Ce mémoire présente une étude de la synthèse par la méthode Vapeur-Liquide-Solide (VLS) de nanofils de silicium et de germanium par Epitaxie par Jets Moléculaires ainsi que de l'effet de la présence de manganèse sur leur croissance. La croissance des nanofils est fortement modifiée par la présence de manganèse. Les nanofils de silicium élaborés sous un faible flux de manganèse présentent des propriétés morphologiques et structurales remarquables. La présence de manganèse modifie le diamètre d'équilibre des gouttes AuSi utilisées pour la croissance par voie VLS et permet l'élaboration de nanofils de silicium de longueurs élevées et de faibles diamètres. De plus, leur qualité cristalline est considérablement améliorée par rapport aux nanofils de silicium formés sans apport de manganèse. Dans ce mémoire nous proposons quelques explications à ce phénomène. Dans le cas des nanofils de germanium, l'incorporation de manganèse n'a pu être obtenue par codépôt. Aussi, (i) le dopage par implantation ionique de nanofils de germanium et (ii) la fonctionnalisation de nanofils de germanium par la formation d'hétérostructures type cœur/coquille Ge/GeMn ont été considérés : - les mesures d'aimantation effectuées sur des nanofils de germanium implantés au manganèse démontrent l'existence de propriétés ferromagnétiques avec des températures de Curie supérieures à 400K. Il s'agit d'un résultat très prometteur en vue d'applications utilisant des nanofils de germanium ferromagnétiques à température ambiante ; - pour accéder aux propriétés magnétiques des nanofils de germanium fonctionnalisés par dépôt de GeMn, nous avons mis au point une procédure de prises de contacts adaptée à la mesure de leurs propriétés de magnétotransport. Les caractéristiques électriques de ces dispositifs montrent que les propriétés de transport sont dominées par la présence de la couche coquille de GeMn, surtout à basse température. Des mesures de magnétotransport effectuées à 100K indiquent l'existence d'effets de magnétorésistance liés aux propriétés ferromagnétiques des nanofils de Ge ainsi fonctionnalisés. / This thesis presents a study of the Vapour-Liquid-Solid (VLS) synthesis of silicon and germanium nanowires by Molecular Beam Epitaxy and the effect of the presence of manganese on the growth properties. The presence of manganese strongly modifies the growth of nanowires and observed behaviours are very different for AuSi and AuGe systems. Silicon nanowires grown in the presence of manganese exhibit very interesting morphological and structural properties. The presence of manganese modifies AuSi droplets' diameter and allows manufacturing long nanowires with relatively small diameters. Moreover, the crystalline quality is dramatically improved as compared to that of silicon nanowires grown without manganese. In this manuscript we propose some explanation for the growth phenomena. In the case of germanium nanowires, manganese incorporation could not be obtained by concomitant deposition of germanium and manganese. Consequently, (i) the doping of germanium nanowires by ion implantation as well as (ii) germanium nanowires functionalization by core/shell Ge/GeMn heterostructures formation were considered: - magnetization measurements performed on implanted germanium nanowires demonstrate ferromagnetic properties with Curie temperatures above 400K. This result is very promising for the processing of devices using room-temperature ferromagnetic germanium nanowires ; - in order to access Ge/GeMn nanowires magnetic properties, we processed samples to probe nanowires magnetotransport properties. Electrical resistivities of devices show that transport properties are dominated by GeMn shell layer even more at low temperature. Magnetotransport measurements done at 100K indicate magnetoresistance effects linked with nanowires ferromagnetic properties.

This thesis deals with the investigations on the nonlinear electrical and manganetotransport properties of polycrystalline multi-phase ceramic composites of Zno/pervoskite manganite. Multifunctional properties are studied such as the enhanced low-field magnetoresistance(LFMR). magnetically tuneable low-voltage nonlinear current-voltage (I-V) characteristics with larger nonlinearity coefficients suitable for semiconducting and magnetoelectric devices. A brief introduction on the structure-property correlations, electronic and magnetic structures, nonlinear electrical conduction, phase separation, grain size and grain boundary effects on transport properties of manganites are presented. The nonlinear current-voltage characteristics of ZnO based varistors are also summarized. The thesis describes the synthesis of the ceramics and the methodology of different techniques utilized in characterizing the samples. The phase conversions in calcium manganite with changing Ca/Mn ratios as well as the oxygen non-stoichiometry and their influence on electrical transport properties were studied. The realization of low-voltage varistors prepared from ZnO+ CaMnO3 ceramic composites was described. An energy band model consisting of n-p-n heterojunctions of n-ZnO1-γ:Mn/p-CMZO/n-ZnO1 γ:Mn has been proposed in order to explain the large nonlinearity coefficients obtained at low field-strengths of 1.8 to 12 V/mm. The detailed investigationos on the structural identification and physico-chemical analyses of Ca4Mn7Zn3O21-δ(CMZO) phases having the beta-alumina or magnetoplumbite-type structures were carried out. The thesis also embodies the magnetically tuneable nonlinear I-V characteristics and the magnetotransport properties of ZnO/La(Sr)MnO3 and ZnO/La(Ca,Sr)MnO3 ceramic composites. The present investigations demonstrate that the ferromagnetic insulating (FMI) La06 Sr04Mn1-yZnyO3(y = 3 to 8 at.%) when present as minor phase in ZnO1- γ:Mn ceramics enables in attaining magnetically tunealbe nonlinear I-V characteristics. Wherein, the dominant ZnO1- γ:Mn phase remains paramagnetic. The results also indicate that the prevalence of ferromagnetism in ZnO1-γ:Mn is not significant for realizing magnetically tuneable I-V curves. The controversial results related to the existence of ferromagnetism in ZnO(doped)leading to diluted magnetic semiconductors(DMS) have been investigated. Another novel aspect of the present work is the low-field magnetoresistive(LFMR) property of ZnO/La(Sr)MnO3 and ZnO/La(Ca.Sr)MnO3 ceramic composites which been explained on the basis of spin-polarised tunneling across the intergrain regions. The influence of Zn2+ as a diamagnetic substitutent in modifying the crystallographic phase content, electrical transport and magnetic properties of Lao6Sro4MnO3 were studied in detail. The results point towards the fact the large decrease of Tc and Ms at lower Zn contents(≤ 8 at.%)is due to the dominant role played by the excess oxygen vacancy (Vo) as an electron donor in p-type Lao6Sro4Mn1-yZnyO3-δ rather than the charge compensatively predictable values. The modifications of electronic and magnetotransport properties were carried out on Lao6Sro4MnO3 substituted with diamagnetic ions such as Mg2+ - Al3+ - Ti4+ - Nb5+ - Mo6+ or W6+ at Mn-sublattice. The TEM studies including HREM results point to the fact the large ΔT(= Tc-TM-1)is accountable in terms of charge conduction within the electronically heterogeneous phase mixtures of charge ordered insulating (CO1) bi-stripes prevailing within the charge disordered FMI phases.

Manipulation of ferromagnetic (FM) spins (and spin textures) using an antiferromagnet (AFM) as an active element in exchange coupled AFM/FM heterostructures is a promising branch of spintronics. Recent ground-breaking experimental demonstrations, such as electrical manipulation of the interfacial exchange coupling and FM spins, as well as ultrafast control of the interfacial exchange-coupling torque in AFM/FM heterostructures, have paved the way towards ultrafast spintronic devices for data storage and neuromorphic computing device applications.[5,6] To achieve electrical manipulation of FM spins, AFMs offer an efficient alternative to passive heavy metal electrodes (e.g., Pt, Pd, W, and Ta) for converting charge current to pure spin current. However, AFM thin films are often integrated into complex heterostructured thin film architectures resulting in chemical, structural, and magnetic disorder. The structural and magnetic disorder in AFM/FM-based spintronic devices can lead to highly undesirable properties, namely thermal dependence of the AFM anisotropy energy barrier, fluctuations in the magnetoresistance, non-linear operation, interfacial spin memory loss, extrinsic contributions to the effective magnetic damping in the adjacent FM, decrease in the effective spin Hall angle, atypical magnetotransport phenomena and distorted interfacial spin structure. Therefore, controlling the magnetic order down to the nanoscale in exchange coupled AFM/FM-based heterostructures is of fundamental importance. However, the impact of fractional variation in the magnetic order at the nanoscale on the magnetization reversal, magnetization dynamics, interfacial spin transport, and the interfacial domain structure of AFM/FM-based heterostructures remains a critical barrier. To address the aforementioned challenges, we conduct a comprehensive experimental investigation of chemical, structural, magnetization reversal (integral and element-specific), magnetization dynamics, and magnetotransport properties, combined with high-resolution magnetic imaging of the exchange coupled Ni3Fe/IrMn3-based heterostructures. Initially, we study the chemical, structural, electrical, and magnetic properties of epitaxially textured MgO(001)/IrMn3(0-35 nm)/Ni3Fe(15 nm)/Al2O3(2.0 nm) heterostructures. We reveal the impact of magnetic field annealing on the interdiffusion at the IrMn3/Ni3Fe interface, electrical resistivity, and magnetic properties of the heterostructures. We further present an AFM IrMn3 film thickness dependence of the exchange bias field, coercive field, magnetization reversal, and magnetization dynamics of the exchange coupled heterostructures. These experiments reveal a strong correlation between the chemical, structural and magnetic properties of the IrMn3-based heterostructures. We find a significant decrease in the spin-mixing conductance of the chemically-disordered IrMn3/Ni3Fe interface compared to the chemically-ordered counterpart. Independent of the AFM film thickness, we unveil that thermally disordered AFM grains exist in all the samples (measured up to 35-nm-thick IrMn3 films). We develop an iterative magnetic field cooling procedure to systematically manipulate the orientation of the thermally disordered and reversible AFM moments and thus, achieve tunable magnetic, and magnetotransport properties of exchange coupled AFM-based heterostructures. Subsequently, we investigate the impact of fractional variation in the AFM order on the magnetization reversal and magnetotransport properties of the epitaxially textured ɣ-phase IrMn3/Ni3Fe, Ni3Fe/IrMn3/Ni3Fe, and Ni3Fe/IrMn3/Ni3Fe/CoO heterostructures. We probe the element-specific (FM: Ni and Co, and AFM: Mn) magnetization reversal properties of the exchange coupled Ni3Fe/IrMn3/Ni3Fe/Co/CoO heterostructures in various magnetic field cooled states. We present a detailed procedure for separating the spin and orbital moment contributions for magnetic elements using the XMCD sum rule. We address whether Mauri-type domain walls can develop at the (polycrystalline) exchange coupled Ni3Fe/IrMn3/Ni3Fe interfaces. We further study the impact of magnetic field cooling on the AFM Mn (near L2,3-edges) X-ray absorption spectra. Finally, we employ a combination of in-field high-resolution magnetic force microscopy, magnetooptical Kerr effect magnetometry with micro-focused beam, and micromagnetic simulations to study the magnetic vortex structures in exchange coupled FM/AFM and AFM/FM/AFM disk structures. We examine the magnetic vortex annihilation mechanism mediated by the emergence and subsequent annihilation of the vortex-antivortex (V-AV) pairs in simple FM and exchange coupled FM/AFM as well as AFM/FM/AFM disk structures. We image the distorted magnetic vortex structures in exchange coupled FM/AFM disks proposed by Gilbert and coworkers. We further emphasize crucial magnetic vortex properties, such as handedness, effective vortex core radius, core displacement at remanence, nucleation field, annihilation field, and exchange bias field. Our experimental inquiry offers profound insight into the interfacial exchange interaction, magnetization reversal, magnetization dynamics, and interfacial spin transport of the AFM/FM-based heterostructures. Moreover, our results pave the way towards nanoscale control of the magnetic properties in AFM-based heterostructures and point towards future opportunities in the field of AFM spintronic devices.:1. Introduction 2. Magnetic Interactions and Exchange Bias Effect 3. Materials 4. Experimental Methods 5. Structural, Electrical, and Magnetization Reversal Properties of Epitaxially Textured ɣ-IrMn3/ Ni3Fe Heterostructures 6. Magnetization Dynamics of MgO(001)/IrMn3/Ni3Fe Heterostructures in the Frequency Domain 7. Tunable Magnetic and Magnetotransport Properties of MgO(001)/Ni3Fe/IrMn3/Ni3Fe/ CoO/Pt Heterostructures 8. Element-Specific XMCD Study of the Exchange Couple Ni3Fe/IrMn3/Ni3Fe/Co/CoO Heterostructures 9. Distorted Vortex Structure and Magnetic Vortex Reversal Processes in Exchange Coupled Ni3Fe/IrMn3 Disk Structures 10. Conclusions and Outlook Addendum Acronyms Symbols Publication List Author Information Acknowledgments Statement of Authorship

In dieser Arbeit werden das Wachstum mittels Molekularstrahlepitaxie und die Eigenschaften der Ferromagneten Fe und Fe_3Si auf niedrig-symmetirschen GaAs(113)A-Substraten studiert. Drei wichtige Aspekte werden untersucht: (i) Wachstum und strukturelle Charakterisierung, (ii) magnetische Eigenschaften und (iii) Magnetotransporteigenschaften der Fe und Fe_3Si Schichten auf GaAs(113)A-Substraten. Das Wachstum der Fe- und Fe_3Si-Schichten wurde bei einer Wachstumstemperatur von = bzw. 250 °C optimiert. Bei diesen Wachstumstemperaturen zeigen die Schichten eine hohe Kristallperfektion und glatte Grenz- und Oberflächen analog zu [001]-orientierten Schichten. Weiterhin wurde die Stabilität der Fe_(3+x)Si_(1-x) Phase über einen weiten Kompositionsbereich innerhalb der Fe_3Si-Stoichiometry demonstriert. Die Abhängigkeit der magnetischen Anisotropie innerhalb der Schichtebene von der Schichtdicke weist zwei Bereiche auf: einen Beresich mit dominanter uniaxialer Anisotropie für Fe-Schichten = 70 MLs. Weiterhin wird eine magnetische Anisotropie senkrecht zur Schichtebene in sehr dünnen Schichten gefunden. Der Grenzflächenbeitrag sowohl der uniaxialen als auch der senkrechten Anisotropiekonstanten, die aus der Dickenabhängigkeit bestimmt wurden, sind unabhängig von der [113]-Orientierung und eine inhärente Eigenschaft der Fe/GaAs-Grenzfläche. Die anisotrope Bindungskonfiguration zwischen den Fe und den As- oder Ga-Atomen an der Grenzfläche wird als Ursache für die uniaxiale magnetische Anisotropie betrachtet. Die magnetische Anisotropie der Fe_3Si-Schichten auf GaAs(113)A-Substraten zeigt ein komplexe Abhängigkeit von der Wachstumsbedingungen und der Komposition der Schichten. In den Magnetotransportuntersuchungen tritt sowohl in Fe(113)- als auch in Fe_3Si(113)-Schichten eine antisymmetrische Komponente (ASC) im planaren Hall-Effekt (PHE) auf. Ein phänomenologisches Modell, dass auf der Kristallsymmetrie basiert, liefert ein gute Beschreibung sowohl der ASC im PHE als auch des symmetrischen, anisotropen Magnetowiderstandes. Das Modell zeigt, dass die beobachtete ASC als Hall-Effekt zweiter Ordnung beschreiben werden kann. / In this work, the molecular-beam epitaxial growth and properties of ferromagnets, namely Fe and Fe_3Si are studied on low-symmetric GaAs(113)A substrates. Three important aspects are investigated: (i) growth and structural characterization, (ii) magnetic properties, and (iii) magnetotransport properties of Fe and Fe_3Si films on GaAs(113)A substrates. The growth of Fe and Fe_3Si films is optimized at growth temperatures of 0 and 250 degree Celsius, respectively, where the layers exhibit high crystal quality and a smooth interface/surface similar to the [001]-oriented films. The stability of Fe_(3+x)Si_(1-x) phase over a range of composition around the Fe_3Si stoichiometry is also demonstrated. The evolution of the in-plane magnetic anisotropy with film thickness exhibits two regions: a uniaxial magnetic anisotropy (UMA) for Fe film thicknesses = 70 MLs. The existence of an out-of-plane perpendicular magnetic anisotropy is also detected in ultrathin Fe films. The interfacial contribution of both the uniaxial and the perpendicular anisotropy constants, derived from the thickness-dependent study, are found to be independent of the [113] orientation and are hence an inherent property of the Fe/GaAs interface. The origin of the UMA is attributed to anisotropic bonding between Fe and As or Ga at the interface, similarly to Fe/GaAs(001). The magnetic anisotropy in Fe_3Si on GaAs(113)A exhibits a complex dependence on the growth conditions and composition. Magnetotransport measurements of both Fe(113) and Fe_3Si(113) films shows the striking appearance of an antisymmetric component (ASC) in the planar Hall effect (PHE). A phenomenological model based on the symmetry of the crystal provides a good explanation to both the ASC in the PHE as well as the symmetric anisotropic magnetoresistance. The model shows that the observed ASC component can be ascribed to a second-order Hall effect.