Charge transfer salts of Tetracyanoquinodimethane (TCNQ) were synthesised and their electrical and magnetic properties were investigated. These salts show unusual electrical and magnetic behaviour in contrast to conventional organic compounds. These salts have crystal structures which in general consist of TCNQ radical ions stacked in chains, isolated from each other by the diamagnetic cations. They are thus regarded as "one-dimensional" electrical and magnetic systems. The ESR spectra of these salts are attributed to triplet excitons showing that the spin-spin and electronelectron correlation effects are important. In the ESR spectra (Chapter III) of some TCNQ salts dipolar splitting is observed confirming the spin-spin interaction. These triplet excitons are regarded as bound electron-hole pairs. The experimentally determined dipolar splitting tensors are presented in Chapter III and the intensity data in Chapter IV. A large number of fine structure lines are observed in the ESR spectra of Pyridinium-TCNQ and 4-Aminopyridinium-TCNQ apart from regular triplet exciton lines (Chapter III). These lines are attributed to the trapping of excitons on an extended formula finit (TCNQ2 )n. In Chapter IV the temperature dependent magnetic susceptibilities are discussed in terms of Heisenberg antiferromagnetism and Pauli paramagnetism. In Chapter V temperature dependent behaviour of electrical conductivity is discussed in terms of an exciton band model, the lattice structure of the salts and one-dimensional lattice consisting of defects giving rise to high and low conducting segments. Low temperature electrical and magnetic phases are discussed (Chapters IV and VII) in terms of a band and hopping mechanisms.In Chapter VI self consistent field calculations are made with reference to the tight binding one electron band theory using simplified Roothaan equations considering CNDO approximations. Theoretical results are related to experimental band gaps, spinspin interactions and charge alteration.

This thesis investigation concerns the optical and nonlinear electrical properties of n-InSb. Two specific areas have been studied. First is the magneto-optical study of magneto-donors, and second is the nonlinear dynamic study of nonlinear and chaotic oscillations in InSb. The magneto-optical study of InSb provides a physical picture of the magneto-donor levels, which has an important impact on the physical model of nonlinear and chaotic oscillations. Thus, the subjects discussed in this thesis connect the discipline of semiconductor physics with the field of nonlinear dynamics.

The synthesis and characterization of two new Schiff base copper(II) complexes are reported. These are Cu(acac: 2-amino-l-phenylethanol) and Cu(acac:2-amino-l-butanol). The ligands, derived from acetylacetone and the appropriate aminoalcohol, are dibasic tridentates with 0,N,O donor atoms. The magnetic properties of the complexes were studied at several temperatures between 78 OK and 296 OK. The magnetic moment of Cu(acac:2-amino-l-phenylethanol) varied little with temperature, and that of Cu(acac:2-amino-lbutanol) increased as the temperature was lowered. This is in contrast to the magnetic moment of Cu(acac:ethanolamine), which decreases as the temperature decreases. Molecular weight data, infrared spectra, magnetic data, electronic spectra, and electron spin resonance spectra of both complexes are reported and discussed.

This thesis reports the synthesis and characterization of two new copper(II) halide complexes with deprotonated N-(2-hydroxyethyl)ethylenediamine behaving as a bidentate. The magnetic properties of the new copper(II) complexes were studied from room temperature to liquid nitrogen temperatures. The magnetic data show that both complexes exhibit antiferromagnetic interactions with a singlet ground state and a thermally accessible triplet excited state. Magnetic data and infrared spectra indicate the complexes are halogenbridged. Deprotonation at an amine nitrogen is based on the presence of a hydroxyl stretching band in the infrared spectra. Electronic spectra and infrared spectra indicate the complexes are square planar. Elemental analyses, infrared spectra, electronic spectra, electron spin resonance spectra, and magnetic data are reported and discussed.

In this work magnetic properties of copper(II) complexes on N-(hydroxyalkyl) pyrrole-2-aldimines were investigated by various techniques, one of which was magnetic susceptibility. Magnetic moments are not directly determined experimentally, but magnetic susceptibilities are.

D.Phil. (Physics) / Please refer to full text to view abstract

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Este trabalho avalia o efeito da deformação plástica sobre as propriedades magnéticas em amostras de aço elétrico de grão não orientado. O material sob estudo foi extraído de uma bobina de aço elétrico GNO, com 0,8 % Si, em estado recozido, com densidade 7781 kg/m3 e tamanho de grão 20 µm. As chapas extraídas foram laminadas a frio para a produção de amostras com deformação real entre 0,03 e 0,29. A densidade de discordâncias desses corpos de prova foi estimada através de difração de raios-X. Através da análise da largura à meia altura dos picos de difração foi possível estimar a densidade de discordâncias das amostras. Os resultados descreveram maior taxa de aumento da densidade de discordâncias para pequenas deformações. A partir de 0,12 de deformação real o incremento foi pequeno. No que tange à caracterização magnética, para os ciclos de histereses obtidos em regime quase estático, verificou-se aumento da perda histerética com o crescimento da deformação plástica. Essa relação foi descrita de acordo com a lei de dependência onde a perda histerética cresce com a raiz quadrada da deformação real. Sobre o efeito da frequência de excitação na dissipação de energia por perdas magnéticas, o material não deformado mostrou-se mais sensível que os laminados a frio ao aumento da frequência. Pois se constatou que os materiais laminados apresentaram perda de excesso desprezível. Este comportamento pode ser explicado assumindo-se que com a deformação plástica eleva-se o número de paredes de domínio por unidade de volume, diminuindo assim as perdas de excesso. Dessa forma, para a faixa de VII frequências analisada, a perda histerética é a parcela que mais contribuiu para o aumento da perda magnética com a deformação. / This study evaluates the effect of plastic deformation on the magnetic properties of electrical steel in electrical steel samples. The material under investigation was obtained from a coil of non-oriented electrical steel sheets, 0.8% Si, annealed, with density 7781 kg/m3 and grain size of 20 µm. The material was cold rolled in order to produce samples with true strain ranging from 3 to 29%. The specimens dislocation density was estimated by X-ray diffraction. Analyzing the full width at half maximum of the diffraction peaks was possible to estimate the dislocation density of the samples. The results described a large increasing rate of dislocation density for small deformations and from 12% true strain the increment was small. On the magnetic characterization, the hysteresis loops obtained at quasi static regime shown a hysteresis loss increasing with true strain. This relationship is described according to the law of dependence where the increasing is linear with the square root of the true strain. All samples suffered total loss increasing with the frequency growth. It was observed that the hysteresis loss was the main contribution for the energy dissipation, for any true strain level. The undeformed material exhibited greather sensibility to frequency excitation than any strained sample. This result occurs due to the cold rolled sheets presented negligible excess loss. This behavior can be explained assuming the hypothesis that the number of domain walls per unit volume increase due to grain fragmentation, thus reducing excess loss.

O crescente interesse em novos métodos para a geração de energia limpa e sustentável contribui para o desenvolvimento de materiais avançados destinados a aplicações estruturais em reatores de fusão nuclear. Os principais resultados obtidos quanto à caracterização microestrutural de dois aços endurecidos por dispersão de óxidos, candidatos potenciais a este tipo de aplicação, são apresentados. Um dos aços contém 9%Cr e o outro 13%Cr (% em massa). Os aços foram laminados até 80% de redução e recozidos até temperaturas próximas a 0,9 Tf, onde Tf é o ponto de fusão. A cinética de amolecimento foi acompanhada por meio de medidas de dureza Vickers em função da temperatura de recozimento e do tempo. Amostras representativas foram caracterizadas via microscopia eletrônica de transmissão e de varredura e por difração de elétrons retroespalhados. As temperaturas das transformações de fases foram determinadas por técnicas como dilatometria e análise térmica diferencial. Curvas de magnetização em função do campo magnético aplicado até 15 kOe foram obtidas para o aço com 9%Cr. Medidas de difração de raios X para determinar a densidade de discordâncias também foram realizadas em amostras representativas do aço com 9%Cr. Os resultados até o momento mostram que o engrossamento microestrutural destes aços só se torna significativo quando o recozimento é feito em temperaturas acima de 0,8 Tf. Em temperaturas mais baixas, ainda no campo ferrítico, a recuperação prevalece e a fração recristalizada é baixa. A resistência destes materiais à recristalização se deve à presença das partículas de óxido de ítrio dispersas na matriz que dificultam a movimentação e o rearranjo das discordâncias e contornos durante sua migração. Por outro lado, partículas grosseiras como as dos carbonetos M23C6 estimulam a recristalização atuando como sítios preferenciais para a nucleação. O recozimento em temperaturas elevadas no campo austenítico resultou no aumento considerável na dureza Vickers e na densidade de discordâncias para o aço com 9%Cr devido à transformação martensítica. No aço com 13%Cr os resultados também sugerem a ocorrência da transformação martensítica. / There is an increasing interest in the development of new, sustainable, clean and safe sources of energy. In view of these requirements, the development of advanced materials for future nuclear fusion reactors becomes essential. This work investigates the annealing behavior and the microstructural stability of two oxide dispersion strengthened steels with 9%Cr and 13%Cr ODS steels. These materials are potential candidates for structural applications in future fusion reactors. The two ODS steels were cold rolled to 80% thickness reduction. Samples were annealed in vacuum at temperatures of about 0.9 Tm, where Tm is the melting point. The microstructural characterization was performed by Vickers hardness testing, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) performed by a highresolution SEM. Important temperatures related to the phase transformations were determined with aid of dilatometry and differential thermal analysis (DTA). Magnetization curves as a function of applied magnetic fields up to 15 kOe were obtained for 9%Cr ODSEurofer. X-ray diffraction experiments were also performed on 9%Cr steel to evaluate the dislocation density in several metallurgical conditions. Results show that microstructural coarsening becomes significant when annealing is performed above 0.8 Tm. Below 800°C, static recovery is the main softening mechanism of these steels and the volume fraction of recrystallized grains is quite low. The resistance of these steels to recrystallization can be explained by the strong interaction between boundaries and mobile dislocations with fine particles of Y-based oxides. Contrastingly, coarse M23C6 particles act as preferential nucleation sites through the well-known particle stimulated nucleation (PSN) mechanism. Samples annealed above 800oC (austenitic phase field) display larger dislocation densities in the 9%Cr ODS steel. Results concerning to 13%Cr steel suggest the occurrence of martensitic transformation.

Neste trabalho de tese é apresentado um estudo sistemático do crescimento e das propriedades magnéticas e estruturais dos compostos semicondutores magnéticos diluídos (SMD) Zn1-xMnxIn2Se4. Amostras monocristalinas com concentrações de Mn $0leqslant xleqslant 1.00$ foram obtidas pela técnica de transporte químico em fase vapor (CVT). Através de técnicas de difração de raios-X (padrões de pó e diagramas de Laue), estudou-se a evolução da estrutura desde a fase romboédrica do MnIn2Se4, até a fase tetraédrica do ZnIn2Se4. Observou-se a existência de uma fase pura romboédrica para $xgeqslant 0.87$, uma fase pura tetragonal para$xleqslant 0.25$ e uma mistura destas duas fases para x entre 0.67 e 0.35. Espectros de ressonância paramagnética eletrônica (EPR) foram obtidos em temperaturas T desde 4 K até 300 K, para diversas concentrações de Mn. Todos os espectros têm o pico de absorção principal centrado em g=2. A largura pico a pico da linha de ressonância, $deltaHpp$, decresce com o aumento da concentração de manganês e com o aumento da temperatura. Este comportamento está intimamente relacionado com a interação de troca entre os íons magnéticos. A partir de análises da dependência de $deltaHpp$ com a temperatura se extraíram estimativas para a temperatura de Curie-Weiss, $ heta$ e para a temperatura de congelamento Tf das fases de vidro de spin. Medidas da susceptibilidade magnética em modo DC foram realizadas para temperaturas $2Kleqslant Tleqslant 300K$. Dos dados de alta temperatura (T>100 K), resultaram estimativas para o parâmetro $ heta$, bem como para a concentração x de Mn. Os valores de $ heta$ obtidos destas medidas concordam bem com os obtidos de EPR e foram utilizados para estimar as constantes de troca antiferromagnéticas nos dois extremos de concentração da série. Nas amostras com alta concentração de Mn, $xgeqslant 0.67$, irreversibilidades características de uma transição para uma fase de vidro de spin foram observadas em temperaturas abaixo de 4K. Para melhor caracterizar estas transições, medidas da susceptibilidade AC e DC para diversas freqüências e campos magnéticos foram realizadas em temperaturas entre 2 e 20 K. A susceptibilidade DC em baixos campos mostra picos agudos em Tf ~ 2.5, 3.0 e 3.5 K, respectivamente para as amostras com x=0,67; 0,87 e 1,00. Nas três amostras, observa-se irreversibilidade entre medidas com esfriamento na ausência de campo (ZFC) e em presença de campo (FC). Evidência de um verdadeiro fenômeno de transição de fase é fornecida pelo crescimento da susceptibilidade não linear, $chiNL$, próximo de Tf.. Análises de escala estáticas de $chiNL$ e análises de escala dinâmicas desenvolvidas a partir de dados da susceptibilidade AC resultaram em expoentes críticos consistentes com os obtidos em outros vidros de spin com interação de curto alcance. / This work reports a systematic study on the crystal growth and magnetic and structural properties of the diluted magnetic semiconductors (SMD) Zn1-xMnxIn2Se4. Crystals with Mn concentrations $0leqslant xleqslant 1.00$ were grown by chemical vapour transport (CVT). Through x-ray powder diffraction patterns and Laue diagrams of single crystal we studied the transformation from the layered rhombohedral structure of MnIn2Se4 to the tetragonal structure of ZnIn2Se4. We observe the presence of a purely rhombohedral phase for $xgeqslant 0.87$, a purely tetragonal phase for $xleqslant 0.25$ and a two-phase mixture for x between 0.67 and 0.35. Electron Paramagnetic Resonance (EPR) spectroscopy has been performed in the temperature range 4.2 K to 300 K for various concentrations of Mn. All the spectra had the main absorption peak centered at g=2. The resonance peak-to-peak line width, ÄHpp, decreases with increasing concentration of manganese and with increasing temperature. This behavior is intimately related with the exchange interaction between magnetic ions. The analysis of the temperature dependence of ÄHpp resulted in estimates for the Curie-Weiss temperature $ heta$ and for the freezing temperature Tf of the spin glass phase. The DC magnetic susceptibility has been investigated in the range between $2Kleqslant Tleqslant 300K$. The high temperature data (T>100 K) gave estimates for the parameter $ heta$, as well as for the concentration x of Mn. The values of $ heta$ obtained in this way are in good agreement with those obtained from EPR and were used to estimate the antiferromagnetic exchange constants for the two extremes of concentration of the system. For the samples with high concentration of Mn, $xgeqslant 0.67$, irreversibilities characteristic of a spin glass transition have been observed at temperatures below 4K. To better characterize this transition, AC and DC susceptibility measurements for different frequencies and magnetic fields were made in the temperature range between 2 and 20 K. The low field AC susceptibility displayed sharp peaks at Tf ~2.5, 3.0 and 3.5 K for samples with x= 0.67, 0.87 and 1.00, respectively. Irreversibility between zero field cooled (ZFC) and field cooled (FC) DC measurements was observed in the three samples. Evidence of a true phase transition phenomenon is given by the steep increase of the nonlinear susceptibility $chiNL$ when approaching Tf from above. Static scaling analysis of $chiNL$ and dynamic scaling analysis of the AC susceptibility data have been carried out, which yielded critical exponents consistent with those obtained in other spin-glasses with short-range interactions.