The precise control of ferromagnetic domain wall formation opens up exciting avenues of research and potential application in spintronics ? the manipulation of charge carriers via their spin properties. Recent experiments on Cobalt-Platinum multilayers containing artificially created domains provide the motivation for this work. In this thesis the electronic structure of CoPt multilayers are calculated by an ab initio multiple scattering method, and attempts are made at replicating the systems used in experiments, including lattice relaxations and the effects of substitutional alloying. The magnetic reversal process in Pt/Co/Pt trilayer systems is studied in the framework of micromagnetics, in which effects such as exchange, magnetocrystalline anisotropy and the demagnetising field are treated phenomenologically. The results are compared to recent experiments and the switching mechanism can be understood in terms of domain growth and domain wall nucleation. A ballistic transport framework is outlined in terms of a tight binding Green function method. The domain wall is modelled as a change in the local spin reference frame. The method is applied to Cobalt Platinum trilayers, and it is found that the resulting domain wall resistances are sensitive to the details of the Fermi energy bands. Subsequently, the angular dependence of domain wall resistivity in Pt/Co/Pt systems is studied by a model based on the anisotropic resistivity tensor that is expected in a domain wall. The results are used to extract resistivity parameters from experimental data, and a positive domain wall resistivity is identified, whilst resulting arguments provide supporting evidence for the Levy-Zhang theory of domain wall resistance. Finally, recent experiments on the dilute magnetic semiconductor (Ga,Mn)As have provided evidence for a negative intrinsic domain wall resistance in this material. By applying a similar model to that used on the CoPt systems, it is shown that the anomalous magnetoresistance effect can also provide a significant negative contribution to the measured resistance via induced eddy current at the domain wall boundary.

The motion of domain walls in magnetic wires is investigated numerically using the program LLG Micromagnetics Simulator. Samples with different dimensions such as 8000x200x5 nm³, 800x200x20 nm³, and 800x40x5 nm³ are studied. The calculations are performed both without and with moving boundary condition, and assuming smooth edge and rough edge samples. The results show that the velocity of the domain wall is affected by the external field, roughness of the edge, the damping constant, and the dimensions of the sample. Two kinds of domain wall vortex structures are identified in addition to simple transverse domain structures: anti-vortex and vortex.

The motion of domain walls due to the spin torque generated by coherent carrier transport is of considerable interest for the development of spintronic devices. We model the charge and spin transport through domain walls in ferromagnetic semiconductors for various systems. With an appropriate model Hamiltonian for the spin– dependent potential, we calculate wavefunctions inside the domain walls which are then used to calculate transmission and reflection coefficients, which are then in turn used to calculate current and spin torque. Starting with a simple approximation for the change in magnetization inside the domain wall, and ending with a sophisticated transfer matrix method, we model the common π wall, the less–studied 2π wall, and a system of two π walls separated by a variable distance. We uncover an interesting width dependence on the transport properties of the domain wall. 2π walls in particular, have definitive maximums in resistance and spin torque for certain domain wall widths that can be seen as a function of the spin mistracking in the system — when the spins are either passing straight through the domain wall (narrow walls) or adiabatically following the magnetization (wide walls), the resistance is low as transmission is high. In the intermediate region, there is room for the spins to rotate their magnetization, but not necessarily all the way through a 360 degree rotation, leading to reflection and resistance. We also calculate that there are widths for which the total velocity of a 2π wall is greater than that of a same–sized π wall. In the double–wall system, we model how the system reacts to changes in the separation of the domain walls. When the domain walls are far apart, they act as a spin–selective resonant double barrier, with sharp resonance peaks in the transmission profile. Brought closer and closer together, the number and sharpness of the peaks decrease, the spectrum smooths out, and the domain walls brought together have a transmission spectrum with many of the similar features from the 2π wall. Looking at the individual walls, we find an interesting interaction that has three distinct regimes: 1) repulsion, where the left wall moves to the left and the right wall to the right; 2) motion together, where the two walls both move to the right, even at the same velocity for one special value of separation; and 3) attraction, where the left wall moves to the right and the right wall moves to the left. This speaks to a kind of natural equilibrium distance between the domain walls. This is of major interest for device purposes as it means that stacks of domain walls could be self–correcting in their motions along a track. Much experimental work needs to be done to make this a reality, however.

In this dissertation, we provide an accurate description of spin transport in magnetic textures and in particular, we investigate in detail, the nature of spin torque and magnetic damping in such systems. Indeed, as will be further discussed in this thesis, the current-driven velocity of magnetic textures is related to the ratio between the so-called non-adiabatic torque and magnetic damping. Uncovering the physics underlying these phenomena can lead to the optimal design of magnetic systems with improved efficiency. We identified three interesting classes of systems which have attracted enormous research interest (i) Magnetic textures in systems with broken inversion symmetry: We investigate the nature of magnetic damping in non-centrosymmetric ferromagnets. Based on phenomenological and microscopic derivations, we show that the magnetic damping becomes chiral, i.e. depends on the chirality of the magnetic texture. (ii) Ferromagnetic domain walls, skyrmions and vortices: We address the physics of spin transport in sharp disordered magnetic domain walls and vortex cores. We demonstrate that upon spin-independent scattering, the non-adiabatic torque can be significantly enhanced. Such an enhancement is large for vortex cores compared to transverse domain walls. We also show that the topological spin currents owing in these structures dramatically enhances the non-adiabaticity, an effect unique to non-trivial topological textures (iii) Antiferromagnetic skyrmions: We extend this study to antiferromagnetic skyrmions and show that such an enhanced topological torque also exist in these systems. Even more interestingly, while such a non-adiabatic torque inuences the undesirable transverse velocity of ferromagnetic skyrmions, in antiferromagnetic skyrmions, the topological non-adiabatic torque directly determines the longitudinal velocity. As a consequence, scaling down the antiferromagnetic skyrmion results in a much more efficient spin torque.

<p> An investigation of domain walls in some uniaxial magnetic materials is reported in this thesis. Firstly, a method for measuring the wall energy anisotropy in orthoferrites, which causes cylindrical magnetic (bubble) domains to be elliptical is described. In Sm0.55Tb0.45FeO3 a measured anisotropy energy of 1.7% of the wall-energy density at room temperature is responsible for eccentricities as large as 0.4 at average bubble radii equal to 85% of the bubble strip-domain transition radius. The relationship between material parameters and wall-energy anisotropy is discussed. The hypothesis that in orthoferrites walls parallel to the a axis are Bloch walls while walls parallel to the b axis are Néel walls is investigated by measuring the wall anisotropy as a function of the quality factor of the material by varying the temperature of the sample. The measurements seem to verify the predicted dependence of wall anisotropy on the quality factor and thus the hypothesis.</p> <p> A method for measuring the temperature dependence of the wall-energy density in orthoferrites and the saturation magnetization in garnets is described. The advantage of the method is that it uses a single isolated bubble domain without the need to destroy the bubble in order to obtain the measurements. This method led to the derivation of the temperature sensitivities of bubble domains in orthoferrites and garnets in terms of the material parameters. Optimum plate thicknesses to minimize the variation of bubble diameter with temperature are considered. Also, the condition for zero temperature sensitivity of bubbles in some uniaxial materials is derived in terms of the material parameters.</p> <p> Finally, a study of the current requirement to cut a bubble domain from a strip domain or another bubble in uniaxial plates is reported in this thesis.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.

We studied the edge states and their roles in conductivity and magnetism of graphene nanoribbions and flakes. we studied the Aharonov-Bohm effect in graphene nanodisks and rings. We described the quantum oscillations of the magnetization of graphene flakes. we have examined the snake-like states of transport electrons in the configurations of graphene ribbons with a domain wall in the centre.

Charged domain walls (CDWs) in proper ferroelectrics are a novel route towards the creation of advancing functional electronics. At CDWs the spontaneous polarization obeying the ferroelectric order alters abruptly within inter-atomic distances. Upon screening, the resulting charge accumulation may result in the manifestation of novel fascinating electrical properties. Here, we will focus on electrical conduction. A major advantage of these ferroelectric DWs is the ability to control its motion upon electrical fields. Hence, electrical conduction can be manipulated, which can enrich the possibilities of current electronic devices e.g. in the field of reconfigurability, fast random access memories or any kind of adaptive electronic circuitry. In this dissertation thesis, I want to shed more light onto this new type of interfacial electronic conduction on inclined DWs mainly in lithium niobate/LiNbO3 (LNO). The expectation was: the stronger the DW inclination towards the polar axis of the ferroelectric order and, hence, the larger the bound polarization charge, the larger the conductivity to be displayed. The DW conductance and the correlation with polarization charge was investigated with a multitude of experimental methods as scanning probe microscopy, linear and nonlinear optical microscopy as well as electron microscopy. We were able to observe a clear correlation of the local DW inclination angle with the DW conductivity by comparing the three-dimensional DW data and the local DW conductance. We investigated the conduction mechanisms on CDWs by temperature-dependent two-terminal current-voltage sweeps and were able to deduce the transport to be given by small electron polaron hopping, which are formed after injection into the CDWs. The thermal activated transport is in very good agreement with time-resolved polaron luminescence spectroscopy. The applicability of this effect for non-volatile memories was investigated in metal-ferroelectric-metal stacks with CMOS compatible single-crystalline films. These films showed unprecedented endurance, retention, precise set voltage, and small leakage currents as expected for single crystalline material. The conductance was tuned and switched according to DW switching time and voltage. The formation of CDWs has proven to be extremely stable over at least two months. The conductivity was further investigated via microwave impedance microscopy, which revealed a DW conductivity of about 100 to 1000 S/m at microwave frequencies of about 1 GHz. / Geladene Domänenwände (DW) in reinen Ferroelektrika stellen eine neue Möglichkeit zur Erzeugung zukünftiger, funktionalisierter Elektroniken dar. An geladenen DW ändert sich die Polarisation sehr abrupt - innerhalb nur weniger Atomabstände. Sofern die dadurch hervorgerufene Ladungsträgeranreicherung elektrisch abgeschirmt werden kann, könnte dies zu faszinierenden elektrischen Eigenschaften führen. Wir möchten uns hierbei jedoch auf die elektrische Leitfähigkeit beschränken. Ein großer Vorteil für die Anwendung leitfähiger DW ist deren kontrollierte Bewegung unter Einwirkung elektrischer Felder. Dies ermöglicht die Manipulation das Ladungstransports, welches zum Beispiel im Bereich der Rekonfigurierbarkeit, schneller Speicherbauelemente und jeder Art von adaptiven elektronischen Schaltungen Anwendung finden kann. In dieser Dissertationsschrift möchte ich diesen neuen Typus grenzflächiger elektronischen Ladungstransports an geladenen DW hauptsächlich am Beispiel von Lithiumniobat/-LiNbO3 (LNO) untersuchen. Die Annahme lautete hierbei: umso stärker die DW zur ferroelektrischen Achse geneigt ist, also desto stärker die gebundene Polarisationsladung und folglich die elektrische DW-Leitfähigkeit. Die elektrische DW-Leitfähigkeit und die Korrelation mit der Polarisationsladung wurde mit verschiedenen experimentellen Methoden wie Rasterkraftmikroskopie, linearer und nichtlinearer optischer Mikroskopie als auch Elektronenmikroskopie untersucht. Es konnte eine klare Korrelation durch Vergleich der dreidimensionalen DW-Aufzeichnungsdaten mit der lokalen Leitfähigkeit gezeigt werden. Wir haben weiterhin den Leitfähigkeitsmechanismus an geladenen DW mittels temperaturabhängiger Strom-Spannungskennlinien untersucht und konnten hierbei einen Hopping-Transport kleiner Elektronenpolaronen nachweisen, welche nach Elektroneninjektion in die geladene DW generiert werden. Der thermisch aktivierte Ladungsträgertransport ist in guter Übereinstimmung mit zeitaufgelöster Polaron-Lumineszenzspektroskopie. Die Anwendbarkeit dieses Effektes für nicht-volatile Speicherbauelemente wurde an Metall-Ferroelektrika-Metall Schichtstrukturen mit CMOS-kompatiblen einkristalliner Filmen untersucht. Die Filme zeigen bisher nichtgesehene Durchhalte- und Speichervermögen, genau definierte Schaltspannung sowie sehr geringe Leckageströme wie dies für einkristalline Materialsysteme erwartet wird. Die Leitfähigkeit konnte mittels entsprechender Wahl der elektrischen Schaltzeiten und -spannungen zielgerichtet manipuliert und geschalten werden. Es konnte darüber hinaus gezeigt werden, dass die hergestellten geladenen DW über eine Zeitspanne von mindestens zwei Monaten stabil sind und hierbei leitfähig bleiben. Die Leitfähigkeit der DW wurde weiterhin mittels Mikrowellenimpedanzmikroskopie untersucht. Dabei konnten DW-Leitfähigkeiten von 100 bis 1000 S/m für Mikrowellenfrequenzen von etwa 1GHz ermittelt werden.

L'étude de la dynamique des parois de domaines dans les nano-systèmes ferromagnétiques est cruciale pour le développement des dispositifs de stockage de l'information basés sur le déplacement et le contrôle des parois. Ces dispositifs ont plusieurs avantages : non-volatilité, rapidité d'exécution, haute densité de stockage, et faible consommation de l'énergie. En utilisant des méthodes micro-magnétiques et analytiques, nous avons constaté que l'interaction entre deux parois affectait les processus de dépiégeage sous champ magnétique, dans des nanofils en nickel à géométrie cylindrique et planaire. Nous avons mis en évidence des comportements non linéaires de la dynamique d'une paroi piégée, qui varient selon le matériau et le type de piège utilisé. Les diagrammes de phases représentant l'exposant de Lyapunov ont permis la distinction entre des zones chaotiques et périodiques, en fonction de la fréquence et de l'amplitude d'une excitation harmonique. Nous avons présenté des résultats sur la manipulation précise d'une paroi transverse sous impulsions de courant dans un nanofil planaire en nickel, structuré par une multitude de défauts artificiels. Nous avons montré que le positionnement exact de la paroi à température ambiante est possible uniquement pour des impulsions symétriques de très courte durée. Des effets inertiels pouvant s'opposer au couple de transfert de spin, ou au contraire l'amplifier ont été observés. Ces derniers résultats ouvrent une route vers le déplacement des parois dans les deux directions par des impulsions unipolaires de courant. / The study of the domain wall dynamics in ferromagnetic nano-systems is crucial for the developement of data-storage devices based on control and displacement of the domain walls. These devices have several advantages : non-volatility, fast execution time, high density, and low power consumption. Using micromagnetics and analytical methods, we have shown that the interaction between two domain walls influences the depinning process under magnetic field, in cylindrical and planar shaped nickel nanowires. We highlighted the nonlinear behaviour of the dynamics of a pinned domain wall, which varies with the material properties and the type of the pinning sites. The Lyapunov phase diagrams display chaotic and periodic regions function of the amplitude and frequency of a harmonic excitation. We have also presented results about the precise manipulation of transverse domain walls by current pulses in a nickel planar nanowire with artificial defects. We have shown that exact positioning of the domain walls at room temperature is possible only for very short symmetric current pulses. We observed inertial effects which can oppose or amplify the spin transfert torque effect. These results open a route to domain wall displacement in both directions with unipolar current pulses.

Dans ce manuscrit, les effets émergeant de la modulation de phase croisée entredeux signaux polarisés orthogonalement dans des fibres optiques à dispersionnormale sont étudiés et leurs applications approfondies.En premier lieu, un générateur d’impulsions cadencées à 40-GHz a été développéen injectant deux ondes sinusoïdales en opposition de phase polariséesorthogonalement au sein d’une fibre optique à dispersion normale. Une grandedifférence de puissance entre les deux ondes induit un potentiel de phasepériodique pour la réplique de plus faible énergie, induisant ainsi dans régimede dispersion normale une dynamique de compression, comprimant ainsi l'ondesinusoïdale en impulsions courtes et correctement séparées.Un échantillonneur et amplificateur est ensuite démontré en utilisant le mêmePrincipe. Une pompe sinusoïdale de forte puissance et à haute fréquence estinjectée dans une fibre optique sur un axe de polarisation, orthogonalement àun long pulse de forme arbitraire et de faible énergie. Nous avons alorsdémontré un échantillonnage à 40-GHz ainsi qu'une amplification jusqu'à unfacteur 4.Nous rapportons ensuite la première observation de solitons en parois dedomaine de polarisation dans des fibres à dispersion normale en transmettanttout d'abord des trains d’impulsions périodiques anticorrélés et polarisésorthogonalement dans une fibre de 10 km, puis en encodant le mot 'PETAL' enASCII sur les ondes lumineuses et le transmettant dans une fibre de 2x25km.L'information encodée est correctement décodée après propagation.L'apparition spontanée de ces parois de domaine en polarisation est ensuiteétudiée en observant le processus de ségrégation de polarisation entre deuxondes incohérentes et décorrélées. Une approche thermodynamique est ensuiteutilisée afin d'expliquer le comportement observé. L'origine de ces parois dedomaine, liée théoriquement à l'instabilité de modulation de polarisationisotrope est ensuite étudiée. En injectant des impulsions de fortes puissancesdans de courts segments de fibre de très faible biréfringence, nous avonsobservé l'apparition des bandes théoriques d'instabilité modulationnellejusqu'à des longueurs de 200m, et mettons en avant leur extinction dans de pluslongues fibres. Enfin, une extension au modèle de Manakov est apportée enprenant en compte le processus de fabrication (spun) pour les fibres utiliséespour les expériences précédentes. / In this Ph.D., physical effects arising from cross-phase modulation between twoorthogonally polarized signals propagating in normal dispersion fibers arestudied and some of their applications investigated.First, a 40-GHz pulse generator is designed by sending two orthogonallypolarized out-of-phase sinusoidal waves in a normally dispersive optical fiber.A large power imbalance between the two waves induces a periodic phasepotential for the weak replica which turns the normal dispersive defocusingregime into a focusing dynamics, thus compressing the sinusoidal wave intoshort and well-separated pulses.A signal sampler and magnifier was then designed using the same principle,sending a high power high frequency sinusoidal pump on one axis ofpolarization, orthogonally to a long arbitrary shaped pulse with low power. Wethen report a 40-GHz sampling as well as a magnification factor up to 4.We then report the first observation of polarization domain walls in normaldispersion fibers by first transmitting anticorrelated orthogonally polarizedperiodic signals in a 10-km long fiber, and then encoding the word 'PETAL' asASCII on the light waves and transmitting the signal in 2x25km fibers. Theinformation encoded was properly decoded after propagation.Spontaneous apparition of such polarization domain walls was then studied byobserving a polarization segregation process of two incoherent and uncorrelatedsignal. A thermodynamic approach was then used to explain the observedbehaviour. The origin of those domain walls, which was theoretically relatedto isotropic polarization modulation instability, was then investigated.Injecting high power pulses in short segments of very low birefringence fibers,we were able to observe the growth of the theoretical bands in a up to200m-long fibers, and highlight their extinction for longer strands, while newand unexplained side-bands appeared. Finally, an extension to the Manakovmodel is explicited taking into account the manufacturing process for thefibers used in the previous experiments.