Submitted by Alison Vanceto (alison-vanceto@hotmail.com) on 2017-05-22T12:58:51Z No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T20:35:23Z (GMT) No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-05-23T20:35:31Z (GMT) No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) / Made available in DSpace on 2017-05-25T19:51:05Z (GMT). No. of bitstreams: 1 DissYJOR.pdf: 3749771 bytes, checksum: 98b6729837eaa69cb6eb598ad207cfcf (MD5) Previous issue date: 2016-03-09 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / We report a systematic study on the structural, morphological and optical properties of Zn1−xCoxO thin films grown by spray pyrolysis. The employed nominal concentrations were up to 15 Co %. The films were deposited on glass substrates between 220-300 o C using precursors such as zinc acetate and cobalt acetate in aqueous solutions. The effects of Co-doping on structural, morphological and optical properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), optical transmittance and absorbance as well as photoluminescence (PL). The XRD results indicated that all films are polycrystalline with wurtzite crystal structure, a preferential growth direction in c-axis, and no secondary phases. SEM images generally showed uniform surface, and few regions have irregularities due to effects of the inclusion of cobalt as well as substrate deposition temperature. The spectra of transmittance and absorbance exhibited the transitions d-d due to Co2+ ion, a redshift of the absorption-edge and changes in the absorption band intensity with increasing of nominal Co-doping. These observations revealed the substitution of Zn ions Zn2+ and the presence of Co in the state 2+ in tetrahedral coordination in the ZnO lattice. Furthermore, there was a narrowing in band gap of 28 meV by percentage of Co inserted into the lattice. The PL spectra of ZnCoO samples presented three bands associated to near-band-edge absorption (NBE), structural defects and intratomic emissions of Co2+. Their optical emissions were characterized by the presence of traps which capture carriers and affect significantly the emissions. Through a qualitative model in a temperature range we interpreted these traps as potential fluctuations up to 17 meV. We also discussed the annealing in selected samples. / Apresentamos um estudo sistemático sobre as propriedades estruturais, morfológicas e ópticas de filmes Zn1−xCoxO com concentrações nominais de Co até 15% obtidos por spray pirólise. Os filmes foram depositados sobre substratos de vidro entre 220 - 300 oC, utilizando como precursores acetato de zinco e acetato de cobalto em solução aquosa. Os efeitos da dopagem com Co nas propriedades estruturais, morfológicas e ópticas foram estudados por difração de raios-X (DRX), microscopia eletrônica de varredura (MEV), transmitância e absorbância óptica, e fotoluminescência (PL). Os resultados de DRX indicaram que todos os filmes são policristalinos, com estrutura cristalina do tipo wurtzita e direção de crescimento preferencial no eixo c, com ausência de fases secundárias. As imagens do MEV, em geral, mostraram que as superfícies são uniformes, com irregularidades em algumas regiões devido aos efeitos da inserção de Cobalto e da temperatura de deposição do substrato. Nos espectros de transmitância e absorbância, observaram-se a presença das transições d-d do íon de Co2+, o deslocamento da borda de absorção para baixas energias, e a variação da intensidade da banda de absorção das transições d-d com o aumento da dopagem nominal de Co. Estas observações revelaram a substituição dos íons de Zn2+ e a presença de Co no estado +2 na coordenação tetraédrica na rede do ZnO. Além disso, houve uma redução da energia do bandgap de 28 meV por porcentagem de Co inserido na rede. Os espectros de PL das amostras de ZnO dopado com Co apresentaram três bandas associadas à emissão próxima à borda de absorção óptica, defeitos estruturais e emissão intratômica do Co2+. A emissão óptica dos filmes foi caracterizada pela presença de armadilhas que localizam os portadores de carga e afetam significativamente as bandas de emissão no intervalo de temperatura estudado. Através de um modelo qualitativo interpretamos estas armadilhas como flutuações de potencial com profundidades de até 17 meV. Discutimos também o tratamento térmico em amostras selecionadas.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Neste trabalho abordamos algumas propriedades físicas associadas ao transporte spintrônico de nanoestruturas formadas por camadas de semicondutor diluído magnético (SDM) e semicondutor convencional submetidas a campos elétrico e magnético cruzados. O campo elétrico é aplicado na direção de crescimento da nanoestrutura e o campo magnético é aplicado perpendicularmente a essa direção. Estuda mos duas configurações de nanoestruturas onde o SDM localiza-se no poço quântico ou nas barreiras. Mostramos que é possível encontrar um potencial efetivo tipo poço de potencial duplo para um intervalo de intensidades de campos externos, altura da barreira de potencial e largura de poço quântico parabólico. Em tal condição esse sistema pode ser visto como um dispositivo spintrônico chamado filtro de spin, pois consegue selecionar polarizações de spin em diferentes regiões da nanoestrutura. / In the present work we studied properties of the spintronic transport of nanostructures formed by layers of diluted magnetic semiconductors (DMS) and conventional semiconductors with crossed fields applied. The electric field is in the growth direction and the magnetic field is perpendicular t o this one. We studied two configurations of nanostructures where the DMS is located in the barriers or in the well. We will show the possibility of the formation of a double quantum well like effective potential for different values of the applied fields intensities, barriers height and quantum well width. In this situation the system can be seen as a spintronic device called spin filter, since it can control the spin polarization in different regions of the nanostructure.

Le développement des lasers impulsionnels a permis l’émergence de l’étude de la dynamique de l’aimantation dans le domaine temporel (fs-ns) par des techniques optiques. Dans les couches minces ferromagnétiques présentant une faible absorption optique les mécanismes de couplage de la lumière avec les propriétés magnétiques permettant l’excitation de la précession de l’aimantation et sa détection ne sont qu’imparfaitement connus. Pour développer leur compréhension il est nécessaire de faire varier les paramètres magnétiques. Les alliages semi-conducteurs ferromagnétiques sont pour cela des matériaux de choix du fait de leurs propriétés magnétiques aisément ajustables grâce au couplage entre les propriétés semi-conductrices et magnétiques. Les conditions pour une excitation optimale ont été précisées. L’excitation par une variation thermique transitoire des constantes d’anisotropie a été quantifiée. Le rôle de l’absorption et du déphasage optique dans la détection a été établi. L’observation de plusieurs ondes de spin a permis la détermination de la constante d’échange en fonction de la concentration en phosphore. J’ai montré que celle-ci variait peu, contrairement aux prédictions théoriques. / Development of impulsional laser enhanced the study of the magnetization dynamic in the temporal domain (fs-ns) with optical techniques. In thin ferromagnetic films with a weak optical absorption, excitation and optical mechanisms are not yet fully understood. To increase our understanding, variations of magnetic parameters are required. To that end, the magnetic semiconductor like (Ga,Mn)(As,P) are a good choice due to their magnetic properties easily adjustable thanks to the coupling between magnetic and semiconductor properties. In this thesis, conditions for optimal excitations were established. Supposing a thermal excitation origin, we assessed the transient change of temperature and anisotropy constant. I also established the role of the optical absorption and dephasing due to the finite magnetic layer thickness in the spin-waves detection process. The observation of several spin-waves gave me the opportunity to assess the spin-stiffness constant as a function of phosphorus. On the contrary of theoretical prediction, I showed that the spin-stiffness hardly vary with the phosphorus concentration.

No description available.

Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique properties in the emerging field of spintronics. Several transition metal defects have been reported to order ferromagnetically in various semiconductors, however, low Curie temperatures and lack of other fundamental material properties have hindered practical implementation in room temperature spintronic applications. In this Thesis, we consider the energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties at various lattice sites and charge states using ab initio Density Functional Theory methods. We find the majority of 3d transition metal impurities in diamond at any charge state to be energetically most stable at the divacancy site compared to substitutional or interstitial lattice sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy site). At each lattice site and charge state, we find the formation energies of transition metals in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to those early or late in the series. The energetic stability of transition metal impurities across the 3d series is shown to be strongly dependent on the position of the Fermi level in the diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype diamond compared to intrinsic diamond. Further, we show that incorporation of isolated transition metal impurities into diamond introduces spin polarised impurity bands into the diamond band gap, while maintaining its semiconducting nature, with band gaps in both the spin-up and spin-down channels. These impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp 3 orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute significantly to hybridization for transition metal atoms at the substitutional site, but not at the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are critically dependent on the lattice site and charge state of the transition metal impurity. By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form a diluted magnetic semiconductor which may successfully be considered for room temperature spintronic applications. In addition, these charge states correspond to p-type diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as B ( will result in an increase of charge concentration, which is likely to enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond. / Physics / D. Litt. et Phil. (Physics)

Ce mémoire présente les travaux réalisés durant ces trois années de thèse. Ils se sont orientés autour de l'étude des propriétés électroniques et magnétiques des matériaux pour la spintronique. Ce domaine d'avenir doit encore trouver les matériaux adaptés qui permettraient de réaliser les concepts liés à la spintronique. Nos résultats sont les suivants. Nous avons proposé un nouveau modèle qui décrit l'arrangement magnétique dans les couches minces ferromagnétiques possédant une anisotropie uniaxiale. Le modèle apporte une meilleure description des largeurs de domaine en fonction de l'épaisseur ainsi qu'une bonne estimation de l'épaisseur critique à partir de laquelle les domaines de Weiss ne sont plus stables. L'ensemble d'équations en unité réduite découlant du modèle apporte quant à lui un outil supplémentaire aux expérimentateurs. L'étude ab-Initio fait sur les semi-Conducteurs magnétiques dilués à mis en évidence l'importance de la corrélation forte et de l'effet Jahn-Teller dans les matériaux tel que le (Ga,Mn)N et le (Zn,Cr)S. Ces calculs confirment l'ensemble des données expérimentales existantes sur le (Ga,Mn)N. La modélisation analytique apporte un complément aux calculs ab-Initio en faisant le lien entre les paramètres expérimentaux et la théorie de Vallin, très largement utilisée pour interpréter les mesures optiques du (Zn,Cr)S. Nos calculs ab-Initio ont montré que le métacinabre est un isolant topologique robuste, qui se distingue par la présence d'un cône de Dirac extrêmement anisotrope. Les effets de passivation à l'hydrogène influencent les états de surface des isolants topologiques de la série HgX (X : S, Se, Te). / This thesis contains the scientific work of three years. The main topic can be defined as follows : study of electronic and magnetic properties of materials for spintronics. That technology of the future has still to find the necessary materials to realize new concepts. Our results are the following : We propose a new model to describe the magnetic configuration in thin ferromagnetic film with uniaxial anisotropy. It gives a better description of domain widths in function of film thickness and permits to obtain a good evaluation of the critical thickness where domains of Weiss type are no longer stable. The set of equations in reduced units is a useful tool to analyze experimental data. The ab-Initio study of diluted magnetic semiconductors has demonstrated the combined effect of strong correlations and Jahn-Teller distortion in (Ga,Mn)N and (Zn,Cr)S. The calculations confirm the experimental results of (Ga,Mn)N. We develop an analytical model that is complementary to the ab-Initio calculations and permits to create a link between several experimental parameters, the ab-Initio calculations and the former theory of Vallin. It has been used to interpret the optical measurements of (Zn,Cr)S. Our ab-Initio studies show that metacinnabar is a strong topological insulator with one peculiarity, it has a highly anisotropic Dirac cone. The passivation with hydrogen atoms influences the surface states of the topological insulator in the series HgX (X : S, Se, Te). When the dangling bonds are saturated by hydrogen, the trivial surface states disappear.

Nous avons étudié les propriétés dynamique d'un spin individuel dans une boite quantique de semiconducteur II-VI (spin d'un atome de Mn ou electron résident). Une boîte quantique comportant un atome de manganese présente six raies qui permettent de sonder optiquement l'état de spin du Manganese. Des expériences pompe-sonde réalisées sur boîte unique ont permit de montrer que le spin du Mn peut être orienté optiquement en quelques dizaines de ns, que le temps de vie $T_1$ de ce spin est supérieur à la $mu$s, et que le pompage optique en champ nul est controlé par une anisotropie magnétique induite par les contraintes. Par ailleurs, dans le but d'identifier les mechanismes du pompage optique, nous avons mis en évidence des processus de relaxation de spin au sein du système exciton-manganese, durant la durée de vie de ce dernier. Enfin, nous avons mis en evidence un effet Stark optique sur chacune des raies d'une boîte quantique magnétique. Concernant la dynamique d'un électron dans une boîte quantique II-VI, nous avons mis en évidence le pompage du spin de l'électron résident ainsi que des noyaux. / We have studied the dynamic properties of a single spin (Mn impurity or resident electron) in a II-VI semiconductor quantum dot. A quantum dot doped with a single Mn atom presents six lines which allow to probe optically the spin-state of the Mn atom. Pump-probe experiments at a single dot level were carried out to demonstrate that the Mn spin could be oriented in a few tens of ns, and that the spin-distribution prepared by such means was perfectly conserved over a few $mu$s. The optical pumping of the Mn spin at zero magnetic field is controlled by a strain-induced magnetic anisotropy. Furthermore, seeking for a microscopic mechanism controlling the optical pumping of the Mn atom, we have evidenced spin relaxation channels within the exciton-Mn complex. At last, we have evidenced an optical Stark effect on any of the lines of a Mn-doped quantum dot. Concerning the dynamics of an electron in a II-VI quantum dot, we have evidenced optical pumping of the resident electron, and dynamic nuclear spin polarization.

i Abstract: This doctoral thesis is dedicated to the study of magnetization dynamics in ferromagnetic semiconductor (Ga,Mn)As using magneto-optical (MO) spectroscopy methods. The character of the magnetization dynamics after the impact of the laser pulse was investigated under different experimental conditions in an extensive set of optimized (Ga,Mn)As samples with Mn doping ranging from 1.5% to 14%. The thorough analysis of the measured MO signal enabled us to develop a new method that can be used to determine the laser pulse-induced real-space magnetization trajectory without any numerical modelling. Moreover, the investigation of the measured MO signals allowed us to determine the basic micromagnetic properties of (Ga,Mn)As, such as the magnetic anisotropy, the Gilbert damping or the spin stiffness. In addition to this, we found out that the light-induced magnetization precession can be caused by three distinct mechanisms - the sample heating due to the energy transfer from the laser pulses, the angular momentum transfer from the circularly polarized photons, and the influence of the non-equilibrium hole polarization induced by the relativistic spin-orbit interaction. The first of these mechanisms is rather well known but the two remaining ones, which are the optical analogues of the spin-transfer torque...

Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique properties in the emerging field of spintronics. Several transition metal defects have been reported to order ferromagnetically in various semiconductors, however, low Curie temperatures and lack of other fundamental material properties have hindered practical implementation in room temperature spintronic applications. In this Thesis, we consider the energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties at various lattice sites and charge states using ab initio Density Functional Theory methods. We find the majority of 3d transition metal impurities in diamond at any charge state to be energetically most stable at the divacancy site compared to substitutional or interstitial lattice sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy site). At each lattice site and charge state, we find the formation energies of transition metals in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to those early or late in the series. The energetic stability of transition metal impurities across the 3d series is shown to be strongly dependent on the position of the Fermi level in the diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype diamond compared to intrinsic diamond. Further, we show that incorporation of isolated transition metal impurities into diamond introduces spin polarised impurity bands into the diamond band gap, while maintaining its semiconducting nature, with band gaps in both the spin-up and spin-down channels. These impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp 3 orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute significantly to hybridization for transition metal atoms at the substitutional site, but not at the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are critically dependent on the lattice site and charge state of the transition metal impurity. By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form a diluted magnetic semiconductor which may successfully be considered for room temperature spintronic applications. In addition, these charge states correspond to p-type diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as B ( will result in an increase of charge concentration, which is likely to enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond. / Physics / D. Litt. et Phil. (Physics)

Heteroestruturas digitais magnéticas (DMHs) são estruturas semicondutoras em que a distribuição de impurezas magnéticas (Mn) restringe-se a alguns arranjos bidimensionais (monocamadas) regularmente espaçados entre si. Na presença de um campo magnético, a interação de troca sp-d entre os momentos magnéticos localizados e os portadores itinerantes é responsável por um desdobramento de spin gigante, da ordem ou até superior que a separação cíclotron dos níveis de Landau. Aqui eu calculo a estrutura eletrônica de poços quânticos digitais magnéticos do grupo II-VI. Resolvo as equações de Kohn-Sham da teoria do funcional da densidade dependente de spin na aproximação de massa efetiva. Eu então calculo diversas propriedades magnetoópticas e de transporte relevantes experimentalmente. Em particular, eu investigo a física dependente de spin presente nestes sistemas sob dois diferentes pontos de vista. Primeiramente o enfoque é no efeito do magnetismo do Mn sobre o potencial dependente de spin da interação de troca sp-d, em particular nos efeitos da aglomeração antiferromagnética e da diluição do seu perfil de concentração (segregação e interdifusão). Ao considerar estes efeitos eu reproduzo resultados experimentais para desdobramento de spin $Delta_E$ e tempos de espalhamento de spin $tau_$ [S. A. Crooker et al., Phys. Rev. Lett. 75, 505 (1995); Phys. Rev. B 61, 1736 (2000)]. Na segunda parte eu mudo o enfoque para a física de gases de elétrons bidimensionais (2DEGs) altamente polarizados e mostro a importância da forte dependência de spin das contribuições de muitos corpos (troca e correlação) presentes nestes sistemas. Em particular, estes efeitos são relevantes para o surgimento de fases de ferromagnetismo de efeito Hall quântico. Eu calculo o magnetotransporte no regime de efeito Hall quântico para DMHs baseadas em ZnSe e CdTe. Meus resultados reproduzem resultados experimentais [R. Knobel et al., Phys. Rev. B 65, 235327 (2002); J. Jaroszynski et al., Phys. Rev. Lett. 89, 266802 (2002)] para a dependência com o campo magnético, com a temperatura, o aparecimento de picos anômalos e o surgimento de curvas de histerese em várias propriedades físicas. / Digital magnetic heterostructures (DMHs) are semiconductor structures with magnetic impurities (Mn) restricted to some planar arrangements (monolayers) regularly spaced. In the presence of an external magnetic field, the sp-d exchange interaction between the localized magnetic moments and the itinerant carriers is responsible for a giant spin splitting, of the order of, or even greater than, the cyclotron separation between Landau levels. Here I calculate the electronic structure of group II-VI digital magnetic quantum wells. I solve the Kohn-Sham equations of the spin-density functional theory within the effective mass approximation. Then I calculate some magneto-optical and transport properties which are experimentally relevant. In particular, I investigate the spin dependent physics of these systems from two different points of view. First, I focus on effects of the Mn magnetism on the sp-d exchange spin dependent potential, particularly the effect of antiferromagnetic clustering and the effect of dilution (segregation and interdiusion) of the Mn content prole. By considering these effects I reproduce experimental results for the spin splitting $Delta_E$ and spin scattering times $tau_$ [S. A. Crooker et al., Phys. Rev. Lett. 75, 505 (1995); Phys. Rev. B 61, 1736 (2000)]. In the second part I move on to the physics of spin-polarized two-dimensional electron gases (2DEGs), and show the relevance of the strong dependence of the many-body contributions (exchange and correlation) with the spin polarization. In particular, these effects are relevant for the development of quantum Hall ferromagnetic phases. I calculate magneto- transport in the quantum Hall eect regime for DMHs consisting of ZnSe and CdTe. My results reproduce experimental results [R. Knobel et al., Phys. Rev. B 65, 235327 (2002); J. Jaroszynski et al., Phys. Rev. Lett. 89, 266802 (2002)] for the dependence with magnetic eld, temperature, development of anomalous resistivities spikes and hysteretic behaviors in many physical properties.