L'objectif de cette thèse est de mettre en œuvre une description multi-échelle adaptée aux écoulements de fluides dans des micro-/nano-conduites. Cette approche doit permettre de décrire, aussi bien les petites échelles relatives aux interactions du fluide avec les atomes du mur, que les grandes échelles de l’écoulement engendrées par les conditions aux limites d'entrée/sortie du canal. Pour cela, nous avons développé une méthode qui couple une modélisation continue des écoulements et des transferts de chaleur dans le cœur du canal avec une modélisation discrète proche des parois, basée sur une représentation atomistique du fluide et du mur.Les équations de Navier-Stokes et de l’énergie, couplées à une équation d’état, sont approximées par une méthode de volumes finis dans le cœur de l’écoulement alors que des simulations de dynamique moléculaire sont utilisées pour représenter finement les interactions entre le fluide et la paroi. Cette approche hybride nécessite la transmission d’informations entre les modélisations : les grandeurs moyennées moléculaires sont imposées comme conditions aux limites pour le modèle continu, et la dynamique sous contrainte, couplée à un thermostat de Langevin, est utilisée pour piloter l’échelle moléculaire. Une représentation par des plots moléculaires locaux de petite taille, intelligemment répartis le long de l’interface entre le fluide et le mur, permet de traiter des écoulements et des transferts dans des canaux de très grands allongements, pour des coûts de calcul raisonnables.Après une partie de validation, des simulations hybrides multi-échelles d’écoulements dans des canaux constitués de parois en platine ont été menées pour de l’argon en phase liquide (incompressible) ou gazeuse (compressible), en tenant compte éventuellement du changement de phase au voisinage de la paroi / The main objective of this thesis is to model the multi-scale heat and fluid flows in micro-/nano channels. This method must be able of capturing at the same time the fluid/solid interaction at the small scale but also the flows induced by the inlet/outlet boundary conditions at the large scale. To this aim, we have adopted an approach coupling the continuum model in the bulks of the channel and the discrete model at the vicinity of the wall, based on an atomistic representation of the fluid and the solid.The Navier-Stokes and energy equations, coupled with an equation of state, are approximated by a finite volume method and the molecular dynamics simulations are used to finely represent the interaction between the fluid and the solid. This hybrid method requires information transmission between the former two regions: averaged quantity in molecular dynamics simulations are imposed as boundary conditions for the continuous model and constrained dynamics, coupled with a thermostat Langevin, is used to control in the molecular level. A set of small molecular dynamics blocks, smartly distributed all along the wall/fluid interface, allows to treat flow and heat transfers in a long micro/nano-channel with a reasonable computational cost.After a validation step, the hybrid multi-scale simulations of complex fluid flows in the channel composed of the platinum wall have been conducted for argon in incompressible liquid or compressible gaseous phase with and without phase change in the vicinity of the wall

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Generating, miniaturizing and controlling spin waves on the nanometer scale is of great interest for magnonics. For instance, this holds the prospect of exploring wave-based logic concepts and reduced Joule heating, by avoiding charge transport, in spin-wave circuitry. In this work, a novel approach is for the first time confirmed experimentally, which allows confining spin-wave transport to nanometre-wide channels defined by magnetic domain walls. This is investigated for different domain wall types( 90deg and180deg Néel walls) in two material systems of polycrystalline Ni81Fe19 and epitaxial Fe. The study covers the thermal, linear and non-linear regime utilizing micro- focused Brillouin light scattering microscopy complemented by micromagnetic simulations. An initially linear dispersion dominated by dipolar interactions is found for the guided spin waves. These are transversally confined to sub-wavelength wide beams with a well-defined wave vector along the domain wall channel. In the non-linear regime, higher harmonic generation of additional spin-wave beams at the sides of the domain wall channel is observed. Furthermore, the possibility to shift the position of the domain wall over several microns by small magnetic fields is demonstrated, while maintaining its spin-wave channeling functionality. Additionally, spin-wave transmittance along domain walls, which change direction at the edges of the structure as well as between interconnected walls of identical and different type is studied. Characterization of spin-wave transmission through interconnected domain walls is an important step towards the development of magnonic circuitry based on domain wall(-networks). With respect to developing flexible and scalable spin-wave sources, the second part of this thesis addresses auto-oscillations in spin Hall oscillators (based on a Pt / Ni81Fe19 bilayer) of tapered nanowire geometry. In these systems, a simultaneous formation of two separate spin-wave bullets of distinct localization and frequency has been indicated. This spin-wave bullet formation is con- firmed experimentally and investigated for different driving currents. Subsequently, control over these bullets by injecting external microwave signals of varying frequency and power is demon- strated, switching the oscillator into single-mode operation. Three synchronized auto-oscillatory states are observed, which can be selected by the frequency of the externally imprinted signal. This synchronization results in linewidth reduction and frequency-locking of the individual bullet modes. Simultaneously the bullet-amplitude is amplified and is found to scale as P2/3 with the injected microwave power P. This amplification and control over position and frequency of the spin-wave bullets is promising for the development of microwave amplifiers/detectors and spin- wave sources on the nanoscale based on spin Hall oscillators.:1 Introduction 1 2 Theoretical background 4 2.1 Energy density of thin film ferromagnets and domain(wall) formation 2.2 Magnetizationdynamicsinthinfilmferromagnets 11 2.2.1 Spin-wavedispersioninthelinearregime 13 2.2.2 Magnetizationdynamicsinthenon-linearregime 17 2.3 SpinHallOscillators 21 2.3.1 Spin Hall effect and spin transfer torque in a ferromagnet/heavy-metal bi- layersystem 21 2.3.2 Characteristics of magnetization auto-oscillations 25 2.3.3 Improvement of monochromaticity, coherence and output power by injec- tionlocking 28 3 Materials and Methods 31 3.1 ElectronBeamLithography,EBL 31 3.2 Ni81Fe19 microstructures 32 3.3 Femicrostructures 34 3.4 TaperedspinHalloscillators 35 3.5 Micro-focused Brillouin Light Scattering Spectroscopy, μBLS 36 3.5.1 μBLSspatialresolution 40 4 Experimental results 43 4.1 Spin-wave dynamics in multi-domain magnetic configurations 43 4.1.1 Spin-wave dynamics of 180◦ Néel walls in rectangular elements 44 4.1.2 Spin-wave dynamics of 90◦ Néel walls in square elements 63 4.1.3 Spin-wave dynamics of interconnected Néel walls in Fe wires 76 4.2 Auto-oscillationintaperedwiregeometries 88 4.2.1 Initial static magnetic configuration and effective field 89 4.2.2 Thermally excited dynamics and spectral properties 91 4.2.3 Direct microwave excitation of spin-wave dynamics 93 4.2.4 Auto-oscillatoryresponse 96 4.2.5 Microwaveamplificationandinjectionlocking 104 5 Summary and outlook 114 Own publications 118 Bibliography 120 Acknowledgement 141 A Appendix 143 A.1 Splitting process in magnetic domains confined by domain walls 143 A.2 reconfigurable remanent states in square structures stabilized by local ion irradiation 144 A.3 Domain wall displacements induced by a scanning laser beam 145 A.4 Magnetic Force Microscopy investigation of the domain wall type and width 147 A.5 Micromagnetic simulations: problem definition and analysis 149 A.6 Current dependence of auto-oscillations in the tapered SHO 152 A.7 Fabrication of Ni81Fe19 microstructures for spin waves in domain walls 153