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.

Diese Arbeit beschäftigt sich mit theoretischen Untersuchung von Oberflächenzuständen in ferromagnetischen Halbleitern. Einleitend wird ein analytisches "tight-binding"-Modell zur Beschreibung der Oberflächenzustände. Es liefert Aussagen zur Existenz von Oberflächenzuständen, zu deren spektralen Gewicht und Position bezüglich der Energie. Das Kondogitter-Modell wird verwendet, um Korrelations- und Temperatureffekte sowie Oberflächenzustände zu beschreiben. Dies erfolgt zunächst für sc-(100)-Modellfilme im Rahmen des sf-Modells. Die temperaturabhängigen der Oberflächenzustände zeigen abhängig vom Ort in der Brillouinzone sowie den "hopping"-Parametern in der Oberfläche sowohl Stoner-artiges als auch "spin-mixing"-Verhalten. Mit wachsender Temperatur werden Lebensdauereffekte in den Spektren sichtbar. Das Kondogitter-Modell (KLM) wird auf die Mehrbandsituation zum df-Modell verallgemeinert, um eine Beschreibung der Prototypen für magnetische Halbleiter EuS und EuO zu erreichen. Durch die Kombination von LDA-Bandstrukturrechnungen mit Vielteilchenrechnungen zum Multiband-KLM ist es gelungen, die ausgeprägte Temperaturabhängigkeit des unbesetzten 5d-Leitungsbandes und das Verhalten der Oberflächenzustände in EuS- und EuO-Filmen realistisch zu beschreiben. Der exakte Grenzfall des Kondogitter-Modells, das magnetischen Polaron (T=0), ermöglicht Kombination von ab-initio-Bandstrukturrechnungen und der Vielteilchentheorie ohne das Auftreten von Doppelzählungen relevanter Wechselwirkungen. Sowohl in EuO als auch in EuS temperaturabhängige Oberflächenzustände beobachtet werden, die im Fall von EuS jedoch schwerer nachzuweisen sind, da sie im Energiebereich des Volumenbandes auftreten. Die für EuS und EuO berechneten Rotverschiebungen sowie die dickenabhängige Magnetisierung von EuS stimmen hervorragend mit experimentellen Befunden überein. Eine Vielzahl von Korrelationseffekten ist mit wachsender Temperatur in den Spektren der unbesetzten Europium-5d-Bänder zu beobachten. / This work is dedicated to the theoretical investigation of surface states in ferromagnetic semiconductors. After the introduction a exact solvable analytical model is presented. It figures out for given conditions if surface states exist, which spectral weight they have, and at which position in energy they can be found. Thereafter the Kondo-Lattice-Model (KLM) is used to describe correlation and temperature effects. The description focuses initially to the sc-(100) model films in the sf-model. The resulting temperature dependent surface states both Stoner behavior and "spin-mixing" behavior dependent on the chosen hopping parameters and the position in the two dimensional Brillouin zone. With increasing temperature (up to Tc) lifetime effects arise in the spectra. In conclusion the KLM is extended to the multi-band situation (df-Model) in order to provide a description of the prototypes of magnetic semiconductor EuS and EuO. We succeed in describing the distinct temperature dependence of the unoccupied 5d-conduction band and the behavior of the surface states in EuS and EuO films realistically by a combination of a LDA band-structure calculation and the manybody theory. The exact limiting case of the KLM (T=0) -the magnetic polaron- allows a combination of a ab-initio band-structure calculation and manybody theory without double counting of any relevant interaction. The presented theory provides numerous results. In EuO and EuS can be found temperature dependent surface states. In case of EuS it''s detection is much more complicated thus the surface state energies are located inside the energy range of the bulk band. The famous redshift in EuS and EuO and the thickness dependent film magnetization of EuS agree very well with the experimental results. A lot of correlation effects are present in the calculated unoccupied Europium 5d bands. With increasing temperature these effects become stronger.

In this study, the oxygen-deficient TiO2, SrTiO3 systems and transition metal ion (Cr or V) doped TiO2, SrTiO3 and SrZrO3 systems have been investigated. We prepared samples as polycrystals, single crystals and thin films for various desires. Their structural, physical and electronic properties were measured by bulk-sensitive techniques (X-Ray Diffraction, SQUID and Electro Paramagnetic Resonance) or surface-sensitive techniques (Photoemission spectroscopy and X-ray absorption spectroscopy). The measurement of SQUID and EPR showed not only their magnetic properties but also the valence state of Cr dopant. We verified the valence state of Cr ions in oxides and found the key parameters of sample synthesis which control the valence state of Cr ions. Segregated phases such as SrCrO4 were formed when the samples were synthesized under O2 rich environment. The surface properties of Cr doped SrZrO3 films are also discussed. We found the synthesis conditions which influence on not only the behavior of Cr ions but also the resistive-switching behaviors. Various resistive-switching behaviors seem to depend on the surface chemistry of films. We found that the accumulation of Cr3+ on film surface provides a clean interface without any non-stoichiometric oxides and that this sharp interface termination results in a good performance of resistive-switching.

Multiferroic materials exhibit unique properties such as simultaneous existence of two or more of coupled ferroic order parameters (ferromagnetism, ferroelectricity, ferroelasticity or their anti-ferroic counterparts) in a single material. Recent years have seen a huge research interest in multiferroic materials for their potential application as high density non-volatile memory devices. However, the scarcity of these materials in single phase and the weak coupling of their ferroic components have directed the research towards multiferroic heterostructures. These systems operate by coupling the magnetic and electric properties of two materials, generally a ferromagnetic material and a ferroelectric material via strain. In this work, horizontal heterostructures of composite multiferroic materials were grown and characterized using pulsed laser ablation technique. Alternate magnetic and ferroelectric layers of cobalt ferrite and lead zirconium titanate, respectively, were fabricated and the coupling effect was studied by X-ray stress analysis. It was observed that the interfacial stress played an important role in the coupling effect between the phases. Doped zinc oxide (ZnO) heterostructures were also studied where the ferromagnetic phase was a layer of manganese doped ZnO and the ferroelectric phase was a layer of vanadium doped ZnO. For the first time, a clear evidence of possible room temperature magneto-elastic coupling was observed in these heterostructures. This work provides new insight into the stress mediated coupling mechanisms in composite multiferroics.

Made available in DSpace on 2016-06-02T20:15:30Z (GMT). No. of bitstreams: 1 5738.pdf: 3104025 bytes, checksum: 27f8126e91dc4b23ddd37a2e733a23fa (MD5) Previous issue date: 2014-01-29 / Universidade Federal de Sao Carlos / In the present thesis, we realize a computational modeling of semiconductor structures based on multiple quantum wells with filter barriers and on quantum wells with semiconductor diluted magnetic layers. We numerically solve the time-dependent Schrödinger s equation within the effective mass approximation, using the Split Operator method. Through the time evolved wave functions we access the dynamics quantities as the light assisted couplings of the states, in which the light is described by the inclusion of an oscillating electric field in the Hamiltonian. Then we determine the probabilities of absorption, oscillator strengths of the intersubband transitions induced by the light. Moreover we analyze the transmission probabilities and, in special, the system s photocurrent. The eigenstates and the eigenfunctions of the stationary states are also obtained within the method by simply making an imaginary time evolution. In the first work, the photocurrent of a multiple quantum well structure with filter barriers modulating the continuum above the wells was analyzed as a function of the applied bias. We find out an interesting dependence of the photocurrent with the applied field, as a differential negative photoconductance controlled by the field. We attribute this negative conductance to the interaction between the localized and extended states in the continuum, expressed by anticrossings between these states and the enhancement of the photocurrent at the crossings by the Landau-Zener-Stückelberg-Majorama like transitions. In the second work, it was evaluated the spin polarized photocurrent arising from quantum well s structures of GaMnAs, under light, electric and magnetic fields of few teslas. The study shows the existence of spectral domains in the THz ranges for which the proposed structure is strongly spin selective. For such photon frequencies, the photocurrent is spin polarized and the application of the external electric field reverts the polarization s signal. This behavior suggests the possibility of conveniently simple switching mechanisms. The physics underlying these results is studied and understood in terms of the spin-dependent coupling strengths emerging from the particular potential profiles of the heterostructures. We present two additional works related to the main ones. In the first additional one, we evaluated the dark current of the multiple quantum well structure with and without filter barriers. For doing this, we add totally the transmission probability through the structure in the Levine s model for the dark current. We observe that dark current is considerably reduced for the structure with the filter barriers when compared to the structure without these barriers. In the second additional work, we calculate the photocurrent in a ZnMnSe structure. We observe the generation of a spin polarized photocurrent controlled by the external electric field, as in the case of the GaMnAs structures. / Na presente tese, realizamos a modelagem computacional de estruturas semicondutoras baseadas em poços quânticos múltiplos com barreiras de filtro e em poços quânticos com camadas de material semicondutor magnético diluído. Para tanto, resolvemos numericamente a equação de Schrödinger dependente do tempo na aproximação de massa efetiva, por meio da evolução temporal das funções de onda do sistema, utilizando o chamado método do Split- Operator. Com as funções de onda evoluídas no tempo temos acesso às variáveis dinâmicas do sistema, como os acoplamentos entre os estados pela presença de luz, descrita na forma de um campo elétrico oscilante. Determinamos assim as probabilidades de absorção, forças de oscilador das transições intersubbandas geradas por essa excitação com luz, as probabilidades de transmissão através da estrutura e, em especial, o espectro de fotocorrente proveniente desses sistemas semicondutores. As autofunções e as autoenergias dos estados estacionários dos sistemas são obtidas pelo mesmo método realizando a evolução em tempo imaginário. No primeiro trabalho, a fotocorrente da estrutura de poços quânticos múltiplos com barreiras de filtro foi analisada em função do campo elétrico aplicado à estrutura. Foi encontrada uma dependência da fotocorrente com o campo elétrico bastante interessante, na forma de uma fotocondutância negativa controlada pelo campo elétrico aplicado à heteroestrutura. Atribuímos essa condutância negativa à interação entre estados localizados e estendidos no continuo se manifestando na forma de anticrossings e o aumento da fotocorrente para os valores de campo elétrico nos quais ocorrem esses crossings foi associado a transições de dois níveis do tipo Landau-Zener-Stückelberg-Majorama. No segundo trabalho, foi calculada a fotocorrente polarizada em spin de estruturas de poços quânticos de GaMnAs, na presença de um campo elétrico varável e um campo magnético de poucos teslas. O estudo mostrou a existência de domínios espectrais na região de THz do espectro eletromagnético, para os quais as estruturas propostas são altamente seletivas em spin. Para tais frequências, encontramos que a fotocorrente é polarizada em spin e a aplicação do campo elétrico é capaz de reverter forma muito eficiente o sinal da polarização. O comportamento observado sugere a possibilidade de mecanismos simples de controle sobre a fotocorrente e a Física por trás de tais efeitos foi entendida em termos dos acoplamentos dependentes de spin dos estados da estrutura, emergentes do perfil de potencial particular das heteroestruturas. Apresentamos dois trabalhos adicionais diretamente relacionados aos trabalhos principais. No primeiro trabalho, calculamos a corrente de escuro proveniente da estrutura de poços quânticos múltiplos com e sem barreiras de filtro, adicionando de forma integral a probabilidade de transmissão através da estrutura no modelo de Levine que determina essa corrente. Observamos que a presença das barreiras de filtro diminui significativamente a corrente de escuro dessa estrutura no regime de altos valores de campo elétrico. No segundo trabalho adicional, foi calculada a fotocorrente de uma estrutura de PQ com camada DMS, composta por ZnMnSe. Observamos a possibilidade de controle da polarização de spin com o campo elétrico, assim como no caso da estrutura composta de GaMnAs.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work we have studied the magnetic, optical and structural properties of ZnO nanoparticles pure and doped with different concentrations of Fe and Mn synthesized by three different synthesis methods; co-precipitation (CP), hydrothermal (SH) and thermal decomposition (DT). The samples were characterized by measurements of X-ray diffraction (XRD) and analyzed allied to Rietveld refinement method, absorption in the UV-Vis region, scanning and transmission electron microscopy (SEM and TEM), and magnetization measurements as a function of field and temperature (MvsH, MvsT). Through XRD analysis we have observed a dependence of the nanoparticle size with increasing temperature for both CP and SH methods well as a change in morphology with temperature for the samples system synthesized by SH. XRD analysis through the Williamson-Hall plot and TEM images show that particles obtained by SH and CP methods present an anisotropic growth, different of the particles obtained by DT method, that they present a spherical-like shape. The XRD results for Zn1-xFexO systems obtained by the CP and SH present only one phase but from point of view magnetic properties we have observed that these samples present transition at low temperature (T = 10 K) similar to an antiferromagnetic ordering. However, the Zn1-xFexO samples obtained by DT synthesis we have observed a paramagnetic behavior evidenced by MvsT curves. Therefore, at room temperature the MvsH curves indicated a ferromagnetic behavior. All Mn-doped ZnO samples present a paramagnetic behavior. The UV-Vis results show for all systems a slow increase in gap band with increases of dopant concentration. / Neste trabalho estudamos as propriedades magnéticas, ópticas e estruturais de nanopartículas de ZnO puras e dopadas com diferentes concentrações Fe e Mn sintetizadas por três diferentes métodos de síntese; co-precipitação (CP), hidrotérmico (SH) e decomposição térmica (DT). As amostras foram caracterizadas por medidas de difração de raios X (DRX) e analisadas juntamente ao método de refinamento Rietveld, absorção na região UV-Vis, microscopia eletrônica de varredura e de transmissão (MEV e MET) e medidas de magnetização em função do campo e da temperatura (MvsH, MvsT). Através das análises de DRX, nós temos observado uma dependência do tamanho da nanopartícula com o aumento da temperatura para ambos os métodos CP e SH, bem como uma variação na morfologia com a temperatura para o sistema de amostras sintetizado pela SH. Análises do gráfico de Williamson-Hall e imagens de MET mostram que as partículas obtidas através dos métodos SH e CP apresentam um crescimento anisotrópico, diferentemente das obtidas pelo método de DT, as quais apresentam morfologia esférica. Além disso, os resultados de DRX mostram que os sistemas Zn1-xFexO obtidos pela CP e SH apresentam somente uma fase, porém do ponto de vista das propriedades magnéticas observamos que as amostras apresentam uma transição em baixa temperatura (T = 10 K) similar a um ordenamento antiferromagnético. Por outro lado, as amostras de Zn1-xFexO obtidas pela síntese de DT observamos um comportamento paramagnético evidenciados pelas curvas de MvsT. No entanto, em temperatura ambiente as curvas de MvsH indicaram um comportamento ferromagnético. Todas as amostras de ZnO dopadas com Mn apresentam um comportamento típico de um material paramagnético. Resultados de absorção de UV-Vis para todos os sistemas estudados mostram um leve aumento na energia de gap com o aumento da concentração do dopante.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this work we study the structural and magnetic properties of pure and TM-doped CeO2 nanoparticles (TM: Mn, Cr, Co and Fe) obtained by the co-precipitation and hydrothermal methods. X-ray diffraction (XRD) results allied to the Rietveld refinement method indicate that all samples present single-phase with structure isomorphous to the CeO2. Average crystallites size was determined using the full width at half maximum from XRD patterns and their sizes were from of 7 to 13 nm. Transmission electron microscopy (TEM) images 10% doped samples show the presence of particles with spherical-like morphology and average sizes in good agreement with the estimated by XRD. From ultraviolet–Visible (UV–Vis) spectroscopy absorption measurements we estimated the optoelectronic gap of the samples, which vary between 2.87 and 3.44 eV. Analyses from magnetization curves as a function of temperature (MvsT) using Curie-Weiss law show that the number of paramagnetic ions per molecule (n) increases with the increasing of the Mn concentration. Already for 10% doped samples with different ions the (n) obeys the following order Mn ˂ Fe ˂ Cr ˂ Co for samples obtained by co-precipitation and Co ˂ Cr ˂ Mn ˂ Fe for the samples obtained by hydrothermal synthesis. Magnetization curves as a function of the magnetic field (MvsH) show that the the systems present a weak ferromagnetic behavior at 5K and paramagnetic at temperature higher than 200 K. / Neste trabalho estudamos as propriedades estruturais e magnéticas de nanopartículas de CeO2 pura e dopadas com diferentes concentrações de metais de transição (MT: Mn, Cr, Co e Fe) obtidas pelo método de co-precipitação e de síntese hidrotérmica. Resultados de difração de raios X (DRX) aliados ao método de refinamento Rietveld indicam a formação de uma única fase isomorfa ao CeO2, cujos tamanhos médios das nanopartículas variam entre 7 e 13 nm quando sintetizadas por co-precipitação e entre 8 e 12 nm quando obtidas por síntese hidrotérmica. Imagens de microscopia eletrônica de transmissão (MET) das amostras dopadas com 10% dos metais de transição mostram a presença de partículas com morfologia tendendo à esferas e tamanhos médios das partículas em bom acordo com os tamanhos estimados por DRX. Através de medidas de absorção na região do ultravioleta ao visível (UV-vis) estimamos o gap optoeletrônico das amostras, os quais variam entre 2,87 e 3,44 eV. Resultados das medidas de magnetização em função temperatura (MvsT) usando a lei de Curie-Weiss indicam que o número de íons paramagnéticos por molécula cresce de forma sistemática com o aumento da concentração de Mn nos dois métodos de síntese. Já para o caso das amostras dopadas com 10% do MT, esse número obedece a seguinte ordem Mn ˂ Fe ˂ Cr ˂ Co para o sistema obtido por co-precipitação e Co ˂ Cr ˂ Mn ˂ Fe para o sistema obtido por síntese hidrotérmica. Medidas de magnetização em função do campo magnético (MvsH) mostram que as nanopartículas apresentam um possível ordenamento ferromagnético fraco em 5K e um comportamento típico de um material paramagnético acima de 200K. / São Cristóvão, SE

Dans la réalisation de nouveaux composants innovants de la spintronique, de grands espoirs sont placés sur les semi-conducteurs magnétiques dilués (DMS). L’enjeu technologique est de développer des matériaux ayant à la fois des propriétés semi-conductrices et ferromagnétiques. Le but de ce travail est de réaliser une étude nanostructurale et magnétique détaillée du système Fe :SiC candidat prometteur pour devenir un semi-conducteur magnétique dilué à température ambiante. Cependant les propriétés magnétiques du matériau (6H-SiC) implanté avec des métaux de transitions (MT) dépendent fortement de sa microstructure (concentration et nature du dopant, précipitation du dopant…). Afin d’appréhender l’ensemble des propriétés nanostructurales et magnétiques, nous avons étudié le système Fe :SiC à l’échelle de l’atome en utilisant la sonde atomique tomographique (SAT) couplée à la spectrométrie Mössbauer 57Fe. Des monocristaux 6H-SiC (0001) de type p et n (~10+18/cm3) ont été multi-implantés en 56Fe et 57Fe à différentes énergies et différentes fluences conduisant à une concentration atomique de (6% et 4%) de 20 à 120 nm de la surface. Dans le cadre de ce travail, nous avons pu suivre l’effet de la nanostructure du système Fe :SiC en fonction de la concentration de fer et des températures d’implantation et de recuit. Nous avons établi de nouveaux résultats : nature et dimension des nanoparticules, évaluation précise du nombre d’atomes de fer dilué dans la matrice SiC. Les différentes contributions ferromagnétiques et paramagnétiques sont identifiées et clairement expliquées grâce au couplage de techniques expérimentales comme la SAT, la spectrométrie Mössbauer, la magnétométrie SQUID (Superconducting Quantum Interference Device). Nous avons réussi à déterminer des conditions optimales pour l’obtention d’un DMS à température ambiante. En effet dans les échantillons implantés 4% Fe à 380°C, plus de 90% des atomes de Fe sont dilués. Ces atomes de Fe dilués contribuent majoritairement aux propriétés ferromagnétiques mesurées par SQUID et par spectrométrie Mössbauer à 300 K. Ces différents résultats expérimentaux mettent en lumière la possibilité de réalisation d’un nouveau (DMS) à température ambiante / Great hopes are placed on diluted magnetic semiconductors (DMS) for new components of spintronics. The challenge is to develop materials with both semiconducting and ferromagnetic properties. The aim of this work is to carry out a detailed nanostructural and magnetic study of the Fe: SiC candidate promising system to become a magnetic semiconductor diluted at room temperature. However, the magnetic properties observed in (6H-SiC) implanted with transition metals (TM) depend strongly on the material microstructure (content and nature of the dopant, precipitation of the dopant, etc.). In order to understand all the nanostructural and magnetic mechanisms, we studied the Fe: SiC system at the atomic scale using atom probe tomography (APT) and Mössbauer spectrometry. p and n single crystalline 6H-SiC near (0001)-oriented samples were submitted to multi-step implantations with 56Fe and 57Fe ions at different energies and fluences leading to an iron concentration (Cat =6 and 4%) at a depth between 20 nm and 120 nm from the sample surface. In this work, we were able to follow the effect of the nanostructure of the Fe: SiC system as a function of the iron concentration and the temperatures of implantations and annealing. We have established new results: nature and size of the nanoparticles, precise evaluation of the number of iron atoms diluted in the SiC matrix. The ferromagnetic and paramagnetic contributions are identified and clearly explained by the coupling of experimental techniques such as APT, Mössbauer spectrometry, SQUID (Superconducting Quantum Interference Device) magnetometry. We were able to put the material in optimal conditions for obtaining a DMS at room temperature. Indeed, the implanted samples (4% Fe) at 380°C more than 90% Fe atoms were distributed homogeneously. These Fe atoms are the main source of the ferromagnetic properties measured by SQUID and Mössbauer spectrometry at 300 K. These experimental results highlight the possibility of obtaining a new (DMS) at room temperature.

Ce travail de thèse porte sur l’étude du carbure de silicium, dopé avec du fer dans le but de réaliser un semi-conducteur magnétique dilué à température ambiante pour des applications à la spintronique. Le dopage en fer a été réalisé par implantation ionique de type multi-énergie (30 - 160 keV) à différentes fluences, conduisant à une concentration atomique constante de 2 % de 20 à 100 nm. Il a été suivi d’un recuit à haute température dans le but d’homogénéiser la concentration en dopants. Les implantations se sont déroulées à une température de 550 °C. L’optimisation des propriétés magnétiques et électroniques du SiC–Fe, de même que la compréhension des mécanismes physiques à l’origine du magnétisme induit, ont nécessité une caractérisation poussée de la microstructure des matériaux implantés. Les objectifs de ce travail ont été d’une part, de réaliser une étude à l’échelle atomique de la nanostructure en fonction des conditions d’implantations (température, fluence) et des traitements thermiques post-implantation, et d’autre part, de déterminer les propriétés magnétiques des matériaux implantés. Dans ce travail, nous avons montré par Sonde Atomique Tomographique, la présence de nanoparticules dont la taille moyenne augmente avec la température de recuit. La cartographie chimique des nanoparticules a permis de révéler l’existence de phases riches en Fe pour les échantillons recuits. L’étude magnétique (spectrométrie Mössbauer et Squid) a montré que la contribution ferromagnétique est due principalement aux nanoparticules magnétiques et/ ou aux atomes de fer magnétiques dilués dans la matrice. La corrélation entre les propriétés structurale et magnétique a permis de montrer que les atomes de fer dilués dans la matrice et substitués sur sites de silicium contribuent au signal ferromagnétique en dessous de 300 K. Nous avons donc montré dans ce travail, que la taille et la nature des phases présentes dans les nanoparticules dépendent des conditions d’implantation et des températures de recuit et qu’il est nécessaire de recuire les échantillons à haute température pour faire apparaître un ordre ferromagnétique. / This PhD thesis focuses on the study of SiC, doped with Fe in order to elaborate a diluted magnetic semiconductor at room temperature for spintronic applications. The iron doping was carried out by ion implantation of multi-energy type (30-160 keV) at different fluences, leading to a 2% constant atomic concentration between 20 to 100 nm, followed by a high temperature annealing in the goal of homogenizing the dopant concentration. The implantation temperature during this process is 550 °C, in order to avoid amorphization. The optimization of the magnetic and electronic properties of SiC-Fe, as well as the understanding of the physical mechanisms at the origin of induced magnetism, require a thorough characterization of the microstructure of the implanted materials. The objectives of this work are, on the one hand, to carry out an atomic scale study of the nanostructure according to the implantation conditions (temperature, fluence) and the post-implantation annealing and the other hand, to characterize the magnetic properties of implanted materials. In this work, we have shown by atom probe tomographic, the existence of nanoparticles whose the average size increases with the annealing temperature. The chemical mapping of the nanoparticles shows the presence of the Fe-rich phases for the annealed samples. Magnetic study (Mössbauer spectrometry and Squid) shows the ferromagnetic contribution is due to the magnetic nanoparticles and/or the diluted Fe atoms in the matrix. The correlation between structural and magnetic properties allowed showing that diluted Fe atoms and substitute to Si sites contribute to the ferromagnetic contribution below 300 K. In coupling many characterization techniques in order to give a detailed description of the different studied samples, we have shown that the size and nature of the phase present in the nanoparticles depend on the implantation conditions and the annealing temperatures and consequently it is necessary to anneal our samples at high temperature to reveal ferromagnetic order.

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