The present thesis deals with the application of ab-initio electronic structure calculations based on density functional theory for material design. The correlation between magnetic properties and electronic structures has been investigated in detail for diluted magnetic semiconductors (DMS), which have promising application for spintronics devices. The point defects, acting as electron donor or electron acceptor, have been studied for their key role in mediating the long ranged ferromagnetic interaction between transition metal (TM) ions. The electron holes induced by electron acceptor are completely spin polarized in semiconductor and exhibit a very significant efficiency to the ferromagnetic coupling between TM ions. While the electron donor shows a negative effect to the ferromagnetism in the system. The common trend of magnetic interaction and electron charge transfer between TM ions and electron acceptors or electron donators have been outlined. The Coulomb correlation U of d electrons, which could change the energy levels of TM d band respective to the host semiconductor band, also shows a significant influence on the magnetic behavior in DMS. The crystallography phase transition under high pressure has also been studied for the iron doped with light element, carbon. Our calculated results show that interstitial carbon defect has little effect on the iron's bcc to hcp phase transition under high pressure. The other carbon iron phases, like Fe3C, has also been studied in a wide pressure range. We also present a first-principles description on the temperature dependence of elastic constant for solids. The total temperature effects are approximated as a sum of two separated parts, the thermal expansion contribution, which gives the normal linearly decreasing effect on the elastic constant with increasing temperature, and the electronic band contribution, which could lead anomalous behavior for thermal elastic constants.

In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements. / text

Spectrally-resolved differential reflectivity experiments on GaMnAs over a broad spectral range (1.4-2.0 eV) are presented, representing the first such measurements in a III-Mn-V diluted magnetic semiconductor. Comparison of the measured nonlinear spectra with results in GaAs and LT-GaAs, together with calculations of the pump probe signal contributions, has allowed an unambiguous identification of the relevant scattering and relaxation processes for optically-excited carriers in this material system. The measured spectra indicate a clear blue shift in the nonlinear optical response, providing support for the valence band model of ferromagnetism in III-Mn-V diluted magnetic semiconductors. / Spectrally-resolved differential reflectivity experiments on GaMnAs over a broad spectral range (1.4-2.0 eV) are presented, representing the first such measurements in a III-Mn-V diluted magnetic semiconductor. Comparison of the measured nonlinear spectra with results in GaAs and LT-GaAs, together with calculations of the pump probe signal contributions, has allowed an unambiguous identification of the relevant scattering and relaxation processes for optically-excited carriers in this material system. The measured spectra indicate a clear blue shift in the nonlinear optical response, providing support for the valence band model of ferromagnetism in III-Mn-V diluted magnetic semiconductors.

This thesis presents the NMR measurements on the single crystals LiMnPO4 and Li0.9FeAs. Therefore, the thesis is divided into two separated sections. The first part reports on the competitive next generation cathode material LiMnPO4 with a stable reversible capacity up to 145 mAh/g and a rather flat discharge voltage curve at 4.1 V. For the basic understanding of the material the magnetic properties have been investigated by a Li and P NMR study in the paramagnetic and antiferromagnetic phase. LiMnPO4 shows a strong anisotropy of the dipolar hyperfine coupling due to the strong local magnetic moments at the Mn site. The corresponding dipole tensor of the Li- and P-nuclei is fully determined by orientation and temperature dependent NMR experiments and compared to the calculated values from crystal structure data. Deviations of the experimentally determined values from the theoretical ones are discussed in terms of Mn disorder which could have an impact on the mobility of the Li ions. The disorder is corroborated by diffuse x-ray diffraction experiments which indicate a shift of the heavy elements in the lattice, namely the Mn atoms. Furthermore, the spin arrangement in the relative strong field of 7.0494 T in the antiferromagnetic state is understood by the NMR measurements. In order to obtain parameters of the Li ion diffusion in LiMnPO4 measurements of the spin lattice relaxation rate were performed. Due to the strong dipolar coupling between the Li-nuclei and the magnetic moments at the Mn site it is difficult to extract parameters which can characterize the diffusive behavior of the Li ions. The second section reports on the AC/DC susceptibility and NMR/NQR studies on Li deficit samples labeled as Li0.9FeAs. LiFeAs belongs to the family of the superconducting Pnictides which are discovered in 2008 by H. Hosono et al. In recent studies the stoichiometric compound reveals triplet superconductivity below Tc ∼ 18 K which demands ferromagnetic coupling of the electrons in the Cooper pairs. In Li0.9FeAs the Li deficit acts like hole doping which suppresses the superconductivity. Then ferromagnetism can arise which is very interesting because of the vicinity to the triplet superconductivity. With the microscopic methods NMR/NQR on the Li and As nuclei, it was investigated where the ferromagnetism can be located in Li0.9FeAs. Recent susceptibility, ESR and µSR studies reveal an internal field due to the ferromagnetism. In contrast, the internal field could not be used to perform zero field NMR measurements. Possible reasons for this discrepancy are discussed. In addition, the automatic insitu AC susceptibility technique by using the NMR radio frequency circuit has been tested by a reference compound Co2TiGa which shows itinerant ferromagnetism. Similar curves are observed for Li0.9FeAs which indicate the existence of itinerant magnetic moments in Li0.9FeAs. Furthermore, in order to determine the size of the dipolar contribution from the magnetic moments of the Fe the dipolar hyperfine coupling tensor was calculated from the crystal structure data. The comparison of the experimental and calculated hyperfine coupling elements reveals transferred hyperfine fields in LiFeAs.

[Truncated abstract] Although exchange bias was discovered more than four decades ago, a satisfactory understanding of every instance of exchange bias observed in experiment has not yet emerged. Understanding exchange bias is complicated by many factors. For example, details of the antiferromagnet interface structure set up during the initial field cooling, thermal activation processes in the ferromagnet and antiferromagnet, and domain formation and domain wall movement in the antiferromagnet are all important in determining features associated with exchange bias. Two exchange bias systems are investigated in this thesis. One is a disordered system: a single layer Co/CoO film with random interfaces prepared by a reactive RF sputtering technique. The other is a ‘model’ system of Fe/KFeF 3 bilayers with compensated interfaces prepared by molecular beam epitaxy (MBE). The central theme of this work is to understand exchange bias and other related magnetic properties in these two very different systems. The Co/CoO exchange bias system studied here is different in structure from conventional exchange bias systems such as bilayer and multilayer systems where interfaces between ferromagnet and antiferromagnet are reasonably well defined. In this Co/CoO system, the Co and CoO is in the form of particles distributed randomly in a sputtered film. The interfaces between the Co and CoO are randomly distributed and may not be continuous over a large length scale. More importantly, the interface area is dependent on the shape and size of the particles and on their distribution. Many unique magnetic properties are related to the random interface in this system. For example, exchange bias and coercivity obtained at low temperatures are very large due to the large interface area between Co and CoO particles. The interface area can be controlled by changing the Co/CoO mass ratio in the film. Unlike in bilayer systems, film thickness in this single layer Co/CoO system turns out not to be critical for exchange bias and coercivity. The independence of film thickness may be technically important. More interestingly, because the interface is random, exchange bias can be setup by field cooling in any direction. Both training and magnetic viscosity effects were studied and provided evidence of thermal activation processes in this Co/CoO system. Training is explained as formation of a domain wall in the CoO with motion limited locally due to limited continuity of Synthesis and Characterization of Magnetic Thin Films - Exchange Bias Systems interfaces between the Co and CoO. Specific magnetization measurements over time were made and studied using viscosity theory. The magnetic viscosity was found to be strongly temperature dependent. There is a broad distribution of blocking temperatures which might be due to a broad distribution of Co particle sizes

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2009. / Title from title screen (site viewed June 26, 2009). PDF text: xxvii, 209 p. : ill. (some col.) ; 17 Mb. UMI publication number: AAT 3350440. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph. D.)--University of Oregon, 1999. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 165-166). Also available for download via the World Wide Web; free to University of Oregon users. Address: http://wwwlib.umi.com/cr/uoregon/fullcit?p9948028.

Made available in DSpace on 2016-12-12T20:15:52Z (GMT). No. of bitstreams: 1 Diovana de Mello Lalis.pdf: 1400139 bytes, checksum: 9a24713cb1bd8642c6db6fc9c49eebbd (MD5) Previous issue date: 2015-02-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Os compostos de férmions pesados são geralmente descritos no âmbito do modelo da rede de Kondo, que pode ser derivado a partir do modelo de Anderson. As suas propriedades peculiares são regidas pela competição entre o efeito Kondo e a interação Ruderman-Kittel-Kasuya-Yosida (RKKY), que permite esboçar o diagrama de Doniach. A investigação de compostos ferromagnéticos de férmions pesados, em particular, tem se intensificado recentemente, com a possibilidade de aproximação do ponto crítico quântico (QCP) por meio do ajuste da pressão, da concentração de elétrons, ou pela aplicação de campo magnético. Neste trabalho, o modelo de rede de Anderson (ou modelo de Anderson periódico) é utilizado diretamente para descrever estes compostos, incorporando o efeito das flutuações de valência, que podem desempenhar um papel importante na vizinhança do QCP. Empregamos o método da equação do movimento para as funções de Green, que permite desacoplamentos sucessivos da cadeia de equações. Exploramos a estabilidade das soluções de AF, FM e não-magnéticas em função dos parâmetros do modelo, da temperatura e da concentração de életrons. O diagrama de fases é obtido a partir das curvas de magnetização e pela comparação das energias das soluções envolvidas.

Cette thèse porte sur la modélisation de nano-fils ferromagnétiques. La première par-tie est consacrée à la dérivation par processus asymptotique d'un modèle uni-dimen-sionnel de nano-fil ferromagnétique fini, courbé, torsadé et de section elliptique non constante, soumis à un courant électrique. Nous utilisons ensuite le modèle asympto-tique de jonction de fils pour considérer deux cas :- celui d'un fil infini présentant un coude dans la deuxième partie.-celui un fil rectiligne infini sur lequel on branche perpendiculairement un fil fini dans la troisième partie.Dans chacun des cas précédents, on explicite toutes les solutions stationnaires. Nous étudions ensuite la stabilité de ces solutions, en concluant que le coude et la jonction sont des points attracteurs du mur. Dans la dernière partie, nous introduisons une mé-thode numérique de type différences finis d'ordre 2 en espace adaptée à la simulation des systèmes de réseaux de nano-fils. Après avoir établi numériquement l'ordre de convergence de la méthode, nous validons le schéma en simulant soit des phénomènes décrits dans la littérature, soit des propriétés décrites de manières théoriques dans les parties précédents.Ainsi, nous calculons d'abord le seuil de Walker pour un fil rectiligne. De plus, nous vé-rifions que la configuration du mur est stable dans un fil pincé même en présence d'un petit champ appliqué dans la direction du fil. Par la suite nous vérifions les résultats de stabilité pour les cas d'un fil coudé de longueur finie et d'un jonction de trois fils finis. Enfin, nous étudions la propagation de plusieurs murs dans un réseau de fils sous forme d'un peigne en injectant un courant électrique. Dans cette partie toutes les simulations numériques sont faites en Python avec quelques visualisations en Matlab. / This thesis focuses on the modeling of ferromagnetic nanowires. In the first part, we derive a one-dimensional asymptotic model for the dynamics of the magnetic moment in a twisted ferromagnetic nanowire with variable elliptical cross-section, curvature and torsion, subjected to an electric current. Then, we use the new one-dimensional model to consider two cases: - the case of an infinite ferromagnetic nanowire having a bend in the second part.- the second case is when we connect perpendicularly a finite straight wire on a straight infinite horizontal wire in the third part.In both cases, we prove the existence of static solutions. We study the stability of these solutions, we conclude that the bend and the junction attract the wall profiles. In the last part, we introduce a finite difference of order 2 in space adapted to the si-mulation of nanowire network systems. After having numerically established the order of convergence of the method,we validate the scheme by simulating either phenomena described in the literature, or properties described in theoretical ways in the previous parts.We calculate the Walker field limit, for a straight wire. In addition, we verify that the wall configuration is stable in a pinched wire even in the presence of a small field ap-plied in the direction of the wire. Then we check the stability results for the case of a finite bent wire and a junction of three finite wires. Finally, we study the propagation of several walls in a network of wires in the form of a comb by injecting an electric current. In this part all the numerical simulations are made in Python with some visua-lizations in Matlab.

Made available in DSpace on 2014-10-09T12:35:09Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:00:32Z (GMT). No. of bitstreams: 0 / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP