Cette thèse s'inscrit dans le contexte de l'électronique de spin et traite plus particulièrement de l'interaction réciproque entre un courant polarisé en spin et l'aimantation des structures magnétiques. Au cours de ce travail, les équations du transport diffusif dépendant du spin ont été couplées de façon auto-cohérente à l'équation de la dynamique d'aimantation dans l'approche micromagnétique au sein du code éléments finis. Cet outil numérique est appliqué à l'étude de la dynamique de parois de domaines dans différentes géométries sous l'action d'un courant polarisé. Il a permis de mettre en évidence plusieurs nouveaux phénomènes liés à l'interaction mutuelle entre l'aimantation et les spins des électrons. Pour des rubans à section rectangulaire, l'impact de cette interaction, habituellement négligée dans les modèles simplifiés, est quantifié via le calcul de la vitesse de déplacement de parois et du courant critique de Walker. Ces paramètres ont été étudiés en fonction de la largeur de paroi, du courant appliqué et des longueurs caractéristiques du transport polarisé en spin. L'augmentation du paramètre de non-adiabaticité du système, liée à l'augmentation du gradient d'aimantation ainsi qu'à une forte non-localité du modèle couplé, a été démontrée. Pour des fils à section circulaire et à diamètre modulable, une contribution supplémentaire à la non-adiabaticité du système liée, à la géométrie confinée, a été mise en évidence. Puis, les différents régimes dynamiques ainsi que les conditions de dépiégage de la paroi ont été caractérisés en fonction de la taille de constrictions. / In the context of spintronics this thesis studies the mutual interaction between a spin polarised current and the magnetization of magnetic structures. During this work, the diffusive spin transport equations were coupled in a self-consistent manner with the magnetization dynamics equations in the micromagnetic approach in our homemade finite element code. This numerical tool applied to the study of domain walls dynamics in different geometries under the action of spin polarized current highlighted several new phenomena related to the mutual interaction between the magnetization and the spins of electrons. For rectangular cross section stripes, the impact of this interaction, usually neglected in simplified models, is quantified by the computation of the domain wall velocity and the Walker critical current. These quantities were studied as a function of the domain wall width, the applied current, and the spin polarised transport characteristic lengths. Increasing the non-adiabatic parameter of the system related to the increase in the magnetization gradient and a strong non-locality of the coupled model was demonstrated. For circular cross section wires with a modulated diameter, an additional contribution to the non-adiabaticity of the system related to the confined geometry is highlighted. Then the different dynamic regimes and domain wall unpinning conditions are characterised according to the constriction size.

Ferroic materials play an increasingly important role in novel (nano-)electronic applications. Recently, research on domain walls (DWs) received a big boost by the discovery of DW conductivity in bismuth ferrite (BiFeO3 ) and lead zirconate titanate (Pb(Zrx Ti1−x )O3) ferroic thin films. These achievements open a realistic and unique perspective to reproducibly engineer conductive paths and nanocontacts of sub-nanometer dimensions into wide-bandgap materials. The possibility to control and induce conductive DWs in insulating templates is a key step towards future innovative nanoelectronic devices [1]. This work focuses on the investigation of the charge transport along conductive DWs in ferroelectric single crystals. In the first part, the photo-induced electronic DC and AC charge transport along such DWs in lithium niobate (LNO) single crystals is examined. The DC conductivity of the bulk and DWs is investigated locally using piezoresponse force microscopy (PFM) and conductive AFM (c-AFM). It is shown that super-bandgap illumination (λ ≤ 310 nm) in combination with (partially) charged 180° DWs increases the DC conductivity of the DWs up to three orders of magnitude compared to the bulk. The DW conductivity is proportional to the charge of the DW given by its inclination angle α with respect to the polar axis. The latter can be increased by doping the crystal with magnesium (0 to 7 mol %) or reduced by sample annealing. The AC conductivity is investigated locally utilizing nanoimpedance microscopy (NIM) and macroscopic impedance measurements. Again, super-bandgap illumination increases the AC conductivity of the DWs. Frequency-dependent measurements are performed to determine an equivalent circuit describing the domains and DWs in a model system. The mixed conduction model for hopping transport in LNO is used to analyze the frequency-dependent complex permittivity. Both, the AC and DC results are then used to establish a model describing the transport along the conductive DW through the insulating domain matrix material. In the last part, the knowledge obtained for LNO is applied to study DWs in lithium tantalate (LTO), barium titanate (BTO) and barium calcium titanate (BCT) single crystals. Under super-bandgap illumination, conductive DWs are found in LTO and BCT as well, whereas a domain-specific conductivity is observed in BTO.

In the present thesis, magnetization reversal in both out-of-plane and in-plane magnetized thin lms and in devices fabricated out of those lms are explored. Pt/Co/Pt stacks with ultrathin Co layer were in-estimated initially for understanding their magnetic properties in this thesis. These perpendicular magnetized systems are good candidates for magnetic hard disc drives due to their large anisotropy, which may allow miniaturization of magnetic data storage devices. The spin Hall e ect mediated current-induced magnetization reversal in patterned Pt/Co/Pt devices were extensively investigated. Investigation of the magnetization reversal by means of a current instead of a magnetic eld is necessary to explore the possibilities of solid state magnetic memory devices. This is the primary motivation behind the investigation of current-induced magnetization reversal in Pt/Co/Pt system, in this thesis. Another important proposal for magnetic data storage is the race track memory, where the domain walls separating magnetic domains (in in-plane or out-of-plane magnetized materials) are moved by using a current. This involves a great deal of understanding of the domain wall motion in Nano-conduits under applied magnetics ends, and currents and also its interaction with engineered geometrical features. In this thesis work, magnetic led-driven domain wall pinning and deepening experiments on in-plane magnetized nanowires of perm alloy were performed to un-distend this interaction and the e act of domain wall chirality. In chapter 1, a general introduction to di errant data storage technologies and the current progress in the leg of spintronic is presented. This will highlight a perspective of this thesis work with respect to the present day research in spintronic and magnetization reversal studies. In chapter 2, a basic background of magnetism using the micromag-netic framework is illustrated. A brief introduction to magnetic domain walls is also presented. The Landau-Lifshitz-Gilbert dynamical equation is discussed and some case studies applied to a single domain particle with uniaxial anisotropy under the effect of spin-orbit torque are illu trated. The basics of spin-orbit coupling leading to spin Hall e ect is also explain In chapter 3, most of the essential experimental tools along with their basic working principles are described. Extensive e orts have been in-vested in designing and building the experimental tools. These include custom designs of a sputter deposition system, an ultra-high vacuum chamber for pulsed laser ablation, a magneto-optic Kerr e ect magne-tometer, a Kerr imaging system and a magneto-transport setup. All of these experimental setups have been automated, details of which are brie y discussed in this chapter. The Kerr imaging system was designed to measure hysteresis loops, observe domain wall motion and to measure domain wall velocity under applied magnetic elds and electric current. The magneto-transport setup was used for studying the domain wall pinning and depinning experiments in permalloy nanowires. In chapter 4, the optimization process for obtaining perpendicular mag-netic anisotropy in Pt/Co/Pt lms is described. The spin reorientation transition with varying thickness of Co (from 1.5 nm down to 0.35 nm) was studied. The magnetization easy axis direction changes from in-plane to out-of-plane as the thickness of Co is reduced. The dependence of Curie temperatures of ultrathin Co lms, with thickness as low as 0.35 nm, on the underlayer Pt thickness and its crystallinity was studied in detail. The e act of Ta but err layer on the texture of the Pt lm, and on the Curie temperature of the Pt/Co/Pt system was evaluated. To gain further insight of the role of the bottom Pt/Co and the top Co/Pt interfaces, ultrathin Cu lbs were inserted at the respective interfaces, and the anisotropy and magnetization reversal behaviour of these lbs were investigated. In chapter 5, studies on current-induced magnetization reversal in mi-corn sized wires of Pt/Co/Pt trilete is presented. The spin Hall e act assisted spin-orbit torque was used to reversibly switch the magnetization of these devices with and without the help of an external magnetic led. Since both the top and bottom layers are Pt, any contribution from Rashia e act towards spin-orbit torque could be ignored. By preparing devices with unequal top and bottom Pt thicknesses, a net spin-orbit torque could be applied to the magnetization of the Co layer. The thickness gradient/induced anisotropy in the Co layer was utilized to experimentally investigate current-induced deterministic switching. Sin-gel domain simulations with spin-orbit torque were also carried out to understand the mechanism of deterministic switching of magnetization in Pt/Co/Pt devices. This study is expected to have made sign cant contributions and to open up the possibilities of further investigation in the studies of spin-orbit torque in Pt/Co/Pt systems for solid state magnetic memory devices. In chapter 6, magnetic led-induced reversal in systems with in-plane magnetic anisotropy is presented. Here the e act of the width of a Nanos-trip on the anisotropy of a soft magnetic material like perm alloy was in-estimated. By introducing a nucleation pad to one end of the perm alloy nanowire, a single domain wall was generated at the junction with apple-cation of a proper magnetic led sequence. This domain wall could be in-jested into the nanowire by a magnetic led and pinned at a geometrical constriction inside the nanowire. The statistics of domain wall pinning and deepening processes indicated two di errant types of domain walls involved in the reversal process. With the assistance of micro magnetic simulations the domain walls were ident end as vortex walls of di errant chirality’s. Thus the interaction of domain walls with a Nano constriction and its dependence on the chirality of domain walls are understood. In chapter 7, a brief summary of the results obtained during the course of investigations is presented. An outlook presented at the end will help the readers of this thesis to understand the important research problems in this area and their potential future aspects.

Ferroic materials play an increasingly important role in novel (nano-)electronic applications. Recently, research on domain walls (DWs) received a big boost by the discovery of DW conductivity in bismuth ferrite (BiFeO3 ) and lead zirconate titanate (Pb(Zrx Ti1−x )O3) ferroic thin films. These achievements open a realistic and unique perspective to reproducibly engineer conductive paths and nanocontacts of sub-nanometer dimensions into wide-bandgap materials. The possibility to control and induce conductive DWs in insulating templates is a key step towards future innovative nanoelectronic devices [1]. This work focuses on the investigation of the charge transport along conductive DWs in ferroelectric single crystals. In the first part, the photo-induced electronic DC and AC charge transport along such DWs in lithium niobate (LNO) single crystals is examined. The DC conductivity of the bulk and DWs is investigated locally using piezoresponse force microscopy (PFM) and conductive AFM (c-AFM). It is shown that super-bandgap illumination (λ ≤ 310 nm) in combination with (partially) charged 180° DWs increases the DC conductivity of the DWs up to three orders of magnitude compared to the bulk. The DW conductivity is proportional to the charge of the DW given by its inclination angle α with respect to the polar axis. The latter can be increased by doping the crystal with magnesium (0 to 7 mol %) or reduced by sample annealing. The AC conductivity is investigated locally utilizing nanoimpedance microscopy (NIM) and macroscopic impedance measurements. Again, super-bandgap illumination increases the AC conductivity of the DWs. Frequency-dependent measurements are performed to determine an equivalent circuit describing the domains and DWs in a model system. The mixed conduction model for hopping transport in LNO is used to analyze the frequency-dependent complex permittivity. Both, the AC and DC results are then used to establish a model describing the transport along the conductive DW through the insulating domain matrix material. In the last part, the knowledge obtained for LNO is applied to study DWs in lithium tantalate (LTO), barium titanate (BTO) and barium calcium titanate (BCT) single crystals. Under super-bandgap illumination, conductive DWs are found in LTO and BCT as well, whereas a domain-specific conductivity is observed in BTO.

Memristor, defined as a two-terminal device which exhibits a pinched hysteresis loop in the current-voltage characteristics, is a main component of the resistive random access memory. Both memristor and memristive phenomena, known also as resistive switching (RS), have been thoroughly investigated in the past nearly two decades. This dissertation investigates YMnO3 thin films and explores a new concept concerning utilization of multiferroic properties for activation and/or enhancement of RS. It is hypothesized that the charged domain walls and/or vortex cores in YMnO3 thin films can act as an effective nanoscale features which support formation of the conductive filaments and, in consequence, enable development of an electroforming-free memristive structure. Results of the electrical characterization of YMnO3-based metal-insulator-metal structures indicate that hexagonal YMnO3 films deposited on metal-coated oxide substrate exhibit electroforming-free unipolar resistive switching (URS) while orthorhombic YMnO3 films grown on the doped oxide substrate show bipolar resistive switching (BRS). Observed URS is assigned to the formation and rupture of conductive, metallic-like filaments induced by the thermo-chemical phenomena. Results of polarity-dependent studies reveal that formation of conductive filaments proceeds in the electrostatic discharge event which is followed by their irreversible rupture during the reset process. Main properties of the observed URS include very good retention of programmed states, large memory window (between 10E+2 and 10E+4), high voltage and current required for set and reset, respectively, and low endurance. BRS is attributed to the complementary electronic and ionic processes within the p-n junction formed at the interface between p-YMnO3 and n-type oxide substrate. Results of ferroelectric characterization reveal that resistively switching YMnO3 films do not exhibit ferroelectric properties. Therefore, observed RS in YMnO3-based structures can not be directly related to the presence of charged domain walls and/or multiferroic vortex cores. Prospective functionality extension of YMnO3-based memristive devices is developed and presented on the example of photodetecting properties of metal-YMnO3-insulator-semiconductor stacks. Studies conducted within the framework of this doctoral dissertation investigate the resistive switching behaviour of YMnO3-based junctions. Obtained results contribute to the better understanding of the resistive switching and failure mechanisms in ternary oxides, and provide hints toward device engineering. / Der Memristor ist definiert als eine Zweipol-Vorrichtung, die eine hysteretische Strom-Spannungs-Charakteristik aufweist. Memristoren sind nichtflüchtige Widerstandsspeicher, deren elektrischer Widerstand mittels elektrischer Spannungspulse verändert werden kann. Sowohl Memristoren als auch memristive Widerstandsschalter (RS) werden seit mehr als zwei Jahrzehnten intensiv untersucht. Diese Dissertation untersucht YMnO3-Dünnschichten mit zirkularen Vorderseiten-Elektroden und unstrukturierten Rückseiten-Elektroden und erforscht ein neues Konzept über die Nutzung der multiferroischen Eigenschaften für die Aktivierung und/oder Verbesserung des memristiven Verhaltens. Es wird angenommen, dass die geladenen Domänenwände und/oder Vortices in YMnO3-Dünnschichten die Bildung leitfähiger Filamente wirksam unterstützt und folglich die Entwicklung eines neuartigen, formierungs-freiem Widerstandsspeichersermöglicht. Die Ergebnisse der elektrischen Charakterisierung von YMnO3-basierten Widerstandsschalter zeigen unipolares RS (URS), wenn eine metallische Rückseitenelektrode verwendet wird und bipolares RS (BRS), wenn als Rückseitenelektrode ein metallisch leitendes Oxid-Substrat verwendet werden. Das URS wird als thermochemisches RS klassifiziert und mit der Bildung und Auflösung metallisch leitender Filamente korreliert. Das BRS wird auf das Einfangen/Freigeben von Defekten in der Raumladungszone des YMnO3 im pn-Übergang von p-YMnO3/n-Nb:SrTiO3-Strukturen zurückgeführt. Die wichtigsten Eigenschaften des formierungsfreien URS sind die sehr gute Retention der programmiertenWiderstandszustände, große Speicherfenster (zwischen 10E+2 und 10E+4), die hohe Schreibspannung für den Set-Prozess und der hohe Schreibstrom für den Reset-Prozess. Die Endurance ist aufgrund der Degradation des Vorderseiten-Elektrode gering. Die Ergebnisse des polaritätsabhängigen Widerstandsschaltens zeigen, dass der Set-Prozess mit elektrostatischer Entladung einhergeht. Die ferroelektrische Charakterisierung zeigt, dass die YMnO3–Dünnfilme keine ferroelektrischen Eigenschaften aufweisen. Daher kann das beobachtete URS nicht direkt auf die Anwesenheit von geladenen Domänenwände und Vortices zurückgeführt werden. Darüberhinaus wurden die photodetektierenden Eigenschaften von Metall-YMnO3-Isulator-Halbleiter-Stacks als potenzielle Erweiterung der Funktionalität von YMnO3-basierten memristiven Bauelementen vorgestellt und vorgeschlagen. Im Rahmen der vorliegenden Dissertation wurde das Widerstandsschalten von multiferroischen, YMnO3-basierten Widerstandsschaltern untersucht. Die erhaltenen Ergebnisse tragen zu einem besseren Verständnis des Widerstandsschaltens von multiferroischen Materialien bei.

In our pursuit of ever smaller transistors, with greater computational throughput, many questions arise about how material properties change with size, and how these properties may be modelled more accurately. Metallic nanocontacts, especially those for which magnetic properties are important, are of great interest due to their potential spintronic applications. Yet, serious challenges remain from the standpoint of theoretical and computational modelling, particularly with respect to the coupling of the spin and lattice degrees of freedom in ferromagnetic nanocontacts in emerging spintronic technologies. In this thesis, an extended method is developed, and applied for the first time, to model the interplay between magnetism and atomic structure in transition metal nanocontacts. The dynamic evolution of the model contacts emulates the experimental approaches used in scanning tunnelling microscopy and mechanically controllable break junctions, and is realised in this work by classical molecular dynamics and, for the first time, spin-lattice dynamics. The electronic structure of the model contacts is calculated via plane-wave and local-atomic orbital density functional theory, at the scalar- and vector-relativistic level of sophistication. The effects of scalar-relativistic and/or spin-orbit coupling on a number of emergent properties exhibited by transition metal nanocontacts, in experimental measurements of conductance, are elucidated by non-equilibrium Green’s Function quantum transport calculations. The impact of relativistic effects during contact formation in non-magnetic gold is quantified, and it is found that scalar-relativistic effects enhance the force of attraction between gold atoms much more than between between atoms which do not have significant relativistic effects, such as silver atoms. The role of non-collinear magnetism in the electronic transport of iron and nickel nanocontacts is clarified, and it is found that the most-likely conductance values reported for these metals, at first- and lastcontact, are determined by geometrical factors, such as the degree of covalent bonding in iron, and the preference of a certain crystallographic orientation in nickel. / Physics / Ph. D. (Physics)

Though ferroelectric HfO₂ thin films are now well characterized, little is currently known about their grain substructure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and phase interconversion. Here, scanning transmission electron microscopy is applied to investigate the atomic structure of boundaries in these materials. It is found that orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic interphase boundaries form throughout individual grains. The results inform how interphase boundaries can impose strain conditions that may be key to phase stabilization. Moreover, the atomic structure near interphase boundary walls suggests potential for their mobility under bias, which has been speculated to occur in perovskite morphotropic phase boundary systems by mechanisms similar to domain boundary motion.

In dieser Dissertation wird die gute Einstellbarkeit von synthetischen Antiferromagneten mit dem kollektiven Surface Spin-Flop-Verhalten kombiniert. Es wird der Einfluss der Gesamtschichtdicke untersucht, welche mit dem Abstand der magnetische Oberflächen korreliert. Zudem werden die Dicken der ferromagnetischen Untereinheiten an den Außenseiten verändert, womit die Beiträge der Oberflächen unterdrückt oder verstärkt werden können. Darauffolgend wird die Kontrolle der Oberflächenbeiträge angewendet, um Exchange Bias-Strukturen auf Basis synthetischer Antiferromagnete zu erzeugen. Da diese nicht aus Heterostrukturen intrinsischer Antiferromagnete und Ferromagnete bestehen, wird nicht nur eine gute Abstimmbarkeit erreicht, sondern auch die Materialwahl wird potentiell vereinfacht. Zudem kann der Exchange Bias in synthetischen Antiferromagneten vollständig bei Raumtemperatur beobachtet und gesteuert werden. Im Weiteren wird ein zuvor untersuchtes Konzept zur Stabilisierung der vom Surface Spin-Flop erzeugten vertikalen antiferromagnetischen Domänenwände erweitert. Es wird demonstriert, wie ein Paar koexistierender antiferromagnetischer Domänenwände in Abwesenheit äußerer Magnetfelder und bei tiefen Temperaturen stabil gehalten werden kann. Damit können in Erweiterung der ursprünglichen Konzeptes nun acht anstatt sechs remanenter Zustände durch geeignete Magnetfeldroutinen eingestellt werden.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste / In this thesis, the high degree of tunability in the SAFs is combined with the collective surface spin-flop reversal. The influence of the total thickness and thus the distance of the magnetic surfaces is explored. Furthermore, the thickness of the ferromagnetic surface subunits is altered to selectively suppress or enhance the surface contribution. The control of the surface contribution is subsequently employed to create magnetic exchange bias structures based on the synthetic antiferromagnets. If compared to conventional exchange bias systems in heterostructures of intrinsic antiferromagnetic and ferromagnetic materials, an exchange bias with full room temperature operation, high tunability and a potential potential much more flexible choice of materials becomes available. Additionally, a previously established concept to stabilize the vertical antiferromagnetic domain walls that originate from the surface spin-flop at remanence is extended to a coexisting pair of antiferromagnetic domain walls. At low temperatures, the coexisting vertical antiferromagnetic domain walls can be stabilized at remanence, too. Furthermore, the total number of different remanent states, which are accessible through different field routines, can be increased from six in the original concept to eight in the more sophisticated concept presented here.:1. Introduction 2. Theoretical background 2.1. Micromagnetic energy terms 2.1.1. Zeeman energy 2.1.2. Demagnetization energy 2.1.3. Anisotropy energy 2.1.4. Exchange energy 2.2. Magnetic multilayers 2.2.1. Magnetic anisotropy in magnetic multilayers 2.2.2. Synthetic antiferromagnets 2.3. Exchange Bias 2.4. The bulk and surface spin-flop 3. Methods 3.1. Sputter deposition 3.2. X-ray diffraction and reflectometry 3.3. Magnetometry 3.4. Magnetic force microscopy 3.5. Micromagnetic simulations 4. Results 4.1. From collective reversal to exchange bias 4.1.1. Total thickness dependency of the surface spin flop 4.1.2. Influence of the surface block thickness 4.1.3. Exchange bias in synthetic antiferromagnets 4.2. Tailoring the surface spin flop 4.2.1. Coexistence of two vertical domain walls 4.2.2. Alternative anisotropy profile 5. Conclusions and Outlook A. Supplemental material A.1. Supplemental material for Section 2.4 A.2. Supplemental material for Section 4.1.1 A.3. Supplemental material for Section 4.1.2 A.4. Supplemental material for Section 4.1.3 A.5. Supplemental material for Section 4.2.1 A.6. Supplemental material for Section 4.2.2 A.7. Supplemental material for the outlook in Chapter 5 A.7.1. Synthetic ferrimagnets ans ferromagnetic resonance A.7.2. Synthetic antiferromagnets based on Co/Ni A.7.3. Initial magneto-resistance measurements A.8. Micromagnetic simulations MuMax3 code B. Bibliography C. List of Samples D. Selbstständigkeitserklärung E. Danksagung F. Lebenslauf G. Publikationsliste