Recentemente, a dopagem de semicondutores (envolvendo compostos II-VI, IV-VI, III-V, e do grupo-IV) com espécies magnéticas tem sido extensivamente investigada em função do seu potencial em spintrônica. Neste contexto, semicondutores magnéticos baseados no Si e no Ge são atraentes devido à sua compatibilidade com a indústria de semicondutores existente. Entretanto, a solubilidade das espécies magnéticas nestes materiais em forma cristalina é muito baixa e, consequentemente, sua atividade magnética é limitada. Este não é o caso para o silício amorfo (a-Si) e o germânio amorfo (a-Ge), que podem conter elementos magnéticos além do limite de solubilidade de seus análogos cristalinos, e apresentar propriedades magnéticas notáveis. Motivado por estes fatos, este trabalho apresenta uma investigação abrangente de filmes finos de Si e Ge contendo diferentes quantidades de Mn e Co, trazendo informações úteis no entendimento das propriedades desta classe de materiais. As amostras foram preparadas por co-sputtering, e possuíram concentrações de Mn na faixa de ~ 0.1-24 at.%, e de Co na faixa de ~ 1-10 at.%. Após a deposição, os filmes foram submetidos a tratamentos térmicos cumulativos até 900 oC, e foram investigados por: espectroscopia de energia dispersiva de raios-x (EDS); espalhamento Raman; difração de raios-x; transmissão óptica; microscopias eletrônica de varredura (SEM), de força atômica (AFM) e de força magnética (MFM); magnetometria SQUID; método de van der Pauw; etc. Para fins comparativos, amostras puras também foram preparadas, tratadas e caracterizadas de forma similar. Os presentes resultados indicam que os átomos de Mn e Co foram incorporados de forma efetiva e homogênea nas matrizes amorfas. Além disso, os filmes sem tratamento (puros ou contendo impurezas) são essencialmente amorfos. Ao contrário, tratamentos em altas temperaturas induzem a cristalização das amostras, e alterações em suas demais características, dependentes da introdução de dopantes. Desta forma: suas propriedades estruturais, ópticas, morfológicas, elétricas, e magnéticas, são notadamente afetadas pela inserção de Mn e Co, e pela temperatura de tratamento térmico. Estas observações foram sistematicamente investigadas e serão apresentadas e discutidas em detalhe. / Along the last few years, the doping of semiconductors (either II-VI, IV-VI, III-V, and group-IV compounds) with magnetic species have been extensively studied due to their potential applications in spintronics. Among them, Si- and Ge-based magnetic semiconductors are very attractive because of their total compatibility with the well-established current semiconductor technology. In the crystalline form, however, these materials exhibit a low solubility limit to magnetic species and, consequently, limited magnetic activity. This is not the case for amorphous (a-)Si and a-Ge, which can contain magnetic elements beyond the solubility limit of their crystalline counterparts, and present improved magnetic properties. Motivated by these facts, this work contains a comprehensive investigation of Si and Ge thin films containing different amounts of Mn and Co, providing useful information concerning the properties of this class of materials. The samples were prepared by co-sputtering, rendering Mn concentrations in the ~ 0.1-24 at.% range, and Co contents in the ~ 1-10 at.% range. After deposition, the films were submitted to isochronal thermal annealing treatments up to 900 oC and investigated by: energy dispersive x-ray spectrometry (EDS); Raman scattering spectroscopy; x-ray diffraction; optical transmission measurements; scanning electron (SEM), atomic force (AFM) and magnetic force (MFM) microscopy techniques; SQUID magnetometry; van der Pauw technique; etc. For comparison purposes, pure samples were also prepared, annealed and characterized in a similar way. The present experimental results indicate that the Mn and Co atoms were effectively and homogenously incorporated into the amorphous hosts. Moreover, the as-deposited films (either pure or doped) are essentially amorphous. On the contrary, thermal annealing at increasing temperatures induces the crystallization of the samples, and changes in their further characteristics, that are dependent of the doping. In this way: their structural, optical, morphological, electrical, and magnetic properties, etc., are notably affected by the insertion of Mn and Co, and by the temperature of thermal annealing. These experimental observations were systematically studied and will be presented and discussed in detail.

Cette thèse porte sur l'étude des ondes de spin dans des puits quantiques CdMnTe dopés n, par rotation Kerr résolue en temps (TRKR) et par mélange à quatre ondes (FWM). Nous avons étudié trois échantillons de haute mobilité et de caractéristiques différentes.La technique TRKR donne accès uniquement aux excitations de vecteur d'onde nul, dans notre cas l'onde spin-flip en q=0. Nous avons étudié l'anticroisement qui apparait entre l'onde spin-flip et l'excitation spin-flip des ions manganèse. Nous avons étudié la variation du gap, et donc de l'énergie de couplage, entre ces modes en fonction de la puissance d'excitation et du champ magnétique. En particulier nous avons étendu les mesures des modes mixtes à plus basse concentration en Mn (jusqu'à 0.07%) et contrairement à ce qui était attendu, nous avons trouvé que le régime de couplage fort persiste à cette concentration.Nous nous sommes ensuite intéressés à la détermination de la polarisation en spin ζ du gaz d'électrons bidimensionnel, qui peut être déduite de l'énergie de couplage entre les modes mixtes. Nous avons trouvé que la polarisation mesurée par cette méthode excède la polarisation théorique calculée en prenant en compte le renforcement de la susceptibilité par les effets à N corps. Nous avons également mesuré les temps de relaxation des électrons confinés dans le puits quantique, et nous avons montré l'influence de l'échauffement de l'échantillon par le laser sur le temps de relaxation de spin des électrons.Dans la deuxième partie de cette thèse, nous avons étudié par FWM l'amortissement et la dispersion des ondes de spin de vecteur d'onde non nul pour l'un de nos échantillons. Nous avons démontré qu'on peut effectivement générer les ondes de spin en excitation femtoseconde, et les détecter en FWM. Nous avons trouvé que leur dispersion est plus faible que celle observée dans les expériences de Raman. Cette faible dispersion pourrait être imputable à la forte densité d'excitation utilisée dans les expériences de FWM (typiquement trois à quatre ordres de grandeur supérieurs à celle du Raman), et/ou au fait que deux ondes de vecteur d'ondes q et –q, ayant des dispersions différentes, sont sondées simultanément en FWM. / This thesis focuses on the study of spin waves in n-doped CdMnTe quantum wells using respectively time-resolved Kerr rotation (TRKR) and four-wave mixing (FWM) techniques. We studied three high mobility samples with different characteristics.The TRKR technique gives access only to zero wave vector excitations, in our case the spin- flip wave q = 0 . We studied the anticrossing that appears between the spin -flip wave and the manganese spin -flip excitation. We studied the gap variation energy between these modes as function on the power excitation and the magnetic field. In particular, we have extended the measurements of mixed modes at lower Mn concentration (up 0.07 %) and contrary to what were expected; we found that the strong coupling regime persists at this concentration.We are then interested in determining the two dimensional electron gas spin polarization ζ, which can be deduced from the energy coupling between the mixed modes. We found that the measured polarization exceeds the theoretical polarization calculated taking into account the increased susceptibility by many-body effects. We also measured the electron spin relaxation time and we have shown that it is influenced by thermal effects inherent to optical pump-probe experiments on this time.In the second part of this thesis, we studied by FWM the damping and the dispersion of the non-zero wave vector spin waves for one of our samples. We have demonstrated that we can actually generate spin waves in femtosecond excitation and deted them by FWM. We found that the dispersion is lower than that observed in the Raman experiments. This low dispersion may be due to the strong excitation density used in the FWM experiments (typically three to four orders of magnitude higher than the Raman ones) and / or the fact that two waves of wave vector q and - q, having different dispersions are simultaneously probed in FWM .

Theoretical studies of point defect interactions and structural stability of compounds have been performed using density functional theory. The defect-related properties, such as activation energy of diffusion, electronic and magnetic structure of selected materials have been studied. The major part of the present work is devoted to a very important material for semiconductor industry, GaAs. The formation energies of intrinsic point defects and the solution energies of 3d transitions in GaAs have been calculated from first principles. Based on the calculated energies, we analysed the site preference of defects in the crystal. The tendency of defects to form clusters has been investigated for the intrinsic defects as well as for impurities in GaAs. The magnetic moment of 3d impurities has been calculated as a function of the chemical environment. The possibility of increasing the Curie temperature in (Ga,Mn)As by co-doping it with Cr impurities has been examined on the basis of calculated total energy difference between the disordered local moment and the ferromagnetically ordered spin configurations. We found that, in order to reach the highest critical temperature, GaAs should be separately doped with either Cr or Mn impurities. Also, we have shown that diffusion barrier of interstitial Mn depends on the charge state of this impurity in (Ga, Mn)As. The formation of defect complexes between interstitial and substitutional Mn atoms, and their influence on the value of diffusion barrier for interstitial Mn, has been studied. The pair interactions energies between interstitial oxygen atoms in hcp Zr, Hf and Ti have been calculated using first principles. Based on the calculated energies, the oxygen ordering structures in IVB transition metal solid solutions have been explained. A prediction of nitrogen ordering in Hf-N solid solution has been made. The thermodynamic description of intermetallic compounds in the Zr-Sn binary system has been obtained. The conclusion has been made that Zr substitution on the Sn sites takes place in the Zr4Sn phase, which accounts for the unusual stoichiometry of this Cr3Si structure type compound. The influence of pressure on the phase stability in the Fe-Si system has been investigated. We have found instability of the hcp Fe0.9Si0.1 random alloy with respect to the decomposition onto the Si-poor hcp Fe alloy and the B2 FeSi under high pressure. The tendency of this decomposition becomes stronger with increasing the applied pressure. / QC 20100624

Semiconductor devices are the building blocks of electronics and communication technology in the modern world. The charge, mass and spin of charge carriers in the semiconductor devices lay the foundations of the technology developments in the modern age. But to date only the electronic charge of the semiconductors has been exploited for such applications. The significance of the spin of charge carriers is completely ignored because in a semiconductor the half of the carriers are in spin-up state and the remainder are in spin-down state. A new electronics termed as spintronics, spin-transport based electronics, is focused to utilise the spin degree of freedom of the charge carriers in addition to its electronic charge. The devices based on these have the potential for various technological advancements like non-volatility, increased data processing speed, decreased electronic power consumption and increased integration densities as compared to the conventional semiconductor devices. In this study, the author intended to study the growth and properties of magnetic impurity doped antimony based III-V compounds and compare these results with those of the films grown by MBE. This thesis is organised into seven chapters. The first introductory chapter gives a brief review of the work on spintronics, diluted magnetic semiconductors, Ferromagnetic / paramagnetic semiconductor hybrid structures with special emphasis on the properties of antimonides which have already been reported in the literature. This is followed by the scope of the thesis. The second chapter deals with technical details of various instruments used in the present research work. Third chapter describes the growth and structural properties of bulk crystals grown by Bridgman method and thin films grown by liquid phase epitaxy (LPE). Bulk crystals of InSb and GaSb doped with magnetic elements such as Mn and Fe are grown with different doping concentrations. Thin films of InSb and GaSb doped Mn with different doping concentrations are grown by LPE. The grown crystals are processed by slicing, lapping, polishing and chemical etching methods. X-ray diffraction studies are carried out to confirm alloy formation and to find the change in lattice parameter if any. From the powder diffraction patterns, the lattice parameter is refined with the help of Retvield refinement program. A systematic change of lattice parameter with the incorporation of magnetic impurities into InSb and GaSb is observed. Scanning electron microscopy and energy dispersive x-ray analysis are carried out to identify the secondary phases and their composition respectively. Chapter 4 gives the detailed magnetotransport studies carried out on InSb and GaSb crystals doped with Mn and Fe. Also, the magnetotransport studies carried out on thin films grown by liquid phase epitaxy are presented here. This chapter is divided into three sections of which the first section deals with Mn doped bulk crystals of InSb and GaSb, the second section deals with Fe doped bulk crystals of InSb and GaSb and the third section deals with Mn doped InSb and GaSb films grown on GaAs by Liquid Phase Epitaxy. Temperature dependence of zero field resistivity, magnetoresistance and Hall measurements are carried out from 1.4 to 300K. All the samples show p type conduction throughout the temperature range studied except for Fe doped InSb. Mn doped crystals show negative magnetoresistance and anomalous Hall effect below 10K. Anisotropy in magnetoresistance is also observed at low temperatures in InMnSb crystals. On the other hand, Fe doped samples exhibit positive magnetoresistance throughout the temperature range and no anomalous Hall effect is observed. Chapter 5 describes the magnetic properties of bulk InMnSb, GaMnSb, InFeSb and GaFeSb crystals so also the thin films of InMnSb/GaAs. DC magnetization measurements are carried out in the temperature range 2 - 300K. The Mn doped InSb and GaSb crystals as well as InMnSb/GaAs films, show a magnetic ordering below 10K which could arise from the InMnSb and GaMnSb alloy formation. Also, saturation in magnetization observed even at room temperature suggests the existence of ferromagnetic MnSb clusters in the crystals which has been verified by scanning electron microscopy studies. In Fe doped InSb crystals, the temperature dependent DC magnetization shows irreversibility under field cooled and zero field cooled conditions below 12K, suggesting a spin glass-like behaviour. Also, magnetization measurement shows the coexistence of ferromagnetic and paramagnetic phases throughout the temperature range studied. Existence of ferromagnetic phase could arise from secondary phases Fe1-xInx or FeSb2 present in the crystal as clusters and paramagnetic phase could arise from the randomly distributed Fe atoms in the InSb matrix. Fe doped GaSb crystals show interesting magnetic property that arises from the FeGa alloy (secondary phase) present in it. The EPR studies on Ga0.98Mn0.02Sb cluster-free (?) crystal suggest that the dominant Mn impurity in GaMnSb is Mn2+ (d5), described as ionized acceptor A−. This conclusion was derived from EPR experiments, which reveal a strong absorption line with an effective g factor very close to 2.00, the value typical for centre A−. The absence of EPR signal typical for neutral Mn acceptor A0 suggests that this center is absent in the crystal under investigation. The observed behavior is similar to that of Ga1-xMnxAs and In1-xMnxAs epilayers. EPR studies also reveal that the competition between antiferromagnetic and ferromagnetic phases exists in the studied crystal. Chapter 6 describes the optical measurements carried out on bulk Ga1-xMnxSb crystals and their films with different Mn doping concentrations. FTIR studies are carried out in the temperature range 4 - 300 K. From the FTIR studies, it is found that intra valance spin – orbit splitting band absorption is dominant compared to the fundamental absorption in doped crystals. In higher doped crystals (x > 0.01), fundamental band absorption merges with split-off band and could not be resolved. Free carrier absorption studies are also carried out in the energy range below the band gap. FTIR studies on GaMnSb/GaAs films suggest band gap narrowing effect due to Mn doping. Furthermore the Photoluminescence measurements are carried out in the temperature range 10 – 300 K for all the Mn doped GaSb crystals. PL studies also support the band gap narrowing and band filling effects. A comprehensive summary of this research investigation and scope for the further work are presented in the last chapter.

The topic of this Thesis is magnetization dynamics on atomic length scales. A computational scheme, Atomistic Spin Dynamics, based on density functional theory, the adiabatic approximation and the atomic moment approximation is presented. Simulations are performed for chemically disordered systems, antiferromagnets and ferrimagnets and also systems with reduced dimensionality The autocorrelation function of the archetypical spin glass alloy CuMn is sampled in simulations following a quenching protocol. The aging regime can be clearly identified and the dependence of the relaxation on the damping parameter is investigated. The time evolution of pair correlation and autocorrelation functions has been studied in simulations of the dilute magnetic semiconductor GaMnAs. The dynamics reveal a substantial short ranged magnetic order even at temperatures at or above the ordering temperature. The dynamics for different concentrations of As antisites are discussed. Antiferromagnets offer opportunities for ultrafast switching, this is studied in simulations of an artificial antiferromagnet. For the right conditions, the cooperative effect of applied field torque and and the torque from the other sublattice enables very fast switching. The dynamics of bcc Fe precessing in a strong uniaxial anisotropy are investigated. It is demonstrated that the magnetization can shrink substantially due to a spin wave instability. The dynamics of a two-component model ferrimagnet at finite temperature are investigated. At temperatures where the magnetic system is close to the magnetic and angular momentum compensations points of the ferrimagnet, the relaxation in a uniaxial easy exis anisotropy resembles results in recent experiments on ferrimagnetic resonance. The different cases of uniaxial or colossal magnetic anisotropy in nanowires at different temperatures are compared. The magnon softening in a ferromagnetic monolayer is investigated, giving results that compare well with recent experiments. The effect of lattice relaxation can be treated in first principles calculations. Subsequent simulations captures the softening of magnons caused by reduced dimensionality and temperature. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 706

Magnetic properties of powders, thin films and single crystals have been investigated using magnetometry methods. This thesis provides analysis and conclusions that are supported by the results obtained from spectroscopic and diffraction measurements as well as from theoretical calculations. First, the magnetic behavior of transition metal (TM) doped ZnO with respect to doping, growth conditions and post annealing has been studied. Our findings indicate that the magnetic behavior stems from small clusters or precipitates of the dopant, with ferromagnetic or antiferromagnetic interactions. At the lowest dopant concentrations, the estimated cluster sizes are too small for high resolution imaging. Still, the clusters may be sufficiently large to generate a finite spontaneous magnetization even at room temperature and could easily be misinterpreted as an intrinsic ferromagnetic state of the TM:ZnO compound. Second, influence of lattice strain on both magnetic moment and anisotropy has been investigated for epitaxial MnAs thin films grown on GaAs substrates. The obtained magnetic moments and anisotropy values are higher than for bulk MnAs. The enhanced values are caused by highly strained local areas that have a stronger dependence on the in-plane axis strain than out-of plane axis strain. Finally, spin glass behavior in Li-layered oxides, used for battery applications, and a double perovskite material has been investigated. For both Li(NiCoMn)O2 and (Sr,La)MnWO6, a mixed-valence of one of the transition metal ions creates competing ferromagnetic and antiferromagnetic interactions resulting in a low temperature three-dimensional (3D) spin glass state. Additionally, Li(NiCoMn)O2 with large cationic mixing exhibits a percolating ferrimagnetic spin order in the high temperature region and coexists with a two-dimensional (2D) frustrated spin state in the mid temperature region. This is one of the rare observations where a dimensional crossover from 2D to 3D spin frustration appears in a reentrant material. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 720

Transition metal and related compounds have been extensively studied over the past several decades. These investigations revealed a wide range of behavior, encompassing colossal magnetoresistance (CMR), high-TC superconductivity, and magnetic semiconductivity, all of which continue to present fundamental challenges to the understanding of such phenomena. There is, however, a close correlation between such characteristics and the appearance of magnetic order. This correlation underlies the present study, which focuses on the magnetic and transport behavior of various Manganese (Mn), Iron (Fe) and Cobalt (Co) containing materials, with particular emphasis on the nature of the magnetic order they display and the critical exponents that characterize the accompanying phase transition. The magnetic and transport properties of two specific systems will be covered: first various doped manganites from the series (La,Pr)1-x(Ca,Ba)xMnO3, and second the magnetic semiconductors Fe0.8Co0.2Si and Ga0.98Mn0.02As. In the manganites, the influence of doping on; (i) the evolution of the metal-insulator transition (MIT) with composition; (ii) the universality class of the magnetic critical behavior associated with the paramagnetic to ferromagnetic transition, which occurs in the vicinity of a MIT with which CMR is associated; (iii) the mechanisms underlying ferromagnetism across the MIT; (iv) the correlation between the appearance of a Griffiths-like phase and CMR, and (v) the origin of Griffiths-like phase have been investigated. Four different systems have been studied: La1-xCaxMnO3 (0.18 ≤ x ≤ 0.27), La1-xBaxMnO3 (x ≤ 0.33), (La1-yPry)0.7Ca0.3Mn16/18O3 (y ≤ 0.85), and Pr1-xCaxMnO3 (x = 0.27, 0.29). In Fe0.8Co0.2Si and Ga0.98Mn0.02As, the scaling between magnetization and conductivity has been the subject of ongoing debate. In bulk Fe0.8Co0.2Si, a novel scaling between the anomalous Hall effect (AHE) and the magnetization enables the anomalous Hall coefficient to be accurately determined. In turn, this enables the universality class for the transition to ferromagnetism to be established independently from the anomalous Hall conductivity. In an epitaxial (metallic) Ga0.98Mn0.02As microstructure, the magnetization has been indirectly determined from the AHE. Subsequent analysis yields magnetic critical exponents consistent with the Mean-Field model, direct support for which had previously been lacking.

Transition metal and related compounds have been extensively studied over the past several decades. These investigations revealed a wide range of behavior, encompassing colossal magnetoresistance (CMR), high-TC superconductivity, and magnetic semiconductivity, all of which continue to present fundamental challenges to the understanding of such phenomena. There is, however, a close correlation between such characteristics and the appearance of magnetic order. This correlation underlies the present study, which focuses on the magnetic and transport behavior of various Manganese (Mn), Iron (Fe) and Cobalt (Co) containing materials, with particular emphasis on the nature of the magnetic order they display and the critical exponents that characterize the accompanying phase transition. The magnetic and transport properties of two specific systems will be covered: first various doped manganites from the series (La,Pr)1-x(Ca,Ba)xMnO3, and second the magnetic semiconductors Fe0.8Co0.2Si and Ga0.98Mn0.02As. In the manganites, the influence of doping on; (i) the evolution of the metal-insulator transition (MIT) with composition; (ii) the universality class of the magnetic critical behavior associated with the paramagnetic to ferromagnetic transition, which occurs in the vicinity of a MIT with which CMR is associated; (iii) the mechanisms underlying ferromagnetism across the MIT; (iv) the correlation between the appearance of a Griffiths-like phase and CMR, and (v) the origin of Griffiths-like phase have been investigated. Four different systems have been studied: La1-xCaxMnO3 (0.18 ≤ x ≤ 0.27), La1-xBaxMnO3 (x ≤ 0.33), (La1-yPry)0.7Ca0.3Mn16/18O3 (y ≤ 0.85), and Pr1-xCaxMnO3 (x = 0.27, 0.29). In Fe0.8Co0.2Si and Ga0.98Mn0.02As, the scaling between magnetization and conductivity has been the subject of ongoing debate. In bulk Fe0.8Co0.2Si, a novel scaling between the anomalous Hall effect (AHE) and the magnetization enables the anomalous Hall coefficient to be accurately determined. In turn, this enables the universality class for the transition to ferromagnetism to be established independently from the anomalous Hall conductivity. In an epitaxial (metallic) Ga0.98Mn0.02As microstructure, the magnetization has been indirectly determined from the AHE. Subsequent analysis yields magnetic critical exponents consistent with the Mean-Field model, direct support for which had previously been lacking.

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)

Cette thèse est principalement consacrée à l'étude des inhomogénéités de taille nanométrique dans les systèmes magnétiques désordonnés ou dilués. La présence d'inhomogénéités, souvent mise en évidence dans de nombreux matériaux, donne lieu à des propriétés physiques intéressantes et inattendues. La possibilité de ferromagnétisme à l'ambiante dans certains matéraux a généré un grand enthousiasme en vue d'application dans la spintronique. Cependant, d'un point de vue fondamental la physique de ces systèmes reste peu explorée et mal comprise. Dans ce manuscrit, on se propose de fournir une étude théorique complète et détaillée des effets des inhomogenéités de tailles nanométriques sur les propriétés magnétiques dans les systèmes dilués. Tout d'abord, on montre que l'approche RPA locale autocohérente est l'outil le plus adapté et fiable pour un traitement approprié du désordre et de la percolation. Nous avons implémenté cet outil et étudié dans un premier temps, les propriétés magnétiques dynamiques d'un modèle Heisenberg dilué (couplages premiers voisins) sur un reseau cubique simple. Nous avons reproduit précisémment la disparition de l'ordre à longue portée au seuil de percolation et comparé ce travail à des études précédentes. Dans le cadre d'un Hamiltonien minimal (modèle $V$-$J$) nous avons ensuite étudié en détails les propriétés magnétiques de (Ga,Mn)As (température critique, excitations magnétiques, stiffness,..). Nous avons obtenu de très bon accords avec les calculs textit{ab initio} et les résulats expérimentaux. Finalement, nous avons étudié les effets des inhomogénéités dans les sytèmes dilués. Nous avons montré, qu'inclure des inhomogenéités pourrait s'averer être une voie très efficace et prometteuse pour dépasser l'ambiante dans de nombreux matériaux. Nous avons pu obtenir une augmentation colossale de la température critique dans certains cas comparée à celle des systèmes dilués homogènes. Nous avons atteint une augmentation de 1600% dans certains cas. Nous avons également analysé les effets des inhomogénéités sur les courbes d'aimantations, elles sont inhabituelles et peu conventionelles dans ces systèmes. Les spectres d'excitations magnétiques sont très complexes, avec des structures très riches, et présentent de nombreux modes discrets à haute energie. De plus, nos calculs ont montré que la ``spin-stiffness" est fortement supprimé par l'introduction d'inhomogénéités. Il reste encore de nombreuses voies à explorer, ce travail devrait servir de base à de futures études théoriques et expérimentales des systèmes inhomogènes. / This thesis is mainly devoted to the study of nanoscale inhomogeneities in diluted and disordered magnetic systems. The presence of inhomogeneities was detected experimentally in several disordered systems which in turn gave rise to various interesting and unexpected properties. In particular, the possibility of room-temperature ferromagnetism generated a huge thrust in these inhomogeneous materials for potential spintronics applications. However, a proper theoretical understanding of the underlying physics was a longstanding debate. In this manuscript we provide a detailed theoretical account of the effects of these nanoscale inhomogeneities on the magnetic properties of diluted systems. First we show the importance of disorder effects in these systems, and the need to treat them in an appropriate manner. The self-consistent local RPA (SC-LRPA) theory, based on finite temperature Green's function, is found to be the most reliable and accurate tool for this. We have successfully implemented the SC-LRPA to study the dynamical magnetic properties of the 3D nearest-neighbor diluted Heisenberg model. The percolation threshold is found to be reproduced exactly in comparison with previous existing studies. Following this, we discuss the essential role of a minimal model approach to study diluted magnetic systems. The one-band $V$-$J$ model, has been used to calculate the Curie temperature and the spin excitation spectrum in (Ga,Mn)As. An excellent agreement is obtained with first principles based calculations as well as experiments. Finally we propose an innovative path to room-temperature ferromagnetism in these materials, by nanoscale cluster inclusion. We find a colossal increase in $T_C$ of up to 1600% compared to the homogeneous case in certain cases. Also the spontaneous magnetization is found to exhibit anomalous non-mean-field like behavior in the presence of inhomogeneities. In addition we observe a complex nature of the magnon excitation spectrum with prominent features appearing at high energies, which is drastically different from the homogeneous case. Our study interestingly reveals a strong suppression of the spin-stiffness in these inhomogeneous systems. The results indicate toward the strong complexities associated with the interplay/competition between several typical length scales. We believe this work would strongly motivate detailed experimental as well as theoretical studies in this direction in the near future.