithm --- p.27 / Chapter 2.2 --- Bayesian maximum entropy method --- p.30 / Chapter 2.2.1 --- Bayesian inference --- p.31 / Chapter 2.2.2 --- Maximum entropy analysis --- p.34 / Chapter 3 --- Magnetic impurity in Bernal stacked bilayer graphene --- p.37 / Chapter 3.1 --- Introduction --- p.37 / Chapter 3.2 --- Density of states and local density of states --- p.38 / Chapter 3.3 --- Results --- p.43 / Chapter 3.3.1 --- Basic thermodynamic quantities --- p.43 / Chapter 3.3.2 --- Spectral densities --- p.48 / Chapter 3.3.3 --- Correlation functions --- p.50 / Chapter 3.4 --- Summary --- p.52 / Chapter 4 --- Magnetic impurity in the vicinity of a vacancy in bilayer graphene --- p.54 / Chapter 4.1 --- Introduction --- p.54 / Chapter 4.2 --- Zero-energy localized states in the vicinity of vacancy --- p.56 / Chapter 4.2.1 --- Monolayer graphene with a vacancy --- p.57 / Chapter 4.2.2 --- Bilayer graphene with vacancy --- p.58 / Chapter 4.3 --- Results --- p.60 / Chapter 4.3.1 --- Basic thermodynamic quantities --- p.61 / Chapter 4.3.2 --- Spin-spin and charge-charge correlations --- p.68 / Chapter 4.4 --- Summary --- p.69 / Chapter 5 --- Indirect exchange of magnetic impurities in zigzag graphene ribbon --- p.71 / Chapter 5.1 --- Introduction --- p.71 / Chapter 5.2 --- Density of states and local density of states --- p.73 / Chapter 5.3 --- Results --- p.76 / Chapter 5.3.1 --- Basic thermodynamic quantities --- p.77 / Chapter 5.3.2 --- Spin-spin correlation --- p.80 / Chapter 5.3.3 --- Weight of different components of the d electron wave function --- p.84 / Chapter 5.3.4 --- Comparison : two magnetic impurities in bulk graphene sheet --- p.85 / Chapter 5.4 --- Summary --- p.88 / Chapter 6 --- Summary and discussions --- p.89 / Bibliography --- p.92 / Chapter A --- Derivation of the input Green’s function G0(τ ) in Bernal stacked bilayer graphene --- p.101 / Chapter B --- Input Green’s function for U = 0 in bilayer graphene in the presence of a vacancy --- p.106

by Lai Yiu Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Abstract in English and Chinese. / Abstract / Abstract (Chinese) / Table of Contents / List of Figures / List of Tables / Chapter Chapter 1 --- Introduction / Chapter 1.1 --- Overview --- p.1-1 / Chapter 1.2 --- Conventional recording --- p.1-2 / Chapter 1.3 --- Superparamagnetism --- p.1-2 / Chapter 1.4 --- Possible solutions --- p.1-4 / Chapter 1.4.1 --- Perpendicular recording --- p.1-5 / Chapter 1.4.2 --- Patterned media --- p.1-6 / Chapter 1.4.3 --- High Ku material for recording media --- p.1-7 / Chapter 1.5 --- FePt-based material research 1 - --- p.1-8 / Chapter 1.6 --- Project goal --- p.1-11 / Reference --- p.1-12 / Chapter Chapter 2 --- Sample preparation and characterization techniques / Chapter 2.1 --- Pulsed filtered vacuum arc deposition (PFVAD) --- p.2-1 / Chapter 2.1.1 --- Sample preparation --- p.2-4 / Chapter 2.2 --- Rutherford backscattering spectroscopy (RBS) --- p.2-4 / Chapter 2.3 --- X-ray diffraction (XRD) --- p.2-6 / Chapter 2.4 --- Vibrating sample magnetometery (VSM) --- p.2-7 / Chapter 2.5 --- Transmission electron microscopy (TEM) --- p.2-9 / Reference --- p.2-10 / Chapter Chapter 3 --- Characterization of FePt-C nanocomposite thin film / Chapter 3.1 --- Experiment design --- p.3-1 / Chapter 3.2 --- Experiment detail --- p.3-1 / Chapter 3.3 --- Results and discussion --- p.3-3 / Chapter 3.3.1 --- NRBS measurements --- p.3-3 / Chapter 3.3.2 --- XRD measurements --- p.3-8 / Chapter 3.3.3 --- VSM measurements --- p.3-14 / Chapter 3.3.4 --- Some preliminary results on effects of post- deposition implantation --- p.3-23 / Chapter 3.3.5 --- TEM images --- p.3-26 / Chapter 3.3.6 --- Overall discussion --- p.3-29 / Chapter 3.3.6.1 --- Total film thickness effect --- p.3-29 / Chapter 3.3.6.2 --- Degree of ordering from XRD (001)/(002) peak intensity ratio --- p.3-33 / Chapter 3.3.6.3 --- C spacer thickness effect --- p.3-34 / Chapter 3.3.6.4 --- Implantation effect --- p.3-35 / Chapter 3.4 --- Summary --- p.3-35 / Reference --- p.3-36 / Chapter Chapter 4 --- Characterization of FePt-Cu nanocomposite thin film / Chapter 4.1 --- Experiment design --- p.4-1 / Chapter 4.2 --- Experiment detail --- p.4-1 / Chapter 4.3 --- Results and discussion --- p.4-3 / Chapter 4.3.1 --- RBS measurements --- p.4-3 / Chapter 4.3.2 --- XRD measurements --- p.4-7 / Chapter 4.3.3 --- VSM measurements --- p.4-9 / Chapter 4.3.4 --- Discussion --- p.4-12 / Chapter 4.3.4.1 --- Total film thickness effect --- p.4-12 / Chapter 4.3.4.2 --- Cu spacer thickness effect --- p.4-13 / Chapter 4.4 --- FePt films without additive --- p.4-16 / Chapter 4.5 --- Summary --- p.4-17 / Reference --- p.4-18 / Chapter Chapter 5 --- Conclusion and future works / Chapter 5.1 --- Conclusion --- p.5-1 / Chapter 5.2 --- Future works --- p.5-3 / Reference --- p.5-4 / Appendix 1 / Appendix 2

by Chiah Man Fat. / "March 2004." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

Xia, Daxue. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Nesta dissertação, foram sintetizados copolímeros à base de poli(metacrilato de metila) (PMMA), divinilbenzeno (DVB) com propriedades magnéticas pela técnica de polimerização em suspensão. O material utilizado para conferir propriedade magnética foi o ferro. Foi estudada a influência das variáveis reacionais para a formação de microesferas (ordem de adição das fases orgânica e aquosa e a dispersão do ferro antes e após a pré-polimerização), o efeito da concentração de ferro adicionado na polimerização, a influência da razão molar MMA/DVB na formação do copolímero, o efeito do tipo de agente de suspensão e a velocidade de agitação para a síntese dos copolímeros. Os copolímeros foram caracterizados quanto à morfologia, estabilidade térmica, teor de ferro incorporado, distribuição de tamanho de partículas, propriedades magnéticas, área superficial, volume e tamanho de poros. As análises de propriedades magnéticas mostraram que os materiais obtidos não apresentaram ciclos de histerese, estando assim próximos de um material com propriedades superparamagnéticas, com magnetização de saturação entre 8,0 e 13,0 emu/g. Os copolímeros sintetizados com velocidade de agitação 500 RPM, temperaturade 90 C, tempo de polimerização de 24h, monômeros MMA/DVB 50/50 (% molar); razão volumétrica fase orgânica/fase aquosa 1/3, diluente heptano, grau de diluição 100% apresentaram melhor controle morfológico, melhor magnetização de saturação, tamanho de partículas menores e maior tamanho de poros / In this dissertation, copolymers based on methyl methacrylate (MMA) and divinylbenzene (DVB) with magnetic properties were synthesized by suspension polymerization. It was employed iron as magnetic material. The influence of reaction variables for the microspheress formation as order of addition of the organic and aqueous phases, dispersion of iron before and after pre-polymerization, iron concentration added to the polymerization, MMA/DVB molar ratio, type of suspending agent and stirring speed were studied. The copolymers were characterized with respect to morphology, thermal stability, the amount of iron incorporated, particle size distribution, magnetic properties, surface area and volume and size of pores. The materials obtained showed no hysteresis loops, standing near a material which superparamagnetic with saturation magnetization between 8.0 and 13.0 emu / g. The copolymers with agitation speed 500 RPM, temperature of 90 C, curing time of 24 hours, molar ratio MMA / DVB = 50/50 (mol%); volume ratio organic phase/aqueous phase = 1/3, heptane and 100% degree of dilution showed better morphologic control, better saturation magnetization, smaller particle size and larger pores

Neste trabalho, estudamos as propriedades magnéticas e a magnetoresistência gigante (GMR) em fitas de Cu IND.80 Ni IND.20-xFe IND.x (x = 2,5 ; 5; 10; 17, 5; 20) produzidas por melt-spinning. As fitas foram estudadas como função da temperatura de tratamento térmico T IND.an 500°C, usando-se um magnetômetro SQUID. Uma grande variedade de estruturas granulares foram obtidas para diferentes razões de Ni/Fe e condições de tratamento térmico. Para Cu80 Ni10 Fe10, a magnetização não apresentou histerese mensurável para T 50K. Este comportamento é consistente com as curvas de susceptibilidade que indicam uma temperatura de bloqueio abaixo de 50K. Nesta temperatura, o maior valor da GMR(19%) foi obtido para as fitas tratadas a 400°C por duas horas. As curvas de magnetização foram comparadas a um modelo teórico que assume uma distribuição de momentos magnéticos. Para Cu80Ni15Fe5,a microestrutura e as propriedades magnéticas são muito sensíveis ao tratamento térmico. Para as ligas com composições 10; 15; 17,5% de Fe, a magnetização de saturação apresentou uma redução com o tratamento a 400°C. Esta redução foi explicada através da formação de partículas perto da região Invar. Um comportamento anômalo linear da amplitude da magnetoresistência foi observado e explicado pelo espalhamento dependente do spin, que acontece quando um elétron se move de uma partícula superparamagnética para uma partícula bloqueada termicamente. / In this work, we study the magnetic properties and giant magnetoresistance (GMR) in ribbons of Cu80 Ni20-xFex (x = 2.5, 5, 10, 17.5, 20) prepared by melt-spinning. The ribbons were studied as a function of annealing temperature Tan 500°C, using a SQUID magnetometer. A wide variety of granular structures é obtained for different Ni/ Fe ratios and annealing conditions. In Cu80 Ni10 Fe10 , the magnetization shows no static hysteresis for T 50K. This behaviour is consistent with the susceptibility curve which indicates a blocking temperature below 50K. At this temperature, the largest GMR value was obtained for the ribbons annealed at 400°C for two hours. The magnetization curves were compared with a theoretical model that takes into account the magnetic moment distribution. In Cu80 Ni15Fe5, the microstructure and magnetic properties are much more sensitive to annealing. For t he ribbons with 10, 15, 17.5 % Fe composition, the magnetization presented a reduction with annealing at 400°C. This redution may be explained in terms of particle formation near the Invar region. An anomalous linear behaviour of the MR was observed and can be explained in terms of spin-dependent scattering when an electron passes from a superparamagnetic particle to a thermally blocked particle.

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