In an effort to study the spectral, magnetic, and stereochemical properties of vanadyl complexes, both a new series of vanadyl complexes derived from type (VII) ligands with subnormal magnetic moment and from type (VIII) ligands with normal magnetic moment are synthesized and characterized.

The electron is known to possess an anomalous magnetic moment, which interacts with the gradient of the electric field. This makes it necessary to compute its effects on the energy spectrum. Even though the Coulomb Dirac equation can be solved in closed form, this is no longer possible when the anomalous magnetic moment is included. In fact the interaction due to this term is so strong that it changes the domain of the Hamiltonian. From a differential equation point of view, the anomalous magnetic moment term is strongly singular near the origin. As usual, one has to resort to perturbation theory. This, however, only makes sense if the eigenvalues are stable. To prove stability is therefore a challenge one has to face before actually computing the energy shifts. The first stability results in this line were shown by Behncke for angular momenta κ ≥ 3, because the eigenfunctions of the unperturbed Hamiltonian decay fast enough near the origin. He achieved this by decoupling the system and then using the techniques available for second order differential equations. Later, Kalf and Schmidt extended Behncke’s results basing their analysis on the Prüfer angle technique and a comparison result for first order differential equations. The Prüfer angle method is particularly useful because it shows a better stability and because it obeys a first order differential equation. Nonetheless, Kalf and Schmidt had to exclude some coupling constants for κ > 0. This I believe is an artefact of their method. In this study, I make increasing use of asymptotic integration, a method which is rather well adapted to perturbation theory and is known to give stability results to any level of accuracy. Together with the Prüfer angle technique, this lead to a more general stability result and even allows for an energy shifts estimate. Hamiltonians traditionally treated in physics to describe the spin-orbit effect are not self adjoint i.e. they are not proper observables in quantum mechanics. Nonetheless, naive perturbation theory gives correct results regarding the spectrum. To solve this mystery, one has to study the nonrelativistic limit of the Dirac operator. In the second part of this study, I have not only given the higher order correction to the Dirac operator but also shown the effects of the spin-orbit term.

Copper(II) and oxovanadium(IV) ions resemble one another magnetically in having one unpaired electron in their complexes irrespective of their geometrical structures and bond types involved. Copper(II) complexes with antiferromagnetic exchange are well known. On the contrary, antiferromagnetic exchange in oxovanadium(IV) complexes is rather new and not well established. Very few oxovanadium(IV) complexes have been reported to have this anomalous magnetic property. In the investigation of the magnetic properties of oxovanadium(IV) complexes, we have successfully prepared two series of new oxovanadium(IV) complexes with N-salicylideneamino acids.

Systems of magnetic nanowires are considered strong candidates in many technological applications as microwave filters, sensors or devices for data storage. Because of their strong potential as candidates in such applications they became lately the object of many studies. However, due to the very complicated nature of the interwire interactions, their magnetic behavior is very difficult to be interpreted. The main parameter controlling the response of magnetic nanowires assemblies is the aspect ratio of the nanowires that is defined as the ratio of the length to the wire's diameter. In our study we choose to modify the aspect ratio by keeping a constant length of nanowires and modifying the wire's diameter while keeping the same interwire distance. The samples were studied at room temperature, using vibrating sample magnetometer and X-band ferromagnetic resonance experiments. The results are explained taking into account the effects of the magnetostatic interactions and shape anisotropy.

The 2008 measurement of the electron magnetic moment is the most precisely measured property of an elementary particle, with an astonishing precision of 0.28 parts per trillion. It makes possible the most precise determination of the fine structure constant and the most precise test of quantum electrodynamics and the Standard Model of particle physics. This thesis describes the installation of a new apparatus designed to have improved stability, more optimal control over the radiation field and inhibited spontaneous emission, and narrower resonance lines. / Physics

Neste trabalho determinamos a estrutura eletrônica em torno de uma impureza substitucional de Fe nos metais de transição Pd, Sc, Y, Ti e Zr. Para realizar estes cálculos utilizamos urn método de primeiros princípios dentro da aproximação de densidade local, o RS-LMTO-ASA (\"Real Space-Linear Muffin-Tin Orbital-Atomic Sphere Approximation\'\'). Os resultados encontrados para os momentos magnéticos são comparados com outros cálculos te6ricos da literatura, e com resultados experimentais, quando existentes. Em particular, para o sistema com uma impureza de Fe em Pd analisamos os mecanismos de formação do momento magnético gigante. No caso da impureza de Fe nos hospedeiros Sc, Y, Ti e Zr, onde dois sítios distintos são observados experimentalmente, os resultados deste trabalho ajudam a identificar o sítio magnético como sendo o sítio substitucional. Calculamos também o comportamento do deslocamento isomérico e do campo hiperfino no sitio de Fe nos vários sistemas. Os valores determinados apresentam, em geral, excelente concordância com os resultados experimentais disponíveis na literatura. / In this work we use a first principles method within the local density approximation, the RS-LMTO-ASA (Real Space-Linear Muffin-Tin Orbitals-Atomic Sphere Approximation) to study the electronic structure around substitutional Fe impurities in Pd, Sc, Y, Ti and Zr hosts. We compare our results for the local magnetic moments at the impurity site with those obtained by other methods in the literature and with experimental results when available. For the substitutional Fe impurity in Pd we investigate the formation of the giant magnetic moment, which appears in this system. For the Fe impurity in Sc, Y, Ti and Zr, where two different sites for the Fe are observed experimentally, our results can help with site identification. We show beyond any doubt that the magnetic site is associated with the substitutional Fe in these hosts. Finally we also calculate the isomer shift and hyperfine contact field at the Fe impurity in all the systems. The values are in general in excellent agreement with experiment and other calculations, when available in the literature.

Neste trabalho determinamos a estrutura eletrônica em torno de uma impureza substitucional de Fe nos metais de transição Pd, Sc, Y, Ti e Zr. Para realizar estes cálculos utilizamos urn método de primeiros princípios dentro da aproximação de densidade local, o RS-LMTO-ASA (\"Real Space-Linear Muffin-Tin Orbital-Atomic Sphere Approximation\'\'). Os resultados encontrados para os momentos magnéticos são comparados com outros cálculos te6ricos da literatura, e com resultados experimentais, quando existentes. Em particular, para o sistema com uma impureza de Fe em Pd analisamos os mecanismos de formação do momento magnético gigante. No caso da impureza de Fe nos hospedeiros Sc, Y, Ti e Zr, onde dois sítios distintos são observados experimentalmente, os resultados deste trabalho ajudam a identificar o sítio magnético como sendo o sítio substitucional. Calculamos também o comportamento do deslocamento isomérico e do campo hiperfino no sitio de Fe nos vários sistemas. Os valores determinados apresentam, em geral, excelente concordância com os resultados experimentais disponíveis na literatura. / In this work we use a first principles method within the local density approximation, the RS-LMTO-ASA (Real Space-Linear Muffin-Tin Orbitals-Atomic Sphere Approximation) to study the electronic structure around substitutional Fe impurities in Pd, Sc, Y, Ti and Zr hosts. We compare our results for the local magnetic moments at the impurity site with those obtained by other methods in the literature and with experimental results when available. For the substitutional Fe impurity in Pd we investigate the formation of the giant magnetic moment, which appears in this system. For the Fe impurity in Sc, Y, Ti and Zr, where two different sites for the Fe are observed experimentally, our results can help with site identification. We show beyond any doubt that the magnetic site is associated with the substitutional Fe in these hosts. Finally we also calculate the isomer shift and hyperfine contact field at the Fe impurity in all the systems. The values are in general in excellent agreement with experiment and other calculations, when available in the literature.

Recognising that the characterisation of actual interfaces in magnetic multilayer systems will provide valuable insight for the integration of spintronics in practical devices, a study of interface effects in various structures is presented. Magnetometry measurements are performed for a range of Fe thicknesses (0.4 - 23 nm) grown by molecular beam epitaxy on GaAs and InAs substrates in order to determine the factors governing the evolution of the magnetic moment of epitaxial Fe grown on a zinc-blende semiconductor. A greater reduction of the Fe magnetic moment is observed for films grown on InAs as compared to GaAs, as the Fe films reach a bulk-like moment (within 10% deviation) at a thickness of ~5.2 nm and ~2.2 nm, respectively. From this direct comparative study it is concluded that interface and interdiffusion effects are the dominant mechanisms influencing the value of the magnetic moment for ultra-thin Fe films on GaAs and InAs. Spin injection at this interface is performed, by detecting optical polarisation in the oblique Hanle geometry from a Fe/AlGaAs/GaAs spin-light emitting diode structure. The electrical and magnetic properties of the system are presented, and a ~1% injection polarisation at room temperature, rising to ~4% at 77 K is reported. A study of the deposition and growth of MgO thin (3 - 39 nm) films in conjunction with magnetic layers is also performed. Crystallinity of MgO grown on GaAs is obtained, and epitaxial growth of Fe and Co on MgO is demonstrated. Polarised neutron reflectivity results again indicate a slight decrease in Fe and Co magnetic moments due to interfacial oxide layers. MgO is also incorporated in a pseudo-spin-valve structure which demonstrates epitaxy-induced magneto-crystalline anisotropy. It is concluded that the interface quality is a critical parameter for spintronic devices. Atomic-scale defects and intermixing in real samples mean that current theoretical estimates of ~100% injection efficiency in perfect systems remain unattainable. However by increasing atomic-level structural control of interfaces, a substantial increase in efficiency might be achieved, similarly to the recent breakthrough in tunnelling magneto-resistance ratios which have reached 1000%.

This dissertation describes the theoretic studies of magnetic moment and spinorbit interaction in vacuum (Dirac wavepacket) and solid state systems, such as semiconductors. The semiclassical approach developed here provides a simple and intuitive picture for the origin of spin and spin-orbit coupling. In the Dirac model, the spin magnetic moment is originated from the self-rotating Dirac wavepacket with a correct g-value. The spin-orbit interaction is related to Berry connection (gauge potential) and the model is generalized to solid state systems. The Rashba effect caused by the spin-orbit coupling in a crystal with asymmetric potential in heterostructure quantum well is calculated by semiclassical spindependent scattering. The exact treatment of interface phase accumulation provides a justification of spin-dependent boundary condition at interface derived in previous treatment using Löwdin decomposition. Other spin-orbit coupling related phenomena in solid state system are also discussed in this thesis. / text

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This work shows a theoretical study of Rh13, Pd13, and Pt13 clusters, using first principles methods. We sought to understand aspects of stability and magnetic moment for these systems. The employed methodology was tested by calculating the structural and electronic properties of bulk Rh, Pd and Pt, by studying the equilibrium lattice constants, bulk modulus, cohesion energies and density of states (DOS). These bulk results also serve as a reference for the study of clusters. To study the properties of clusters we initially search for the atomic structures of minimum energy for these systems. We implement a strategy to obtain the clusters structures which is based on high-temperature molecular dynamics calculations. This search was complemented by the addition high-symmetry structures or structures with similarity to those already proposed in the literature. We obtained among 25 and 40 structures for each of the Rh13, Pd13 and Pt13 clusters. For Rh13 and Pd13 we find for the ground state the same structures reported in the literature, but for Pt13 a new lowest energy structure was found. The resulting set of clusters, including ground state and higher energy structures show characteristic patterns as: (i) packed structures, (ii) open cubic-like structures, (iii) layered structures and (iv) low symmetry structures. The structures obtained as the most stable for Rh13, Pd13 and Pt13 show that the increasing importance of d states for the binding of clusters leads to structures with low symmetry and small packing factors. The obtained magnetic moment values are excellent agreement with literature, with a nonlinear dependence on the average bond length. The high density of states at the Fermi level shows both the large magnetic moments and low energetic stability for the clusters, following the Stoner criterion. The decomposition of the DOS shows the large contribution of d levels to the close to Fermi energy, thus confirming a well established feature for transition metals. Positive vibrational frequencies for the whole set of the studied clusters revealed that the obtained structures are local minimum. The analysis of the average bond length, magnetic moment, vibrational frequencies, and effective coordination number show that the obtained structures follow a structural pattern that depends on the occupation of d states valence shell. / Neste trabalho realizou-se o estudo de clusters de Rh13, Pd13 e Pt13, através de um estudo teórico utilizando métodos de primeiros princípios. Procurou-se entender aspectos relacionados à estabilidade e ao momento magnético desses sistemas. Um teste da metodologia adotada foi realizado calculando-se as propriedades estruturais e eletrônicas dos bulk de Rh, Pd e Pt, com o estudo do parâmetro de rede de equilíbrio, bulk modulus, energia de coesão e densidade de estados (DOS). Estes resultados serviram ainda de referência para o estudo dos clusters. Para o estudo das propriedades dos clusters buscou-se inicialmente as estruturas atômicas de mínima energia desses sistemas. Aplicou-se uma estratégia baseada em cálculos de dinâmica molecular de altas temperaturas, que foi complementada com a inclusão de estruturas de alta simetria ou que possuíssem similaridade às estruturas propostas na literatura. Obteve-se de 25 a 40 estruturas para cada um dos clusters de Rh13, Pd13 e Pt13. Para Rh13 e Pd13 encontrou-se como estruturas mais estáveis as mesmas reportadas como mais estáveis na literatura, mas para Pt13 obteve-se uma nova estrutura, ainda não reportada. Observou-se padrões estruturais no conjunto total de clusters obtidos, incluindo estados fundamentais e os de mais alta energia, tais como: (i) estruturas mais empacotadas, (ii) estruturas cúbicas mais abertas, (iii) estruturas em camadas e (iv) estruturas de baixa simetria. As estruturas obtidas como sendo as mais estáveis para Rh13, Pd13 e Pt13 mostram que o aumento da importância dos estados d para as ligações dos clusters leva a estruturas de baixa simetria e menores fatores de empacotamento. Aplicou-se o conceito de número de coordenação efetivo (ECN) para obtenção do número de coordenação e do comprimento médio de ligação dos clusters. O estudo do momento magnético resultou em valores que possuem ótima concordância com trabalhos da literatura. Constatou-se também a dependência não-linear do momento magnético em relação ao comprimento de ligação médio. As altas densidades de estado no nível de Fermi obtidas mostraram, ao mesmo tempo, a existência de grandes valores de momento magnético e baixa estabilidade energética para os clusters, seguindo o Critério de Stoner. A decomposição da DOS auxiliou na constatação da maior contribuição dos estados d, confirmando assim uma característica bem estabelecida para metais de transição. Por fim, a obtenção de freqüências vibracionais positivas para todo o conjunto de estruturas obtidas revelou que estas constituem mínimos locais. As análises de comprimento médio de ligação, momento magnético, freqüências vibracionais, DOS e número de coordenação efetivo mostraram que as estruturas resultantes obtidas seguem um padrão estrutural que depende da ocupação dos estados d da camada de valência.