My dissertation research focuses on the investigation of the transport and magnetic properties of transition metal and rare earth doped oxides, particularly SnO2 and HfO2 thin films. Cr- and Fe-doped SnO2 films were deposited on Al2O3 substrates by pulsed-laser deposition. Xray- diffraction patterns (XRD) show that the films have rutile structure and grow epitaxially along the (101) plane. The diffraction peaks of Cr-doped samples exhibit a systematic shift toward higher angles with increasing Cr concentration. This indicates that Cr dissolves in SnO2. On the other hand, there is no obvious shift of the diffraction peaks of the Fe-doped samples. The magnetization curves indicate that the Cr-doped SnO2 films are paramagnetic at 300 and 5 K. The Fe-doped SnO2 samples exhibit ferromagnetic behaviour at 300 and 5 K. Zero-field-cooled and field-cooled curves indicate super paramagnetic behavior above the blocking temperature of 100 K, suggesting that it is possible that there are ferromagnetic particles in the Fe-doped films. It was found that a Sn0.98Cr0.02O2 film became ferromagnetic at room temperature after annealing in H2. We have calculated the activation energy and found it decreasing with the annealing, which is explained by the increased oxygen vacancies/defects due to the H2 treatment of the films. The ferromagnetism may be associated with the presence of oxygen vacancies although AMR was not observed in the samples. Pure HfO2 and Gd-doped HfO2 thin films have been grown on different single crystal substrates by pulsed laser deposition. XRD patterns show that the pure HfO2 thin films are of single monoclinic phase. Gd-doped HfO2 films have the same XRD patterns except that their diffraction peaks have a shift toward lower angles, which indicates that Gd dissolves in HfO2. Transmission electron microscopy images show a columnar growth of the films. Very weak ferromagnetism is observed in pure and Gd-doped HfO2 films on different substrates at 300 and 5 K, which is attributed to either impure target materials or signals from the substrates. The magnetic properties do not change significantly with post deposition annealing of the HfO2 films.

Nenhuma / O estudo das correlações entre as propriedades estruturais e magnéticas de filmes finos e ultrafinos é hoje assunto de grande interesse tanto científico como tecnológico, sendo que, dada a disponibilidade de materiais e a importância das aplicações atuais e potenciais, filmes magnéticos compostos por metais e ligas de metais de transição 3d estão entre os materiais mais investigados. O Co é um metal de transição 3d, ferromagnético, e que, em volume, apresenta estrutura hexagonal compacta (hc). Entretanto, tanto esta fase como as fases cúbica de face centrada (cfc) e cúbica de corpo centrado (ccc) podem ser estabilizadas à temperatura ambiente na forma de filmes ultrafinos, ou ainda em forma de estruturas multicamadas, e as propriedades magnéticas dos mesmos apresentam uma complexa correlação com a estrutura cristalina. Existe um grande numero de trabalhos a respeito de Co crescido sobre superfícies de Cu (cfc, parâmetro de rede a = 3,615 Å) e outros monocristais, mas nada há na literatura sobre monocamadas de Co depositadas sobre Cu90Au10, uma liga cfc com parâmetro de rede (3,66 Å) expandido de 1,0% em relação ao Cu. Investigamos aqui o crescimento, em condições de epitaxia de feixe molecular (MBE), e a estrutura de filmes de Co depositados a temperatura ambiente, com espessuras entre uma e cinco monocamadas atômicas (ML) depositadas sobre a superfície (100) da liga Cu90Au10. A composição química e a pureza da superfície do monocristal e dos filmes foram determinadas por espectroscopia de elétrons excitados por raios X (XPS). A cristalinidade da superfície do substrato, bem como a forma de crescimento e a estrutura dos filmes foram determinados por difração de elétrons de baixa e alta energia, LEED e RHEED. A morfologia da superfície do cristal de Cu90Au10(100) e das primeiras monocamadas do filme de cobalto foram determinadas através de medidas de microscopia de varredura por tunelamento (STM). Medidas de magnetometria por efeito Kerr magneto- óptico foram utilizadas para se estabelecer, em caráter preliminar, uma correlação entre a estrutura e o magnetismo dos filmes de Co sobre Cu90Au10(100) Nossos resultados indicam o crescimento de Co com estrutura tetragonal de face centrada (tfc), e uma evolução, com o aumento da espessura de cobalto, de formação de ilhas para crescimento camada a camada. Além disso, nota-se para os filmes de Co uma rápida contração do parâmetro de rede no plano, atingindo aproximadamente 2,5% para ~ 4,0 ML, quando comparado ao substrato de Cu90Au10 (100). Medidas de magnetometria por Efeito Kerr Magneto-ótico indicam magnetização no plano do filme. Este trabalho eminentemente experimental representou um amplo aprendizado no uso e exploração das potencialidades de técnicas múltiplas (LEED, RHEED, XPS, AES, STM e MOKE) para a adequada caracterização e investigação das propriedades estruturais e magnéticas de superfícies e nanoestruturas heteroepitaxiais preparadas em UHV, em condições de epitaxia de feixe molecular (MBE), bem como o primeiro estudo do crescimento de monocamadas de Co depositadas sobre Cu90Au10(100). / The investigation of correlations between structural and magnetic properties of thin and ultrathin films is of great scientific and technological interest presently. Due to importance of their actual and potential applications, films of 3d metals and their alloys are among the most investigated materials. Co is a ferromagnetic 3d metal that in bulk has a hexagonal compact structure (hcp). For Co films or multilayers, the hcp as well as the face-centered (fcc) and bodycentered cubic (bcc) phases can be stabilized at room temperature, depending on the used substrate. It is known that the preparation method can affect decisively the structural and magnetic properties of Co monolayers. There is a big amount of work on Co films grown on different Cu surfaces (fcc; lattice parameter a = 3.615 Å) and other substrates but, to the best of our knowledgement, there is no studies on Co monolayers deposited on Cu90Au10(100), a fcc alloy with lattice parameter of about 3.66 Å. In this work we investigate the epitaxial grow and the structure of Co films with thickness up to 5 atomic monolayers (ML) deposited on Cu90Au10(100). The goal of the study was to investigate the modifications in the magnetic properties of the Co films provided by small distortions in the lattice, since Cu90Au10 presents cfc structure with a lattice parameter ~1% larger than the one of the pure Cu. The sample preparation and the majority of the experimental analysis has been done in ultra high vacuum under molecular beam epitaxy conditions. The studies were conducted in situ, in UHV, by using x-ray photoelectron spectroscopy (XPS), high and low energy electron diffraction (RHEED and LEED), and scanning tunneling microscopy (STM). Preliminary magnetic measurements on the correlation structure - magnetism were conducted by magneto-optical Kerr effect (MOKE). Our results indicate the growth of a tetragonal distorted face centered (fct) Co lattice and an initial formation of islands followed by a layer-by-layer grow starting from 2 ML Co. The lateral lattice parameter shows a fast contraction with increasing thickness when compared to the CuAu substrate, reaching ~ 2.5% at 4 ML Co. Surface magnetometry by Magneto-optical Kerr effect indicated in-plane magnetization of the Co films. This experimental work represented a broad and extensive learning process on preparation and characterization of heteroepitaxial nanostructures by multiple techniques (LEED, RHEED, XPS, AES, STM, and MOKE) under MBE conditions (UHV) and the first investigation of Co monolayers on Cu90Au10(100). SUMÁRIO

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.

"Apr 2002." / The numerals in title is subscript. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2002. / 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.

Ferroelectric materials continue to find increasing use in actuator, sensor and transducer design. Questions regarding lifetime and reliability remain a concern due to the inherent low fracture toughness and complex material behavior. The poling procedure required for use in actuator and sensing devices introduces anisotropy in elastic and dielectric coefficients as well as piezoelectric coupling between the mechanical and electrical fields. This introduces complex fracture behavior which necessitates advanced analytical techniques and fracture characterization. In this dissertation, fracture mechanics of ferroelectric materials is evaluated by employing different analytical techniques and experimental methodology. The theoretical work has focused on linear piezoelectric coupling that accounts for the influence of anisotropy and heterogeneity on fracture. A new orthotropic rescaling technique is presented that explicitly solves the anisotropic linear elastic piezoelectric crack problem in terms of material coefficients. The effects of heterogeneities on electric field induced microfracture are analyzed by implementing a crack at the edge of a heterogeneous piezoelectric inclusion. A positive, flaw-localized driving force is realized when permeable crack face boundary conditions are considered. The experimental portion of the work evaluates fracture behavior in the ferroelectric ceramic, lead zirconate titanate (PZT), and the ferroelectric relaxor single crystal PZN-4.5%PT. Relative humidity and electric boundary conditions are shown to have significant effects on crack kinetics in PZT. Fracture anisotropy in single crystal PZN-4.5%PT is characterized using the Single-Edge V-notch Beam (SEVNB) method and Vickers indentations. Scanning electron micrographs are used to determine the crack profile which leads to a prediction of crack tip toughness and local energy release rate. A weak cleavage plane is identified in the single crystal relaxor which contains a significantly lower toughness in comparison to the ferroelectric ceramic PZT.

This thesis describes a research of suppression of superconducting gap in a superconducting island of a Ferromagnetic-Superconducting-Ferromagnetic Single-Electron-Transistor due to the fringing magnetic fields produced by the ferromagnetic leads. The devices are working below the critical temperature of the superconducting gap. A model is proposed to explain how the fringing magnetic field produced by the leads is strong enough to suppress the superconducting gap. The peak of the fringing magnetic field produced by one lead reaches 5000 oe. It is observed an inverse tunneling magneto resistance during the suppression of the superconducting gap, obtaining a maximum absolute value 500 times greater than the TMR in the normal state where the efficiency of the spin injection is low. It is concluded that the suppression of the superconducting gap is due to fringing magnetic field and not to the spin accumulation because the low efficiency of the spin injection. It is suggested a new geometry to reduce the effect of the fringing magnetic field so it can be obtained a suppression of the superconductivity due to the spin accumulation. It is described the qualitatively behavior of the IV characteristic when the suppression of the superconductivity is due to spin accumulation.

Ferroelectric and ferromagnetic alloy clusters are produced and studied in molecular beams. Nb clusters doped with 1-3 impurity atoms are ferroelectric with low transition temperatures. The alloy clusters with an even number of valence electrons have larger dipole moments than those with odd number of valence electrons. The ferroelectricity is suppressed by magnetic impurities or thermal excitations, and is enhanced by Au and Al doping. The observations strongly suggest that electron-pairing interactions exist in Nb clusters, which indicates Cooper pairing in clusters. The magnetic moments of Co clusters doped with small fraction of Mn,V and Al are studied and compared with those of the bulk alloys. CoMn alloy clusters have enhanced average magnetic moments with Mn doping, which is opposite to the behavior of bulk CoMn. CoV and CoAl alloy clusters behave similarly to their bulk counterparts. We explain the experimental results using the virtual-bound-state model. Finally, the magnetic properties of BiMn clusters are studied in molecular beams. The Mn local moments are found to couple ferromagnetically or ferrimagnetically depending on the composition of the clusters.

Magnetic properties of cobalt clusters (20 N 200) were studied in molecular beams. The magnetization of cobalt clusters is studied at a broad range of temperatures, magnetic fields and clusters sizes. It is shown that the agnetization of ferromagnetic clusters in a cluster beam can be understood as an adiabatic process using the avoided crossing theory. Besides the ground state that bears magnetic moment of about 2 Bohr magneton per atom, an excited state that has 1 Bohr magneton per atom was discovered for every cobalt cluster observed. The energy separations between the two states was investigated by photo-ionization experiments. The ionization threshold shows that the energy gap between the two states is on the order of 0.1 eV for small clusters (N 100) and vanishes for larger clusters. Experiments also show that the polarizability of the excited state is lower than that of the ground state, which indicates a significant electronic tructure difference between the two states. Two states are also found for iron clusters (20 N 200) for which the magnetic moments per atom are about 3 Bohr magneton for the ground state and 1 Bohr magneton for the excited states. This explains the fractional magnetic moments as well as the local magnetic order observed above the Curie temperatures for iron group ferromagnets. Further experiments show two states for manganese clusters for which the ground state has magnetic moment of 1 Bohr magneton per atom in about the same size range. This suggests that the two states are a universal phenomenon of 3d transition metal clusters, which originate from the interaction between 3d and 4s electrons.

No description available.

Thermally activated shape memory polymers are a desirable material for use in dynamic structures due to their large strain recovery, light weight, and tunable activation. The addition of ferromagnetic susceptor particles to a polymer matrix provides the ability to heat volumetrically and remotely via induction. Here, remote induction heating of magnetite filler particles dispersed in a thermoset matrix is used to activate shape memory polymer as both solid and foam composites. Bulk material properties and performance are characterized and compared over a range of filler parameters, induction parameters, and packaging configurations. Magnetite filler particles are investigated over a range of power input, in order to understand the effects of particle size and shape on heat generation and flux into the matrix. This investigation successfully activates shape memory polymers in 10 to 20 seconds, with no significant impact of filler particles up to 10wt% on mechanical properties of shape memory foam. Performance of different particle materials is dependent upon the amplitude of the driving magnetic field. There is a general improvement in heating performance for increased content of filler particles. Characterization indicates that heat transfer between the filler nanoparticles and the foam is the primary constraint in improved heating performance. The use of smaller, acicular particles as one way to improve heat transfer, by increasing interfacial area between filler and matrix, is further examined.