Recently, different studies on Weyl semimetals have shown some great potential for applications in spintronics. Indeed, spin-chiral Weyl nodes are perfect sources or sinks of the Berry curvature, which give new transport properties due to their topological nature, such as the chiral anomaly, and a large anomalous Hall response. Moreover, type-II Weyl semimetals, such as WTe2, have a specific band structure with tilted Weyl cones and overlapping electron/hole bands that can result in a perfect charge compensation and an extremely large magnetoresistance (XMR) . Yet, in WTe2 , Weyl nodes are usually located about 50 meV above the Fermi energy, a situation that questions the observation of both a large positive XMR and a negative magnetoresistance attributed to the chiral anomaly in some studies. In this work, we investigate the magneto-transport properties of WTe2 nanos- tructures obtained by different methods (mechanical exfoliation, chemical vapor transport), considering both the real electronic band structure and scattering by dis- order. Although the XMR amplitude also depends on charge mobilities, it is shown that the subquadratic response is not strongly influenced by the degree of disorder. Taking carrier densities infered from quantum oscillations into account, a three-band model explains this behavior by a large difference in hole mobilities, as confirmed by numerical simulations. At low temperatures and for small magnetic fields, an isotropic negative magneto-resistance is observed and attributed to a topological property of the band structure far away from the Weyl nodes. This new mechanism, different from the chiral anomaly, allows us to reproduce the experimental results by numerical calculations based on the real band structure of WTe2. / In den vergangenen Jahren haben verschiedene Untersuchungen von Weyl Halb- metallen gezeigt, dass sich diese sehr gut als Spintronische Geräte eignen. In der Tat sind die Spin-chiralen Weyl Quasiteilchen perfekte Quellen und Abflüsse der Berrykrümmung, was auf Grund ihrer topologischen Natur neue Transporteigen- schaften hervorruft, wie beispielsweise die chirale Anomalie und einen großen, anomalen Hall Effekt. Außerdem haben Typ II Weyl Halbmetalle wie WTe2 eine spezifische Bandstruktur mit gekippten Weylkegeln und überlappenden Elektronen-/Lochbändern, die dazu führen können, dass die Ladungsträgerkompensation ideal wird und ein sehr starker Magnetowiderstand (XMR) entsteht. Dennoch befinden sich die Weylknoten in WTe2 ca. 50 meV über dem Ferminiveau, eine Beobachtung die sowohl den starken positiven Magnetowiderstand, als auch den negativen Mag- netowiderstand, der meist mit der chiralen Anomalie in Verbindung gebracht wird, in Frage stellt. In dieser Arbeit untersuchen wir die Magnetotransporteigenschaften von WTe2 Nanostrukturen, die durch verschiedene Wachstumsarten hergestellt werden (mech- anische Exfoliation, chemische Gasphasenabscheidung), um sowohl die reale Band- struktur, als auch Streuung an Störstellen in Betracht ziehen zu können. Es wird gezeigt, dass der extrem große Magnetowiderstand nicht direkt vom Grad der Un- ordnung abhängt und dass das typisch subquadratische Verhalten im Rahmen eines Multibandmodells, was über das Zweibandmodell hinaus geht, verstanden wer- den kann und sich auch mit numerischen Simulationen bestätigt lässt. Bei tiefen Temperaturen und für kleine Magnetfelder kann ein isotropisch negativer Magne- towiderstand beobachtet werden, der der topologischen Eigenschaft der Bandstruk- tur weit weg von den Weylknoten geschuldet ist. Dieser neue Mechanismus, der sich von der chiralen Anomalie unterscheidet, erlaubt es uns die experimentellen Ergebnisse mit numerischen Berechnungen, die auf der realen Bandstruktur basieren, zu reproduzieren. / Récemment, différentes études sur les semimétaux de Weyl ont montré leur large potentiel pour des applications en spintronique. En effet, les noeuds de Weyl avec leur chiralité de spin sont des sources ou puits parfaits de la courbure de Berry, ce qui peut conduire à de nouvelles propriétés de transport, dues à la nature topologique de la structure de bande, comme l’anomalie chirale et une large réponse liée à l’effet Hall anormal dit intrinsèque. De plus, les semimétaux de Weyl de type II, comme WTe2, ont une structure de bande particulière avec des cônes de Weyl inclinés et un chevauchement des bandes de trous et d’électrons qui résulte en une forte compensation de charge et une magnétorésistance extrêmement large (XMR) associée. Cependant, dans WTe2, les noeuds de Weyl se trouvent environ 50 meV au-dessus de l’énergie de Fermi, ce qui remet en cause la possibilité d’observer à la fois une XMR positive à fort champ et une magnétorésistance négative à champ faible due à l’anomalie chirale. Dans ce travail, nous étudions les propriétés de magnéto-transport de nanostructures WTe2 obtenues par différentes méthodes (exfoliation mécanique, transport en phase vapeur), avec des degrés de désordre microscopique différents, en considérant à la fois la structure de bande réelle du matériau et les processus de diffusion liés au désordre. Il est montré que la XMR présente un comportement subquadratique, qui peut être compris dans le cadre d’un modèle multi-bandes, au-delà de deux bandes, comme confirmé par des simulations numériques. A très basse température et faible champ magnétique, une magnétorésistance négative et isotrope est observée et attribuée à une propriété topologique de la structure de bandes loin des noeuds de Weyl. Ce nouveau mécanisme, différent de celui de l’anomalie chirale, nous permet de reproduire nos résultats expérimentaux par des simulations numériques basées sur la structure de bande réelle de WTe2.

In dieser Arbeit werden Messungen der elektrischen und thermischen Transportkoeffizienten an einem antiferromagnetisch ordnenden Iridat und FeAs-basierten Hochtemperatursupraleitern vorgestellt und analysiert. Iridate sind Materialien mit starker Spin-Bahn-Kopplung. In dem zweidimensionalen Vertreter Sr_2IrO_4 führt diese Kopplung zu isolierendem Mott-Verhalten mit gleichzeitiger antiferromagnetischer Ordnung der gekoppelten Spin-Bahn-Momente. Somit stellt Sr2IrO4 ein Modellsystem für die Untersuchung magnetischer Anregungen dieser Momente in Iridaten dar. Die Analyse der Wärmeleitfähigkeit von Sr_2IrO_4 liefert erstmals klare Hinweise auf magnetische Wärmeleitung in den Iridaten. Die extrahierte magnetische freie Weglänge gibt Aufschluss über die Streuprozesse der zum Wärmetransport beitragenden Magnonen und lässt Schlüsse über die Anregungen des gekoppelten Spin-Bahnsystems zu. Die FeAs-Hochtemperatursupraleiter haben aufgrund ihrer geschichteten Kristallstruktur einen hauptsächlich zweidimensionalen Ladungstransport. Die Phasendiagramme dieser Materialien setzen sich aus Ordnungsphänomenen zusammen, die Magnetismus, Supraleitung und eine Strukturverzerrung umfassen. Das Hauptaugenmerk richtet sich auf die Reaktion der Transportkoeffizienten mit den sich ausbildenden Phasen in Vertretern der 111- und 122-Familien unter chemischer Dotierung innerhalb und außerhalb der Schichtstruktur. Mithilfe von Widerstand und magnetischer Suszeptibilität lassen sich Phasendiagramme der verschiedenen Supraleiterfamilien konstruieren. In ausgewählten Fällen werden der Hall-Koeffizient und elektrothermische Transporteffekte genutzt, um das Phasendiagramm näher zu erforschen. Der Großteil der Untersuchungen zeigt omnipräsente elektrische Ordnungsphänomene, die als nematische Phase bezeichnet werden. Die Messdaten zeigen, dass die Wärmeleitfähigkeit und der Nernst-Koeffizient dominant von Fluktuationen, die der nematischen Phase vorausgehen, beeinflusst werden. Aus den Ergebnissen der Nernst-Daten an dotiertem BaFe_2As_2 werden Schlüsse über die der nematischen Phase zugrunde liegenden Mechanismen des korrelierten Elektronensystems gezogen.

Manganese-based Heusler compounds display intriguing fundamental physical properties, determined by the delicate balance of magnetic interactions that give rise to real and reciprocal-space topology, sparking the interest in their potential application in the spin-based technology of the future. In this thesis, a thorough study of thin films of two Mn-based Heusler compounds, the hexagonal MnPtGa and inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system, was performed. The observation of Néel-type skyrmions in single-crystalline MnPtGa motivated our interest in the growth and characterization of thin films of this compound. The films were deposited by magnetron sputtering on (0001)-Al2O3 single crystalline substrates, achieving the epitaxial growth of the Ni2In-type hexagonal crystal structure (P6_3/mmc space group, no. 194). Two thermally-induced magnetic transitions were identified in MnPtGa thin films: below the ordering temperature (T_C=273 K) the system becomes ferromagnetic, followed by a spin-reorientation transition at T_sr=160 K, adopting a spin-canted magnetic structure. Resorting to single-crystal neutron diffraction (SCND), we were able to resolve the magnetic ground state of our MnPtGa thin films. The Mn magnetic moments were found to tilt 20 degrees away from the c-axis, forming a commensurate magnetic structure with a ferromagnetic component along the crystallographic c-axis and a staggered antiferromagnetic one in the basal plane. This further demonstrated the applicability of a bulk technique, such as SCND, to the study of magnetic structures in thin films. Additionally, the perpendicular magnetic anisotropy (PMA) in the system was determined by magnetometry technique. Electrical magnetotransport measurements were performed in a thickness series of MnPtGa thin films. A non-monotonous anomalous Hall conductivity (AHC) was observed, whose intrinsic Berry-curvature origin was elucidated by means of first-principle calculations. We further observed by magnetic force microscopy technique the nucleation of irregular magnetic bubbles under the application of a magnetic field. We tentatively link their appearance to the onset of an additional electron scattering mechanism contributing to the transverse resistivity. In the second part of this thesis, the inverse tetragonal Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) system was investigated. The films were grown on MgO(100) single crystalline substrates, promoting the epitaxial growth of the tetragonal structure (I-4m2 space group, no. 119). We primarily focused on the impact of the systematic substitution of iridium on the structural, magnetic and electrical (magneto)transport properties of the system. A compression of the basal lattice parameters and elongation of the c-axis, accompanied by larger crystallographic disorder, was observed as the Ir content (x) increased, altering the Mn-Mn exchange interactions and therefore the magnetic properties of the compound. Mn2RhSn have two thermally-induced magnetic transitions: first, to a collinear ferrimagnetic state below the Curie temperature (T_C=280 K), followed by a spin-reorientation transition at T_sr=80 K to a noncollinear state, determined by two inequivalent Mn sublattices. A reduction of both T_C and T_sr was observed, as well as a tendency towards a hard-axis ferromagnet and therefore larger PMA as the Ir content of the films was increased. Additionally, a reduction of the saturation magnetization suggest a change of the magnitude of the spin canting upon Ir-substitution. The electrical magnetotransport properties of the Mn2Rh(1-x)Ir(x)Sn (0 < x < 0.4) thin films were acquired and analyzed in a wide temperature and magnetic field range. A strongly temperature and composition dependent non-monotonous AHC was found, suggesting two regimes in the electronic transport: (i) a nearly x-independent regime dominated by intrinsic Berry-curvature and (ii) a strongly x-dependent regime suggesting a more relevant role from extrinsic mechanisms contributing to the AHC. On the other hand, the Mn2Rh(0.95)Ir(0.05)Sn bulk system is known to host magnetic skyrmion and antiskyrmion phases. We indirectly assessed the impact of the systematic Ir-substitution on the (anti)skyrmionic phases through the analysis of the sign of the topological Hall effect in our thin films. A tendency towards the suppression of the low-T skyrmion phase stabilized by magnetic dipole-dipole interaction, and subsistence of the high-T antiskyrmion phase in Mn2Rh(1-x)Ir(x)Sn thin films was found as x > 0.2, which can be interpreted as a change of magnitude of the anisotropic DMI in this tetragonal D_2d system upon Ir-substitution. We have thus demonstrated that the magnetic and topological properties of the Mn2Rh(1-x)Ir(x)Sn system can be tailored upon chemical substitution, showing a strongly intertwined relation between composition, crystal and electronic structure, with the emergence of exotic magnetic phases, ultimately reflected in their electrical transport signatures.:Abstract iii Abbreviations iv Symbols vi Preface xii 1 Fundamentals 1 1.1 Noncollinear magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Magnetic interactions in solids . . . . . . . . . . . . . . . . . . . 2 1.1.1.1 Exchange interaction . . . . . . . . . . . . . . . . . . . 2 1.1.1.2 Dzyaloshinsky-Moriya interaction . . . . . . . . . . . . 3 1.1.1.3 Magnetic anisotropy . . . . . . . . . . . . . . . . . . . 4 1.1.1.4 Magnetic dipolar interaction . . . . . . . . . . . . . . . 5 1.1.2 Spin-reorientation transition . . . . . . . . . . . . . . . . . . . . 5 1.1.3 Magnetic skyrmions and antiskyrmions . . . . . . . . . . . . . . 6 1.1.3.1 Antiskyrmions in Heusler compounds . . . . . . . . . . 8 1.2 Magnetic Heusler compounds . . . . . . . . . . . . . . . . . . . . . . . 9 1.2.1 Cubic crystal structure . . . . . . . . . . . . . . . . . . . . . . . 10 1.2.2 Distorted crystal structures . . . . . . . . . . . . . . . . . . . . 10 1.2.2.1 Tetragonal Heusler compounds . . . . . . . . . . . . . 11 1.2.2.2 Hexagonal Heusler compounds . . . . . . . . . . . . . 11 1.3 Charge and spin transport in ferromagnets . . . . . . . . . . . . . . . . 13 1.3.1 The two-current model . . . . . . . . . . . . . . . . . . . . . . . 13 1.3.2 The Hall effect . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 1.3.2.1 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 15 1.3.2.2 Topological Hall effect . . . . . . . . . . . . . . . . . . 17 1.4 Neutron scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1 Thermal Neutrons . . . . . . . . . . . . . . . . . . . . . . . . . 18 1.4.1.1 Scattering cross sections . . . . . . . . . . . . . . . . . 19 1.4.1.2 The four-circle diffractometer . . . . . . . . . . . . . . 23 xv 1.4.2 Magnetic neutron scattering . . . . . . . . . . . . . . . . . . . . 24 2 Experimental Techniques 29 2.1 Magnetron sputtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.1.1 Thin films growth modes . . . . . . . . . . . . . . . . . . . . . . 32 2.1.2 Thin films microstructure . . . . . . . . . . . . . . . . . . . . . 33 2.2 X-ray characterization of thin films . . . . . . . . . . . . . . . . . . . . 34 2.2.1 Geometry of the X-ray diffractometer . . . . . . . . . . . . . . . 35 2.2.2 Radial θ-2θ scan . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.3 ϕ -scans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2.2.4 Rocking curves (ω-scans) . . . . . . . . . . . . . . . . . . . . . . 36 2.2.5 X-ray reflectivity (XRR) . . . . . . . . . . . . . . . . . . . . . . 37 2.3 Composition analysis: energy dispersive X-ray spectroscopy (EDS) . . . 38 2.4 Surface characterization: atomic and magnetic force microscopy . . . . 38 2.5 D10 thermal neutron diffractometer . . . . . . . . . . . . . . . . . . . . 39 2.6 SQUID-VSM magnetometry . . . . . . . . . . . . . . . . . . . . . . . . 40 2.7 Electrical (magneto-)transport measurements . . . . . . . . . . . . . . 41 3 Noncollinear magnetism in MnPtGa epitaxial thin films 43 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.2 MnPtGa thin films: growth and characterization . . . . . . . . . . . . . 45 3.2.1 Growth conditions . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2.2 Crystal structure . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.3 Magnetic properties of MnPtGa thin films . . . . . . . . . . . . . . . . 49 3.3.1 Thermal evolution of the magnetic structure . . . . . . . . . . . 49 3.3.2 Field dependent magnetization . . . . . . . . . . . . . . . . . . 50 3.3.3 Single-crystal neutron diffraction in MnPtGa thin films . . . . . 52 3.3.3.1 Ferromagnetic phase . . . . . . . . . . . . . . . . . . . 54 3.3.3.2 Noncollinear phase . . . . . . . . . . . . . . . . . . . . 55 3.4 Electronic band structure of h-MnPtGa . . . . . . . . . . . . . . . . . . 57 3.5 Electrical magnetotransport properties of MnPtGa thin films . . . . . . 59 3.5.1 Zero field longitudinal resistivity . . . . . . . . . . . . . . . . . . 60 3.5.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.5.3 Magnetic transitions under a magnetic field . . . . . . . . . . . 64 3.6 Intrinsic origin of the anomalous Hall effect . . . . . . . . . . . . . . . . 65 3.6.1 Scaling of the anomalous Hall conductivity vs. σxx . . . . . . . 68 3.7 Spin textures in MnPtGa thin films . . . . . . . . . . . . . . . . . . . . 73 3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 4 Tuning the magnetic and topological properties of Mn2Rh1−xIrxSn epitaxial thin films 83 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.2 Growth and characterization of Mn2Rh1−xIrxSn thin films . . . . . . . 86 4.2.1 Growth conditions and Ir substitution . . . . . . . . . . . . . . 86 4.2.2 Crystal structure of Mn2Rh1−xIrxSn . . . . . . . . . . . . . . . . 87 4.3 Tuning the magnetic properties of the Mn2Rh1−xIrxSn system . . . . . 91 xvi 4.3.1 Thermal magnetic transitions . . . . . . . . . . . . . . . . . . . 91 4.3.2 Increasing the magnetic anisotropy under Ir-substitution . . . . 92 4.4 Electrical (magneto-)transport properties of Mn2Rh1−xIrxSn thin films 94 4.4.1 Zero-field longitudinal resistivity and spin reorientation transition 94 4.4.2 Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . . 95 4.4.3 Hall effects: from ordinary to anomalous & topological . . . . . 96 4.4.3.1 Ordinary Hall effect . . . . . . . . . . . . . . . . . . . 97 4.4.3.2 Anomalous Hall effect . . . . . . . . . . . . . . . . . . 98 4.4.3.3 Competing mechanisms in the AHC of the Mn2Rh1−xIrxSn system . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.4.3.4 Scaling of the AHC with the magnetization . . . . . . 101 4.4.3.5 Topological Hall effect . . . . . . . . . . . . . . . . . . 102 4.5 Tuning the (Anti-)Skyrmion phases . . . . . . . . . . . . . . . . . . . . 106 4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 5 Conclusions & Outlook 111 List of Figures 117 List of Tables 120 List of Publications 124 Aknowledgements 124 Bibliography 127 Eigenständigkeitserklärung 147

Les nanotubes de carbone forment une structure quasi-unidimensionnelle de diamètre nanométrique, dont les propriétés mécaniques et électroniques, en particulier leur remarquable conductivité électrique, présentent un grand potentiel pour la conception de dispositifs électroniques. Les nanotubes fonctionnalisés, c’est-à-dire dont la paroi a été chimiquement modifiée, présentent aussi un intérêt majeur pour leur mise en œuvre facilitée et pour la formation d’une interface active entre le nanotube et l’environnement, cette dernière étant essentielle pour la conception de nanocapteurs chimiques et biologiques. La présente thèse porte sur l’étude des mécanismes gouvernant le transport électrique dans les nanotubes de carbone et leurs dérivés fonctionnalisés. Les travaux, de nature expérimentale, ont été réalisés sur des dispositifs électroniques constitués d’un nanotube individuel monoparoi ou biparoi, additionné de groupes fonctionnels au besoin. En première partie, on s’intéresse à l’effet de la dimensionnalité sur les mécanismes d’injection des porteurs de charge au niveau des contacts électriques avec le nanotube. En seconde partie, on étudie l’effet de la fonctionnalisation covalente sur les propriétés de transport électrique des nanotubes, et on montre notamment que l’impact de l’addition des greffons varie fortement selon leur valence et qu’il est possible d’obtenir des nanotubes fonctionnalisés avec une bonne conductance. En troisième partie, on explore les phénomènes de saturation du courant et de claquage électrique survenant à haut voltage. Enfin, on discute de l’impact des résultats obtenus sur l’avancement de la compréhension des mécanismes de transport électrique dans les systèmes hautement confinés, ainsi que des perspectives fondamentales et technologiques ouvertes par ces travaux. / Carbon nanotubes are highly promising for building electronic devices because of their quasi-unidimensional nanometer-sized geometry, and their mechanical and electronic properties, including their remarkable electrical conductance. Their functionalized derivatives, in which the nanotube sidewall is chemically modified, are also interesting for their better processability and for creating a chemically active interface between the nanotube and the environment, which is essential for applications such as nanosensors or biosensors. In this thesis, we study the mechanisms governing electrical transport in carbon nanotubes and their functional derivatives. Our experimental work was performed on electronic devices made of individual single-walled or double-walled carbon nanotubes, with or without functional adducts. In the first part, we focus on the effect of reduced dimensionality on the physics of charge injection at electrical contacts. In the second part, we study the effect of covalent functionalization on carbon nanotubes electrical transport properties. We show that the impact of chemical addition is strongly dependent on graft valence, and that it is possible to produce covalently functionalized carbon nanotube devices with excellent electrical conductance. In the third part, we explore current saturation and electrical breakdown phenomena occurring at high bias. Finally, the impact of our results on the global understanding of electrical transport in highly confined systems is discussed, along with fundamental and technological perspectives unveiled by our work.

This thesis deals with the studies of magnetization and electrical transport properties of iron nanoparticles embedded in the carbon matrix. The synthesis and characteristics of the nanoparticle systems studied, are also presented. Carbon-iron (C-Fe) based systems are of growing interest due to their improved magnetic properties as well as in their potential application as sensors, catalysts, and in various other applications. In particular, nanocomposites of iron carbide, such as the cementite phase Fe3C, are further suited to diverse technological exploitations due to their enhanced mechanical properties and importance in ferrous metallurgy. The recent interest in magnetic nanostructures lies in the emergence of novel magnetic and transport properties with the reduction of size. As the dimension approaches the nanometer length scale, interesting size-dependent properties like enhanced coercivity, enhanced magnetic moment, super paramagnetism etc. are seen. Thermal assisted chemical vapour deposition (CVD) is used to decompose and chemically react the introduced precursors, maleic anhydride and ferrocene. This method provides relative size control over the individual particles by varying C/Fe concentration in precursors and the pyrolysis temperature during the co-deposition process. Ferrocene has been used actively for the production of nanoparticle composites and in the production of nanostructured carbon. The temperature of preparation, reaction rate, and the time duration of annealing directly effects the nanoparticle compositions. The catalytic effect of transitional elements are well documented in literature. This thesis is an effort to understand the growth of ferromagnetic nanocrystallites in carbon matrix, which undergo partial graphitization due to the catalytic effect of transitional elements. The effect of transitional metal on the degree of graphitization of the carbon matrix, morphology of the nanoparticle and the carbon matrix are studied. The phase of the ferromagnetic iron nanoparticles and the structural investigation forms part of the study. Here X-Ray diffraction (XRD) is employed to study the presence of different phases of iron in the partially graphitized carbon matrix. The matrix morphology and the particle size distribution were studied using Transmission Electron Microscopy (TEM) and High-Resolution TEM (HRTEM). The ferromagnetic states of the iron nanoparticles are investigated using Mossbauer spectroscopy. The results from these studies, are used to correlated the macroscopic properties to the microscopic studies. The enhanced magnetization, coercivity and the temperature dependence of the magnetization value is understood within the frame work of ferromagnetic Bloch law and surrounding carbon spins. The logarithmic temperature dependence of conductivity of the nanoparticle composites is analyzed in the framework of interference models as well as the many-body Kondo interaction effect. This thesis contains seven chapters: In chapter 1, a brief introduction to mesoscopic physics and the size-dependent phenomenon are given. Special attention is paid to magnetic nanoparticle and its composites, and the various finite-size effects exhibited by them are discussed in detail. The relevance of carbon matrix and its importance on the growth of iron nanoparticles with high thermal stability is also discussed. The ballistic and diffusive transport phenomena observed in low-dimensional systems are briefly discussed. The interplay of localization and various interaction effects at nanoscale are examined. In disordered metals the low temperature conductivity is dominated by the interference effects. A brief discussion is made on the conductivity in disorder systems, with the presence of magnetic impurities and how the classic many-body Kondo problem, is effected by various interactions. Chapter 2, mainly deals with the experimental techniques employed in the thesis. The thermal-assisted chemical vapour deposition setup used to decompose and chemically react the introduced organometallic precursors, for the preparation of C:Fe composites are discussed and its advantage over other preparation methods are emphasized. The method is optimized to provide relative size control over the nanoparticles composites and the phase compositions by varying C/Fe concentration in precursors and the pyrolysis temperature, during the co-deposition process. The various structural characterization tools used in the present study are summed up concisely in this chapter. The SQUID magnetometer system; its working principle and the various protocol used for the low temperature magnetization measurements are elaborated. Further, details regarding superconducting magnetic cryostat, utilized for the low temperature conductivity and magneto resistance measurements, are discussed. Films of C:Fe composites are grown on substrates to study the effect of disorder and sample size on the conductivity behaviour of the composites at low temperature. Chapter 3, presents the outcome of the structural studies undertaken on the C:Fe composites using XRD, TEM, and HRTEM. X-ray diffraction measurements performed on the powder composites reveal that, in addition to the presence of sharp diffraction peak from nanographite, peaks corresponding to the different phases of Fe are also seen. The effect of preparation temperature on the matrix morphology is revealed from the estimation of degree of graphitization. Iron carbide is the predominant phase in all the prepared composites. For low concentration of iron, iron carbide alone is present but as the percentage of iron in the samples increased other phases of iron are also seen. The microscopic studies on the prepared compositions revealed the presence of nanosized iron particles well embedded in the partially graphitized matrix. Here again, with the increase in iron percentage, agglomeration of ferromagnetic nanoparticles are seen. The kinetics of the particle growth and the filamentous nature of the carbon matrix are also discussed. Mossbauer investigation on C:Fe composites are presented in chapter 4. The measurements revealed the iron atom occupation in the crystal lattice. In the lower Fe concentration samples, the room temperature Mossbauer spectrum revealed the presence of sextet from Fe3C (cementite) phase. As the percentage of iron increased, sextet from α-Fe, Fe3O4 are also seen in some of the prepared compositions. Effect of carbon atoms on the structure and magnetic properties of the nanoparticle species are obvious from the isomer shift measurements. Chapter 5 comprises of the various magnetic properties and interactions present in small particle system such as magnetic anisotropy, coercivity, enhanced magnetization, inter-and intra-particle interactions etc. Magnetization measurements carried out in SQUID magnetometer on the C:Fe composites and carbon flakes (prepared from organic precursor, maleic anhydride alone) are presented. The enhanced magnetic properties of the nanoparticle assembly is discussed in detail. The hysteresis loops trace, with a finite coercivity at room temperature, indicates the ferromagnetic nature of the samples. At room temperature the magnetization value saturates at high magnetic field, indicating negligible effect from super paramagnetic particles on the hysteresis loop. The squareness ratio, saturation magnetization, coercivity and remanence magnetization values are analyzed in detail. The temperature dependence of magnetization shows a combination of Bloch law and Curie-Weiss behaviour, consistent with the picture of ferromagnetic clusters embedded in a carbon matrix. The Bloch’s constant is found to be larger by an order of magnitude compared to the bulk value, implying stronger dependence of magnetization with temperature. Effort to understand the enhanced magnetic moment in the light of magnetism in carbon was taken up. The proximity effect of ferromagnetic metal on the carbon and the hydrogen bonding with the dangling bonds, both studied in detail in literature, in connection with the induced magnetic moments in carbon, are invoked. In chapter 6, the different conductivity regimes are identified, to study the conduction mechanisms in composites and films. For the transport measurements pelletized samples are used for the resistivity and magneto resistance measurements. The conductivity data are analyzed based on the interplay of localization and Kondo effect in the ferromagnetic disordered system. In order to understand the effect of disorder and thickness on the Kondo problem, transport measurements are carried on thin films of C:Fe composites grown on quartz and alumina substrate. Disorder induced metal-insulator transition is observed in the prepared samples. The zero-field conductivity and magneto resistance data is fitted to variable range hopping (VRH) in strong localization regime. Chapter 7 summarizes the thesis and presents some perspectives for the future.

Les nanotubes de carbone forment une structure quasi-unidimensionnelle de diamètre nanométrique, dont les propriétés mécaniques et électroniques, en particulier leur remarquable conductivité électrique, présentent un grand potentiel pour la conception de dispositifs électroniques. Les nanotubes fonctionnalisés, c’est-à-dire dont la paroi a été chimiquement modifiée, présentent aussi un intérêt majeur pour leur mise en œuvre facilitée et pour la formation d’une interface active entre le nanotube et l’environnement, cette dernière étant essentielle pour la conception de nanocapteurs chimiques et biologiques. La présente thèse porte sur l’étude des mécanismes gouvernant le transport électrique dans les nanotubes de carbone et leurs dérivés fonctionnalisés. Les travaux, de nature expérimentale, ont été réalisés sur des dispositifs électroniques constitués d’un nanotube individuel monoparoi ou biparoi, additionné de groupes fonctionnels au besoin. En première partie, on s’intéresse à l’effet de la dimensionnalité sur les mécanismes d’injection des porteurs de charge au niveau des contacts électriques avec le nanotube. En seconde partie, on étudie l’effet de la fonctionnalisation covalente sur les propriétés de transport électrique des nanotubes, et on montre notamment que l’impact de l’addition des greffons varie fortement selon leur valence et qu’il est possible d’obtenir des nanotubes fonctionnalisés avec une bonne conductance. En troisième partie, on explore les phénomènes de saturation du courant et de claquage électrique survenant à haut voltage. Enfin, on discute de l’impact des résultats obtenus sur l’avancement de la compréhension des mécanismes de transport électrique dans les systèmes hautement confinés, ainsi que des perspectives fondamentales et technologiques ouvertes par ces travaux. / Carbon nanotubes are highly promising for building electronic devices because of their quasi-unidimensional nanometer-sized geometry, and their mechanical and electronic properties, including their remarkable electrical conductance. Their functionalized derivatives, in which the nanotube sidewall is chemically modified, are also interesting for their better processability and for creating a chemically active interface between the nanotube and the environment, which is essential for applications such as nanosensors or biosensors. In this thesis, we study the mechanisms governing electrical transport in carbon nanotubes and their functional derivatives. Our experimental work was performed on electronic devices made of individual single-walled or double-walled carbon nanotubes, with or without functional adducts. In the first part, we focus on the effect of reduced dimensionality on the physics of charge injection at electrical contacts. In the second part, we study the effect of covalent functionalization on carbon nanotubes electrical transport properties. We show that the impact of chemical addition is strongly dependent on graft valence, and that it is possible to produce covalently functionalized carbon nanotube devices with excellent electrical conductance. In the third part, we explore current saturation and electrical breakdown phenomena occurring at high bias. Finally, the impact of our results on the global understanding of electrical transport in highly confined systems is discussed, along with fundamental and technological perspectives unveiled by our work.

Les propriétés électriques et physiques de structures à faible dimensionalité ont été étudiées pour des applications dans des domaines divers comme l’électronique, les capteurs. La mesure du bruit bruit à basse fréquence est un outil très utile pour obtenir des informations relatives à la dynamique des porteurs, au piègeage des charges ou aux mécanismes de collision. Dans cette thèse, le transport électronique et le bruit basse fréquence mesurés dans des structures à faible dimensionnalité comme les dispositifs multi-grilles (FinFET, JLT…), les nanofils 3D en Si/SiGe, les nanotubes de carbone ou à base de graphène sont présentés. Pour les approches « top-down » et « bottom-up », l’impact du bruit est analysé en fonction de la dimensionalité, du type de conduction (volume vs surface), de la contrainte mécanique et de la présence de jonction metal-semiconducteur. / Electrical and physical properties of low-dimensional structures have been studied for the various applications such as electronics, sensors, and etc. Low-frequency noise measurement is also a useful technique to give more information for the carrier dynamics correlated to the oxide traps, channel defects, and scattering. In this thesis, the electrical transport and low-frequency noise of low-dimensional structure devices such as multi-gate structures (e.g. FinFETs and Junctionless FETs), 3-D stacked Si/SiGe nanowire FETs, carbon nanotubes, and graphene are presented. From the view point of top-down and bottom-up approaches, the impacts of LF noise are investigated according to the dimensionality, conduction mechanism (surface or volume conduction), strain technique, and metal-semiconductor junctions.

No description available.

Au cours de ce travail de thèse à caractère expérimental, nous nous sommes attachés à mesurer les spectres absolus de section efficace d’absorption de nanotubes de carbone individuels placés dans différents environnements. Pour ce faire, nous avons développé un dispositif expérimental basé sur la technique de spectroscopie à modulation spatiale qui permet d’accéder de manière directe à la section efficace d’absorption de nano-objets individuels. Cette méthode ne requière aucun a priori sur les propriétés des nanotubes, et très important nous affranchit des effets d’ensemble. Ainsi, nous avons pu étudier les propriétés d’absorption de nanotubes individuels simple et double parois dans les environnements suivants : suspendus librement, agrégés en petit fagot, et déposés sur substrat. Plus précisément, l’évolution de l’absorption excitonique des nanotubes est analysée en fonction des paramètres structuraux (diamètre, nombre de parois, chiralité) et de l’environnement de ces derniers. Un autre aspect de cette thèse a pour objet l’analyse des propriétés de transport électronique des nanotubes de carbone soumis à des pressions hydrostatiques de gaz de l’ordre du gigapascal, avec la possibilité d’accéder au régime des basses températures. Ici, nous nous sommes concentrés sur l’étude de transistors à effet de champ composés de petits fagots de nanotubes de carbone contactés à leurs extrémités par des électrodes en palladium. Dans ce cadre, nous avons notamment réalisé la première observation de l’effet de blocage de Coulomb sous pression. Au final, ce travail de thèse a permis d’analyser les propriétés optiques et électroniques intrinsèques aux nanotubes de carbone et leur évolution sous l’effet de différents environnements (écrantage diélectrique, dopage chimique, contrainte mécanique et pression hydrostatique). Ce travail a pu être réalisé grâce au développement de nouvelles techniques permettant de sonder ces propriétés au niveau du nanotube individuel / In this dissertation, we report on the experimental investigation of the optical properties of single- and double-wall carbon nanotubes. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. In particular direct quantitative measurement of their absorption cross-section has not been achieved yet. Using spatial modulation spectroscopy we have determined, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of identified individual single- and double-wall carbon nanotubes. These quantitative measurements permit the determination of the oscillator strength of the different excitonic resonances. Furthermore, investigation of the same nanotube, either a single-wall or double-wall nanotube, freestanding or deposited on a substrate shows large broadening with increase of oscillator strength of the excitonic resonances, as well as stark weakening of polarization dependent antenna effects, due to nanotube-substrate interaction. Similar study on nanotube bundles and double-wall nanotubes demonstrate the importance of inter-tube and inter-wall exciton coupling effects which seem to be of different nature in these two types of sample. The second part of this thesis studies electrical transport in carbon nanotube bundles under high pressure condition and low temperature. The behavior of nanotubebased field-effect transistors has been investigated, in the classical and Coulomb blockade regime, under gas-pressure up to 0.9 GPa. Overall, this dissertation communicates on the quantitative analysis of the absorption and electronic properties of carbon nanotubes and how they are influenced by various environmental effects such as dielectric screening, stress induced strain, hydrostatic pressure, or chemical doping. The novelty of this work is to address these issues at the single nanotube level

(ULSI) causes an increase of the resistance of the wiring system by increased scattering of electrons at side walls and grain boundaries in the state of the art Cu technology, which increases the RC delay of the interconnect system and thus degrades the performance of the device. The outstanding properties of carbon nanotubes (CNT) such as a large mean free path, a high thermal conductance and a large resistance against electromigration make them an ideal candidate to replace Cu in future feature nodes. The present thesis contributes to the preparation and properties of CNT based vertical interconnections (vias). In addition, all processes applied during the fabrication are compatible to ULSI and an interface between CNT based vias and a Cu metallization is studied. The methodology for the evaluation of CNT based vias is improved; it is highlighted that by measuring the resistance of one multiwall CNT and taking into account the CNT density, the performance of the CNT based vias can be predicted accurately. This provides the means for a systematic evaluation of different integration procedures and materials. The lowest contact resistance is obtained for carbide forming metals, as long as oxidation during the integration is avoided. Even though metal-nitrides exhibit an enhanced contact resistance, they are recommended to be used at the bottom metallization in order to minimize the oxidation of the metal-CNT contact during subsequent processing steps. Overall a ranking for the materials from the lowest to the highest contact resistance is obtained: Ta < Ti < TaN < TiN « TiO2 « Ta2O5 Furthermore the impact of post CNT growth procedures as chemical mechanical planarization, HF treatment and annealing procedures after the CNT based via fabrication are evaluated. The conductance of the incorporated CNTs and the applicable electrical transport regime relative to the CNT quality and the CNT length is discussed. In addition, a strong correlation between the temperature coefficient of resistance and the initial resistance of the CNT based vias at room temperature has been observed. / Die kontinuierliche Miniaturisierung der charakteristischen Abmessungen in hochintegrierten Schaltungen (ULSI) verursacht einen Anstieg des Widerstandes im Zuleitungssystem aufgrund der erhöhten Streuung von Elektronen an Seitenwänden und Korngrenzen in der Cu-Technologie, wodurch die Verzögerungszeit des Zuleitungssystems ansteigt. Die herausragenden Eigenschaften von Kohlenstoffnanoröhren (CNT), wie eine große mittlere freie Weglänge, hohe thermische Leitfähigkeit und eine starke Resistenz gegenüber Elektromigration machen diese zu einem idealen Kandidaten, um Cu in zukünftigen Technologiegenerationen zu ersetzen. Die vorliegende Arbeit beschreibt die Herstellung und daraus resultierenden Eigenschaften von Zwischenebenenkontakten (Vias) basierend auf CNTs. Alle verwendeten Prozessierungsschritte sind kompatibel mit der Herstellung von hochintegrierten Schaltkreisen und eine Schnittstelle zwischen den CNT Vias und einer Cu-Metallisierung ist vorhanden. Insbesondere das Verfahren zur Evaluierung von CNT Vias wurde durch den Einsatz verschiedener Methoden verbessert. Insbesondere soll hervorgehoben werden, dass durch die Messung des Widerstandes eines einzelnen CNTs, bei bekannter CNT Dichte, der Via Widerstand sehr genau vorausgesagt werden kann. Dies ermöglicht eine systematische Untersuchung des Einflusses der verschiedenen Prozessschritte und der darin verwendeten Materialien auf den Via Widerstand. Der niedrigste Kontaktwiderstand wird für Karbidformierende Metalle erreicht, solange Oxidationsprozesse ausgeschlossen werden können. Obwohl Metallnitride einen höheren Kontaktwiderstand aufweisen, sind diese für die Unterseitenmetallisierung zu empfehlen, da dadurch die Oxidation der leitfähigen Schicht minimiert wird. Insgesamt kann eine Reihenfolge beginnend mit dem niedrigsten zum höchsten Kontaktwiderstand aufgestellt werden: Ta < Ti < TaN < TiN « TiO2 « Ta2O5 Desweiteren wurde der Einfluss von Verfahren nach dem CNTWachstum wie die chemischmechanische Planarisierung, eine HF Behandlung und einer Temperaturbehandlung evaluiert, sowie deren Einfluss auf die elektrischen Parameter des Vias untersucht. Die Leitfähigkeit der integrierten CNTs und die daraus resultierenden elektrischen Transporteigenschaften in Abhängigkeit der CNT Qualität und Länge werden besprochen. Ebenso wird die starke Korrelation zwischen dem Temperaturkoeffizienten des elektrischen Widerstandes und des Ausgangswiderstandes der CNT basierten Vias bei Raumtemperatur diskutiert.