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Cobalt doped anatase TiO2 films show room temperature ferromagnetism. Doping was provided by implantation using a MEVVA ion source. The enhancement of ferromagnetic properties was obtained by post-implantation annealing. The microstructure, magnetic properties and the dependence on the annealing conditions have being studied using various characterization techniques. Interestingly, the output referring to the saturation magnetization per Co atom with a value as high as 3.16 muB/Co atom, exceeds considerably that of the bulk cobalt which suggests that contribution to the overall magnetic behavior is not only a function of the concentration of inherently magnetic elements, but there must exist also sources of magnetisms. One of these sources are oxygen vacancies as discussed within this work. It is also interesting that instead of the more commonly observed hcp structure, the Co nanoclusters are found in fcc structure probably being stabilized by the TiO2 matrix. / In this work, we study the properties of cobalt-implanted titanium dioxide, a room temperature dilute ferromagnetic semiconductor discovered in 2001. The ferromagnetic interaction mechanism is however controversial. By using metal vapor vacuum arc (MEVVA) ion source, different doses of cobalt ions were implanted into anatase structures of titanium dioxide (TiO2) thin films. The TiO2 films which were sputtered on SiO2 (100nm)/Si (110) substrates and rutile structure of TiO2. The cobalt implanted TiO2 thin films were prepared with different atomic fraction and then thermally treated at different temperature after ion implantation. The structural properties of the anatase titanium dioxide were also studied as a comparison to rutile titanium dioxide. / Rutherford backscattering spectrometry (RBS) was performed to determine the composition of cobalt. The crystal structure of the thin films and rutile single crystal was mainly anatase as detected in XRD spectra. X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) were also used in sample analysis. Vibrating sample magnetometer (VSM) was employed to study the magnetic properties of the cobalt implanted films. Ferromagnetic behaviors of these films were observed at room temperature. / Semiconductor spintronics is a promising new field of study in the ongoing quest to make electronic devices faster, cheaper, and more efficient. While current spintronics utilize the spin property of electrons to achieve greater functionally, the integration of spintronics into conventional semiconductor electronics will lead to advances optoelectronics, quantum computing, and other emerging fields of technology. This integration relies on effective generation; injection, transport, and detection of spin polarized electron current. To these end, mastering synthesis of room temperature ferromagnetic semiconductors is inevitable. / Luk, Wing Yan. / Adviser: H. P. Ho. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3730. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

I have measured the magnetic and transport properties of Fe and Co in the variable SDW host, CrMo. Analyzing the behaviour of the Neel temperature shows that Co interacts with the SDW, almost an order of magnitude more strongly than does Fe. Also that it magnetically polarizes the SDW more strongly than Fe. Resistivity measurements show that Co reduces the nesting area of the Fermi surface while Fe increases it. Overall, the Neel temperature is strongly reduced by Fe and much less reduced by Co. / Magnetic susceptibility measurements show that Fe supports an effective magnetic moment below T(,N) while Co does not. However, as suggested by Friedel, only nearest neighbour Fe pairs contribute to the effective Fe moment, while isolated Fe moments, like Co, are locked into the SDW antiferromagnetism. / In the absence of SDW formation, the strong interaction between Co and the CrMo host leads to a Kondo state, while the smaller interaction with Fe allows the onset of spin glass ordering.

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The main goals of this research were to fabricate magnetic cobalt nanoparticles and study their structural, crystal structure, optical, and magnetic properties. Cobalt nanoparticles with average particle size 8.7 nm were fabricated by the method of high temperature reduction of cobalt salt utilizing trioctylphosphine as a surfactant, oleic acid as a stabilizer, and lithium triethylborohydride as a reducing reagent. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the formation of cobalt nanoparticles. High resolution transmission electron microscopy images show that Co NPs form both HCP and FCC crystal structure. The blocking temperature of 7.6 nm Co NPs is 189 K. Above the blocking temperature, Co NPs are single domain and hence showed superparamagnetic behavior. Below the blocking temperature, Co NPs are ferromagnetic. Cobalt nanoparticles with a single-domain crystal structure support a sharp plasmon resonance at 280 nm. Iron nanoparticles with average particle size 4.8 nm were fabricated using chemical reduction method show plasmon resonance at 266 nm. Iron nanoparticles are ferromagnetic at 6 K and superparamagnetic at 300 K.

Orientadores: Abner de Siervo, Richard Landers / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-25T18:41:09Z (GMT). No. of bitstreams: 1 DeLima_LuisHenrique_D.pdf: 24142455 bytes, checksum: b13ea70fe8434614a9b9ec40d25b3770 (MD5) Previous issue date: 2014 / Resumo: Monocamadas sp2 de grafeno e nitreto de boro hexagonal (h-BN) têm atraído muita atenção devido ao descobrimento de importantes propriedades, por exemplo, alta resistência mecânica, boa condutividade térmica e excelente estabilidade química e térmica. Porém, enquanto o grafeno é um semicondutor de gap nulo com alta mobilidade dos portadores de carga; o h-BN é um isolante com um largo band gap. Além disso, quando estas monocamadas sp2 são obtidas na superfície de uma variedade de substratos, elas podem exibir superestruturas corrugadas com parâmetro de rede no plano da ordem de nanômetros. Estas superestruturas são importantes para o autoordenamento de moléculas, átomos ou aglomerados de átomos e também para a intercalação de partículas e átomos em posições específicas na interface entre a monocamada sp2 e o substrato. Nesta tese, realizou-se um estudo, básico e aplicado, de monocamadas sp2 de grafeno e h-BN obtidas sobre a superfície do SiC(0001) e do Rh(111), respectivamente. Do ponto de vista básico, foi aplicada a técnica de Difração de Fotoelétrons (XPD) para um estudo da estrutura atômica do grafeno obtido por aquecimento na superfície do SiC(0001) e para a camada de interface entre o grafeno e o SiC, denominada buffer layer (BL). Os resultados de XPD mostraram particularidades distintas na estrutura atômica dessas monocamadas, o que explicaria a diferen_ca na estrutura eletrônica entre a BL e o grafeno. Do ponto de vista aplicado, foi mostrada a viabilidade do nanotemplate de grafeno/BL/SiC(0001) para a obtenção de aglomerados de Co e subsequente estudo das suas propriedades magnéticas por Dicroísmo Circular Magnético de Raios X (XMCD). Os aglomerados de Co foram obtidos sobre a camada de grafeno e os resultados evidenciam uma possível interação cluster-cluster de longo alcance, com influência nas propriedades magnéticas das partículas. Foi investigada também a intercalação dos átomos de Co entre o grafeno e a BL, formando uma rede quase periódica de clusters 2D. O grafeno forma uma barreira de proteção contra oxidação, preservando o caráter metálico das partículas. A monocamada de h-BN sobre a superfície do Rh(111) foi utilizada para a implantação de átomos de Rb. Para a implantação, foi construída uma evaporadora de íons de Rb que permite acelera-los numa faixa de energia possível para penetrar a monocamada de h-BN. Imagens de STM mostraram que os átomos de Rb termalizam entre a monocamada de h-BN e a superfície do Rh(111) em posições especificas da superestrutura, formando o que se denominou de nanotent. A formação dos nanotents e dos defeitos de vacância gerados pelo choque dos íons é uma forma de funcionalização do h-BN, sendo estas estruturas possíveis pontos de ancoragem de moléculas, átomos ou clusters de átomos / Abstract: Graphene and hexagonal boron nitride (h-BN) sp2 monolayers have attracted much attention due the discoveries of their important properties, such as high mechanical strength, good thermal conductivity and excellent chemical and thermal stability. However, while graphene is a zero band gap semiconductor with high carrier mobility; h-BN is a wide band gap insulator. Furthermore, when these sp2 monolayers are obtained on the surface of a variety of substrates, they can exhibit corrugated superstructures with a few nanometers in-plane lattice constants. Such superstructures are important for the self-assembly of molecules, atoms or clusters of atoms and also for the intercalation of these structures at specific positions between the sp2 monolayer and the substrate. In this thesis, we performed a study, fundamental and applied, of sp2 monolayers of graphene and h-BN obtained on the surface of SiC(0001) and Rh(111), respectively. From a fundamental point of view, the Photoelectron Di_raction (XPD) technique was applied for the study of the atomic structure of graphene obtained by heating the surface of the SiC(0001) and for the interface layer between the SiC and graphene, named buffer layer (BL). The XPD results showed distinct peculiarities in the atomic structure of these monolayers, which would explain the difference in electronic structure between BL and graphene. From the applied point of view, it has shown the feasibility of graphene/BL/SiC(0001) nanotemplate to obtain Co clusters and subsequent study of their magnetic properties by X-ray Magnetic Circular Dichroism (XMCD). The Co clusters were obtained on the graphene layer and the results suggest a possible clustercluster long-range interaction, that has influence on the magnetic properties of the particles. It was also investigated the intercalation of Co atoms between graphene, forming a quasi-periodic lattice of 2D-clusters. Moreover, graphene acts as a barrier to oxidation, preserving the metallic character of the clusters. The h-BN monolayer on the surface of Rh(111) was used for the implantation of Rb atoms. For the implantation, it was constructed an evaporator that allows the acceleration of Rb ions to an energy that enables the penetration through the h-BN monolayer. STM images show that the Rb atoms thermalize between the h-BN monolayer and the surface of the Rh(111) at specific positions of the superstructure, forming what is called a \\nanotent\". The formation of the nanotents and the vacancy defects generated by the collision of the ions is a form to functionalize the h-BN, with these structures being possible points for the anchoring of molecules, atoms or clusters of atoms / Doutorado / Física / Doutor em Ciências

The research in permanent magnet thin films focuses on the search of new materials and methods to increase the prevalent data storage limit. In the recent past the work towards the application of these films to micro systems have also gained momentum. Materials like samarium cobalt with better magnetic properties and temperature stability are considered to be suitable in this regard. The essential requirement in miniaturization of these films is to deposit them on silicon substrates that can alleviate the micro fabrication process. In this work, an effort has been made to deposit SmCo films with better magnetic properties on silicon substrates. In the deposition of SmCo, the composition of the deposited films and the structural evolution are found to play an important role in determining the magnetic properties. Proper control over these parameters is essential in controlling the magnetic properties of the deposited films. SmCo being a two component material the composition of the films is dependent on the nature of the source and the transport of the material species from source to substrate. On the other hand, structural evolution is dependent on the energetical considerations between the SmCo lattice and substrate lattice. This most often is dominated by the lattice match between the condensing lattice and the substrate lattice. As such Si does not provide good lattice match to SmCo lattice. Hence suitable underlayers are essential in the deposition of these films. Materials like W, Cu, Mo and Cr were used as underlayers. Out of all these Cr is found to provide good lattice match and adhesion to SmCo lattice. Sputtering being the common deposition tool, SmCo could be sputtered either from the elemental targets of Sm and Co or from the compound target of SmCo5. Sputtering of elemental targets of Sm and Co provides the flexibility of varying the composition whereas sputtering from the SmCo alloy target provides to flexibility of controlling the structural evolution by different process parameters. In this work two different techniques namely Facing Target Sputtering (FTS) and Ion Beam Sputter Deposition (IBSD) were followed in depositing SmCo films. In FTS technique, SmCo films were directly deposited on silicon substrates by simultaneous sputtering of samarium and cobalt targets facing each other. This sputtering geometry enabled to achieve films with a wide composition range of 55 – 95 at. % of cobalt in single deposition. The resulting composition variation and material property variation were investigated in terms of process parameters like pressure, temperature, SubstrateTarget Distance (STD) and InterTarget Distance (ITD). The composition distribution of the films was found to be dependent on the thermalisation distances and the mean free path available during the transport. To explain the process and the composition variation, a simulation model based on Monte Carlo method has been employed. The simulated composition variation trends were in good agreement with that of the experimental observations. IBSD, known for its controlled deposition, was employed to deposit both Cr (as an underlayer) and SmCo films. Cr with close epitaxial match with SmCo induces structural evolution in deposited films. The initial growth conditions were found to play a dominant role in the structural evolution of these Cr films. Hence, initial growth conditions were modified by means of oblique incidence and preferential orientation of (200) plane was obtained. With three different angles of incidence, three different surface orientations of Cr films were achieved. These films were then used as structural templates in the deposition of SmCo films. The influence of parameters like composition, impurities, film thickness, beam energy, ion flux, annealing, angles of incidence and underlayer properties on the structural and magnetic properties of SmCo was studied. The structural evolution of SmCo has been found to depend on the structural orientation of Cr underlayers. This followed the structural relation of SmCo(100)||Cr(110)||Si(100) and SmCo(110)||Cr(100)||Si(100). A mixed surface plane orientation was observed in the case of mixed orientation Cr template. The magnetic coercivities were found to increase from 50 Oe to 5000 Oe with the change in the structure of the deposited films.