Complete understanding of the physics underlying the changes in viscoelasticity, relaxation time, and phase transitions that mesoscopic fluid-like systems undergo at solid-liquid interfaces or under confinement remains one of the major challenges in condensed matter physics. Moreover, studies of confined mesoscopic fluid films are relevant to technological areas like adhesion, wetting processes and nanotribology. This thesis addresses the interaction between two sliding solids interfaces separated by a nanometer sized gap, with emphasis on the role of the mesoscopic fluid film trapped between them. For this purpose we integrated two acoustic techniques, recently introduced by our group, into a sub-nanometer precision and thermal drift corrected scanning probe microscope (SPM): the shear-force/acoustic near-field Microscope (SANM) and the whispering gallery acoustic sensing (WGAS). The SANM monitors the sound waves originating in the probe-layer interaction while the motion of the probe is monitored by the WGAS. Additionally, we decouple the interaction forces by using frequency modulation and measure the local tunneling current to help establish the location of the substrate. Our results show a strong correlation between the elastic component of the probe's interaction and the SANM amplitude, as well as between the phase lag response of the fluid relative to the probe's excitation (represented by the SANM phase) and the onset of the probe-sample contact region. Frequency modulation SANM-WGAS brings a new acoustic sensing mechanism to the challenging characterization of fluid-like physical systems at the nanometer scale.

<p>The work presented in this thesis concernes two sorts of nanostructures: energetic-ion-impact-induced surface tracks and gas-deposited WO₃ nanoparticles. Our aims to characterise these nanostuctures and understand the physical principles behind their formation are of general interests for basic science as well as of the field of nanotechnology.</p><p>AFM studies of irradiated organic surfaces showed that individual ion impacts generate craters, most often accompanied by raised plastically deformed regions. Crater sizes were measured as a function of ion stopping power and incidence angle on various surfaces. Observed crater volumes were converted into estimates of total sputtering yields, which in turn were correlated with data from collector experiments. The observations were compared to predictions of theoretical sputtering models. The observed plastic deformations above grazing-incidence-ion penetration paths agree with predictions of the pressure pulse model. However, closer to the ion track, evaporative sputtering can occur.</p><p>AFM images of gas-deposited WO₃ nanoparticle-films indicated the formation of agglomerates. The size distribution of the agglomerates was measured to be log-normal, <i>i.e.</i> similar to the size distribution of the gas-phase nanoparticles forming the deposit. By simulations we could relatively well reproduce this observation. The agglomerates exhibited high thermal stability below 250°C when considering their size, implying that these porous films can be useful in applications involving elevated temperatures in the 250°C range. The appearance of the nanoparticles in the tapping-mode AFM images was sensitive to the free amplitude of the oscillating tip. We could show by model calculations that the high adhesion between the tip and the sample could account for some of these observations.</p>

The work presented in this thesis concernes two sorts of nanostructures: energetic-ion-impact-induced surface tracks and gas-deposited WO3 nanoparticles. Our aims to characterise these nanostuctures and understand the physical principles behind their formation are of general interests for basic science as well as of the field of nanotechnology. AFM studies of irradiated organic surfaces showed that individual ion impacts generate craters, most often accompanied by raised plastically deformed regions. Crater sizes were measured as a function of ion stopping power and incidence angle on various surfaces. Observed crater volumes were converted into estimates of total sputtering yields, which in turn were correlated with data from collector experiments. The observations were compared to predictions of theoretical sputtering models. The observed plastic deformations above grazing-incidence-ion penetration paths agree with predictions of the pressure pulse model. However, closer to the ion track, evaporative sputtering can occur. AFM images of gas-deposited WO3 nanoparticle-films indicated the formation of agglomerates. The size distribution of the agglomerates was measured to be log-normal, i.e. similar to the size distribution of the gas-phase nanoparticles forming the deposit. By simulations we could relatively well reproduce this observation. The agglomerates exhibited high thermal stability below 250°C when considering their size, implying that these porous films can be useful in applications involving elevated temperatures in the 250°C range. The appearance of the nanoparticles in the tapping-mode AFM images was sensitive to the free amplitude of the oscillating tip. We could show by model calculations that the high adhesion between the tip and the sample could account for some of these observations.

Combined atomic force microscopy-scanning electrochemical microscopy (AFM-SECM) is capable of providing simultaneous topographical and electrochemical imaging at sample surfaces. Integration of amperometric biosensors at tip-integrated electrodes recessed from the apex of the AFM tip further enhances the versatility of such bifunctional probes. Of particular interest to this work was the detection of adenosine triphosphate (ATP) at a cellular level, since ATP is involved in many biologically relevant processes. There are challenges concerning the integration of biosensors into bifunctional AFM-SECM probes. This thesis focuses on addressing and advancing several of these limitations. Thin insulation layers are important for AFM-SECM based applications to enhance AFM and SECM performance. Plasma-polymerized fluorocarbon membranes are introduced as novel thin film insulation materials for AFM-SECM probes. Insulation layers with a thickness of < 300 nm were found to exhibit excellent insulating properties and satisfying temporal stability for successful application in AFM-SECM experiments. Furthermore new approaches for increasing the electrode area in conventionally focused ion beam (FIB) fabricated AFM-SECM probes were implemented, since enhancement of the current response in conjunction with biosensing experiments is required. Ion beam induced deposition (IBID) was used to generate platinum carbon (PtC) deposits at AFM-SECM probes, thereby successfully increasing the tip-integrated electrode area. PtC composites were thoroughly characterized in terms of their physical and electrochemical properties. Since a high carbon fraction in the PtC composite was inhibiting the charge transfer kinetics at the electrode surface for certain analytes, several pre-treatment strategies were investigated including annealing, UV/ozone treatment, and FIB milling. FIB milling proved to be the most promising procedure improving charge transfer properties at the electrode along with fabrication compatibility at AFM-SECM probes. The last part of this thesis aimed at providing fundamental studies on AFM-SECM application at live epithelial cell monolayers. AFM was used in different imaging modes to characterize the topography of epithelial cells. ATP detection at epithelial cells was achieved with amperometric biosensors combined with non-invasive SECM. Biosensors were further miniaturized at batch-fabricated AFM-SECM probes enabling laterally-resolved detection of ATP at epithelial cells. Additionally, PtC composite materials were evaluated for applicability as transducer platforms for enzymatic biosensors.

This dissertation focuses on quantitative subsurface and mechanical properties imaging potential of AFM probes. Extensive modeling of AFM probes are presented for thorough understanding of capabilities and limitations of current techniques, these models are verified by various experiments, and different methods are developed by utilizing force-sensing integrated read-out active tip (FIRAT), which is an active AFM probe with broad bandwidth. For quantitative subsurface imaging, a 3-D FEA model of AFM tip-sample contact is developed and this model can simulate AFM tip scan on nanoscale-sized buried structures. FIRAT probe, which is active and broadband, is utilized for interaction forces imaging during intermittent contact mode and mechanical characterization capability of this probe is investigated. It is shown that probe dynamics, stiffness, stiffness ambiguity, assumed contact mechanics, and noise are important parameters for the accuracy of mechanical properties imaging. An active tip control mechanism is introduced to limit contact forces during intermittent contact mode. In addition to these, a combined ultrasonic AFM and interaction forces imaging method is developed and modeled to solve the reduced elasticity measurement sensitivity on composite materials. This method is capable of imaging a broader range of elasticity on combination samples such as metal nanoparticles in polymers at nanoscale.

No description available.

Nanostructuring functional materials can lead to a variety of enhanced intrinsic material properties. In particular, nanowires (NWs) have large surface-to-volume ratio and large aspect ratio (length / diameter), which makes them sensitive to low-amplitude vibrations and have increased flexibility compared to the bulk form of the material. In this thesis, piezoelectric, ferroelectric, ferromagnetic and magnetoelectric (ME) NWs have been explored in the context of vibrational energy harvesting and magnetic energy harvesting and sensing; because of their increased piezoelectric coefficients and ME coupling compared to bulk. Low-temperature, solution-processable and hence scalable fabrication techniques have been used throughout this work. Electrochemical deposition or electrodeposition (ED) in conjunction with nanoporous templates i.e. template-assisted electrodeposition (TAED) have been used to grow piezoelectric zinc oxide (ZnO) and ferromagnetic nickel (Ni) NWs and three template-wetting based techniques have been used to grow ferroelectric poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) NWs and nanotubes (NTs). Both techniques have been optimised and subsequently combined to synthesise core-shell or (1-1) Ni - P(VDF-TrFE) composite NWs. The structural and crystalline properties of each type of nanostructure has been studied using a variety of techniques including: scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM) and all of the NWs have been shown to be polycrystalline. The energy harvesting performance of vertically aligned ZnO NW arrays embedded in flexible, polycarbonate (PC) templates when incorporated into a flexible nanocomposite nanogenerator (NG), has been tested via periodic impacting and flexing of the NG at different frequencies. The voltage ($V$), current ($I$) and power were recorded during testing and measured across a range of external load resistances. The aligned nature of the embedded NWs ensures good piezoelectric performance across the entire device under impacting, while the PC template ensures mechanical stability and longevity of the device, confirmed by good fatigue performance over 24 hours of continuous testing, which is rarely studied in this field. The power density ($P_\mathrm{d}$) was found to be 151 mW m$^{-3}$ for low-amplitude (0.68 mm) and low-frequency (5 Hz) impacting, resulting in energy conversion efficiencies ($\chi$) and device efficiencies ($\chi$') of $\approx$ 4.2 \% and $\approx$ 3.76 x 10$^{-3}$ \% respectively. The nanoscale or surface piezoelectric charge coefficient ($d_{33}$) was measured to be $\approx$ 12.5 pm V$^{-1}$ on an individual ZnO NW, using a combination of Kelvin probe force microscopy (KPFM) and non--destructive piezoresponse force microscopy (ND-PFM). Both nanoscale and bulk ME measurements have been performed on Ni - P(VDF-TrFE) ME composite (1-1) NWs, nanocomposite (1-3) films and (2-2) laminates. The latter two structures have been fabricated using TAED and ED for the Ni NW and film respectively, in combination with drop-casting and spin-coating for the P(VDF-TrFE) films. The scanning probe microscopy (SPM) measurements used here include atomic force microscopy (AFM), KPFM, magnetic force microscopy (MFM) and piezoresponse force microscopy (PFM) and it has been found that the ME coupling in the (1-1) composites NWs is enhanced compared to the other structures, confirmed by approximating the converse ME coupling coefficient ($\alpha^\mathrm{C}$) of each composite. Additionally, vibrating sample magnetometry (VSM) has been used to confirm the ferromagnetic nature of the Ni phases in the composite structures. ME composite devices based on (2-2) and (1-3) composite materials and have been fabricated and preliminary bulk ME measurements of the ME coupling coefficient ($\alpha^\mathrm{E}$) plus energy harvesting measurements have also been performed as a proof of concept that the nanoscale ME coupling translates to the bulk, to some extent.

Le graphène héberge un gaz d'électrons bi-dimensionnel, sujet à un potentiel électrostatique désordonné dû aux impuretés de charge dans le substrat. Ce potentiel désordonné induit des inhomogénéités de la densité de porteurs de charge dans le graphène. Par ailleurs, l'écrantage dans le graphène mono-feuillet de ce potentiel dépend lui-même de la densité de porteurs de charge. L'effet du désordre de charge peut donc être modulé avec un potentiel de grille global, ce qui se manifeste en particulier dans la transconductance de dispositifs à base de graphène. Nous combinons des mesures par Microscopie/Spectroscopie à effet tunnel avec des mesures de transport in situ sur des dispositifs à base de mono-feuillets de graphène sur SiO2, à basse température. Les cartes de la densité locale d'états du graphène, à diverses tensions de grille, mettent en évidence l'augmentation progressive des dimensions latérales ainsi que de l'amplitude des inhomogénéités au voisinage du point de Dirac. Alors que la dépendance en grille de la taille des inhomogénéités est en bon accord avec les prédictions, leur amplitude est plus forte qu'attendue au point de Dirac. Nous expliquons ce désaccord en prenant en compte l'effet de grille local produit par la pointe elle-même, qui a pour effet d'amplifier expérimentalement toute variation de la densité de porteurs de charge lorsque celle-ci elle faible. Cette expérience est ainsi la première mesure qui relie quantitativement les propriétés de désordre de charge à l'échelle microscopique aux propriétés de transport macroscopiques d'un dispositif à base de graphène. / Graphene presents a two-dimensional system whose charge carriers are subjected to a disordered potential created by random charge impurities trapped in the substrate. This impurity potential induces an inhomogeneous carrier concentration. On the other hand, the ability of single-layered graphene to screen this potential strongly depends on the charge carrier density. Thus the effect of the resulting charge disorder can be tuned with the backgate which manifests also in the transport properties of the device. By combining Scanning tunneling microscopy and spectroscopy with in-situ transport at dilution temperature, we probe a system of single-layered graphene on SiO2. Local density of states maps on graphene, acquired at various carrier concentrations show gradual increase of spatial extent and amplitude of inhomogeneities as the Dirac point is approached. While the variations of the spatial extent of the fluctuations with back-gate show very good agreement with predictions, the observed amplitude of inhomogeneities show a larger than expected increase at low densities. We explain this as a result of the local gating effect exerted by the tip on graphene which amplifies any change in the intrinsic doping at low carrier concentrations. This is the first experiment bridging the gap between microscopic disorder and macroscopic transport properties of a graphene device.

Ce travail s’inscrit dans le contexte des microscopies optiques à très haute résolution. Nous proposons un nouveau concept de sonde active pour la microscopie optique en champ proche (SNOM), exploitant les effets de génération de second harmonique (SHG) de molécules. L’idée développée vise à s'affranchir de l’une des principales limitations des sondes actives fluorescentes réalisées jusqu'à présent : l'accrochage des sondes à l'extrémité de la pointe SNOM, étape toujours délicate et souvent peu fiable. Pour ce faire, nous avons mis en œuvre une technique qui consiste à utiliser la localisation du champ électrique au sein d’une jonction pointe métallique-substrat conducteur immergée dans une solution de molécules non-linéaires dipolaires. L’interaction champ-molécules entraine l’orientation locale un nano-volume de ces molécules dont l’excitation par un laser permet ensuite la génération d’un signal de second harmonique. Après avoir validé ce concept dit de « nano-EFISHG » (Electric Field Induced SHG) nous avons conçu un nouveau banc expérimental, dédié à l'imagerie de second harmonique haute résolution : celui-ci a permis d'obtenir les premières images présentant un contraste de second harmonique sur un échantillon structuré à l'échelle micronique.Nous avons ensuite travaillé à l’optimisation de la résolution de l’expérience mise en place : nous avons notamment démontré la possibilité de tirer parti d’effets d’exaltation locale du champ électromagnétique se produisant à l'extrémité de pointes ou de nano-objets métalliques. L’extrapolation des résultats obtenus montre que de telles exaltations devraient permettre d’atteindre des résolutions de l’ordre de 50 nm. / This work was achieved within the context of high resolution optical microscopy. We propose a new concept of active probe for near-field optical microscopy (SNOM), exploiting the effect of second harmonic generation (SHG) of molecules. The idea intends to avoid one of the main limitations of currently developed fluorescent active probes: the anchoring of the probes at the end of a SNOM tip which is a very delicate and often unreliable step. The technique implemented here consists in using the electric field localization in a metallic tip – conducting substrate junction immersed in a solution containing dipolar non-linear molecules. The interaction between the molecules and the electric field gives rise to the local orientation of a nano-volume of these molecules whose excitation by a laser allows generation of a second harmonic signal.After validation of this concept named as “nanoEFISHG” (Electric Field Induced SHG) we have designed a new experimental setup, dedicated to high resolution second harmonic imaging. Successful implementation of this setup has leaded to the recording of the first images presenting a second harmonic contrast on a sample structured at the micronic scale. Next step has consisted in working towards optimization of the experimental resolution: we have especially study the possibility of taking advantages of local field enhancement effects occurring at metallic nano-structures or sharp tip’s apex. The extrapolation of the obtained results shows that such effects should allow to reach resolutions about 50 nm.

Skyrmions are spin textures with particle character that order themselves into so-called “skyrmion lattices” (SkLs). A skyrmion is topologically nontrivial, which adds stability against external perturbations and attracts tremendous interest from the theoretical side. Since skyrmions can be moved with small electrical currents, they are being discussed for novel spintronic applications, such as racetrack memory. Further interest has been spurred by the discovery of multiferroic compounds that also host SkLs, resulting in additional properties that are highly interesting both for applications and for fundamental research. The scope of this thesis encompasses the investigation of two completely different exemplary SkL-hosting multiferroic systems using a broad set of scanning probe microscopy techniques. These can probe multiple properties on a local scale in real space with a single measurement, examining details not resolved by non-local techniques. In the first part, there is a brief introduction to magnetic skyrmions and scanning probe microscopy with a short review of the theoretical background. The materials of interest and their known properties are then introduced. These are Cu2OSeO3, an insulator exhibiting the emergence of Bloch-type skyrmions as well as type-II multiferroicity, and the lacunar spinel chalcogenides, which were recently found to exhibit multiferroic Néel-type skyrmions pinned to magnetic easy-axes/planes together with type-I multiferroicity originating from a structural Jahn–Teller transition. The second part first presents various scanning probe studies and their results for Cu2OSeO3, where, aside from the magnetic textures of the various magnetic phases, the magnetoelectric effect and the magnetic phase transitions are investigated and described with basic theoretical models. Results show a good correlation between observations and theory, as well as with other experimental methods. Various lacunar spinels are then investigated, mostly GaV4S8 and GaMo4S8. Observation of the structural phase transition leads to the observation of {100}-type domain boundaries compatible with the compatibility critera based on crystal geometry. Furthermore, measurements of the magnetic textures of the different magnetic phases for GaV4S8 are presented and analysed. Results highlight a pinning of the pitch vector to the magnetic hard plane, and that the structural domain boundaries are by necessity magnetic domain boundaries. Analysing the influence of surface anisotropy and structural domain boundaries reveals a strong effect of both on the formation of magnetic patterns in their vicinity. Finally, the magnetoelectric effect of different lacunar spinels is investigated by measuring the surface potential with changing magnetic fields leading to a hysteretic behaviour in all materials.:Abstract/Kurzdarstellung 1 Introduction – Skyrmions meet Multiferroicity 2 Magnetic Skyrmion Lattices 2.1 What is a Skyrmion? 2.2 Formation of Skyrmion Lattices 2.2.1 Basic Considerations 2.2.2 Emergence of Skyrmion Lattices 2.3 General Properties of Skyrmions 2.4 Ways to Observe Skyrmions 3 Scanning Probe Microscopy 3.1 General Aspects 3.2 SPM in Contact Mode 3.2.1 Atomic Force Microscopy 3.2.2 Conductive Atomic Force Microscopy 3.2.3 Piezoresponse Force Microscopy 3.3 SPM in Non-Contact Mode 3.3.1 Atomic Force Microscopy 3.3.2 Kelvin Probe Force Microscopy 3.3.3 Magnetic Force Microscopy 3.4 About Scanning Dissipation Microscopy 3.4.1 Possible Origins of Dissipation 3.4.2 Measuring Dissipation 3.4.3 Mathematical Background 3.5 Experimental Setup 4 Investigated Materials 4.1 Cubic copper(II)-oxo-selenite Cu2O(SeO3) 4.2 Lacunar Spinel Chalcogenides 4.2.1 General Aspects and Materials Chosen 4.2.2 Structural Phase Transition and Expected Piezoresponse 4.2.3 Magnetic Phase Transition 4.2.4 Investigated Crystals 5 Investigations on Cu2OSeO3 5.1 Observing the Different Magnetic Phases 5.1.1 Analysis of Magnetic Textures with Magnetic Force Microscopy 5.1.2 Analysis of Magnetic Textures with Scanning Dissipation Microscopy 5.2 Analysis of the Magnetoelectric Effect 5.2.1 Observing the Magnetoelectric Effect with KPFM 5.2.2 Heuristic Description of the Magnetoelectric Effect 5.3 Analysing the Magnetic Phase Transitions with SPM 5.3.1 Motivation from Theory 5.3.2 Distinguishing the Helical, Conical and Field-Polarised Phases 5.3.3 The Helical–Conical Phase Transition 5.3.4 Passing through the Conical Phase 6 Investigations on GaV4S8 6.1 Observing the Structural Phase Transition 6.1.1 Results from nc-AFM 6.1.2 Results from ct-AFM and PFM 6.2 Observing the Magnetic Phases 6.3 Analysing the Magnetic SDM Images 6.3.1 Theoretical Considerations 6.3.2 Rescaling from the Measured to the Magnetic Hard Plane 6.3.3 Influence of the Surface on the Patterns Observed 6.4 Influence of Structural Domain Walls on Magnetic Patterns 7 Further Investigation on Lacunar Spinels 7.1 Investigations on GaMo4S8 7.1.1 Experimental Results 7.1.2 Theoretical Considerations 7.1.3 Evaluation of the Experimental Data 7.2 Magnetoelectric Effect of Lacunar Spinels 8 Remarks About Magnetic Non-Contact Dissipation 9 Summary and Outlook 9.1 Synopsis 9.2 Outlook – Probing the Future A Permissions For Usage of Content B Some Additional Information on Non-Contact Dissipation C Bonus Images Bibliography Publications Acknowledgements Erklärung / Skyrmionen sind teilchenartige Spintexturen, welche sich in sogenannten Skyrmionengittern anordnen. Jedes Skyrmion besitzt eine topologische Ladung. Dieses Konzept ist von bedeutendem Interesse für die Theorie und führt zu zusätzlicher Stabilität gegen externe Störungen. Da Skyrmionen mit geringen elektrischen Strömen bewegt werden können, sind sie auch Kanditaten für neuartige, spintronische Anwendungen wie den Racetrack-Speicher. Zusätzlich wurden vor einiger Zeit multiferroische Materialien entdeckt, welche ebenso Skyrmionengitter bilden und aufgrund dessen weitere, interessante Eigenschaften besitzen, welche sowohl für Anwendungen als auch für die Grundlagenforschung interessant sind. Inhalt dieser Dissertation ist die Untersuchung zweier verschiedener, exemplarischer multiferroischer Materialien mit Skyrmiongitterphasen mittels verschiedener Rastersondentechniken. Dies erlaubt das gleichzeitige Erfassen mehrerer Parameter auf einer lokalen Skala im Realraum mit einer einzigen Messung und somit die Untersuchung von Details, welche durch nicht-lokale Techniken nicht erfasst werden können. Im ersten Teil wird eine kurze Einleitung über magnetische Skyrmionen und die Rastersondenmikroskopie sowie Abrisse über deren theoretischen Hintergrund gegeben. Im Anschluß werden die untersuchten Materialien und deren Eigenschaften vorgestellt. Das erste System ist Cu2OSeO3, ein Isolator, welcher Bloch-artige Skyrmionengitter formiert und ein Typ-II Multiferroikum ist. Weitere Systeme gehören zur Klasse der lakunären Spinell-Chalkogenide, welche nach neuesten Erkenntnissen multiferroische Néel-artige Skyrmionen formieren, deren Modulationsvektor zur magnetisch harten Achse/Ebene fixiert ist. Ebenso sind diese aufgrund eines strukturellen Jahn-Teller Überganges Typ-I Multiferroika. Im zweiten Teil werden verschiedene Rastersondenuntersuchungen und ihre Ergebnisse präsentiert. Beginnend mit Cu2OSeO3, werden, neben den den magnetische Texturen der verschiedenen magnetischen Phasen, der magnetoelektrische Effekt und der helisch-konische Phasenübergang untersucht sowie mit grundlegenden theoretischen Modellen verglichen. Die Ergebnisse zeigen eine gute Übereinstimmung zwischen den Beobachtungen und der Theorie sowie mit anderen Meßmethoden. Im Anschluß werden verschiedene lakunäre Spinell-Chalkogenide, vor allem GaV4S8 und GaMo4S8, untersucht. Beobachtungen des strukturellen Phasenüberganges ergeben die Formierung von {100}-artigen Domänenwänden, welche mit den Vorhersagen der Kompatibilitätskriterien resultierend aus der Kristallgeometrie übereinstimmen. Des Weiteren werden Messungen der magnetischen Texturen der verschiedenen magnetischen Phasen von GaV4S8 präsentiert sowie analysiert. Die Ergebnisse heben hervor, daß der Modulationsvektor an der magnetisch harten Ebene fixiert ist und daß die strukturellen Domänengrenzen notwendigerweise auch die magnetischen Domänengrenzen sein müssen. Eine Analyse des Einflusses der Oberflächenanisotropie sowie der strukturellen Domänengrenzen zeigt eine starke Wirkung beider auf die Formierung magnetischer Texturen in ihrer Nähe. Schließlich wird der magnetoelektrische Effekt der lakunären Spinell-Chalkogenide durch Messung des Oberflächenpotentiales als Funktion des angelegten Magnetfeldes untersucht. Beobachtungen ergeben ein hysteretisches Verhalten in allen Materialen.:Abstract/Kurzdarstellung 1 Introduction – Skyrmions meet Multiferroicity 2 Magnetic Skyrmion Lattices 2.1 What is a Skyrmion? 2.2 Formation of Skyrmion Lattices 2.2.1 Basic Considerations 2.2.2 Emergence of Skyrmion Lattices 2.3 General Properties of Skyrmions 2.4 Ways to Observe Skyrmions 3 Scanning Probe Microscopy 3.1 General Aspects 3.2 SPM in Contact Mode 3.2.1 Atomic Force Microscopy 3.2.2 Conductive Atomic Force Microscopy 3.2.3 Piezoresponse Force Microscopy 3.3 SPM in Non-Contact Mode 3.3.1 Atomic Force Microscopy 3.3.2 Kelvin Probe Force Microscopy 3.3.3 Magnetic Force Microscopy 3.4 About Scanning Dissipation Microscopy 3.4.1 Possible Origins of Dissipation 3.4.2 Measuring Dissipation 3.4.3 Mathematical Background 3.5 Experimental Setup 4 Investigated Materials 4.1 Cubic copper(II)-oxo-selenite Cu2O(SeO3) 4.2 Lacunar Spinel Chalcogenides 4.2.1 General Aspects and Materials Chosen 4.2.2 Structural Phase Transition and Expected Piezoresponse 4.2.3 Magnetic Phase Transition 4.2.4 Investigated Crystals 5 Investigations on Cu2OSeO3 5.1 Observing the Different Magnetic Phases 5.1.1 Analysis of Magnetic Textures with Magnetic Force Microscopy 5.1.2 Analysis of Magnetic Textures with Scanning Dissipation Microscopy 5.2 Analysis of the Magnetoelectric Effect 5.2.1 Observing the Magnetoelectric Effect with KPFM 5.2.2 Heuristic Description of the Magnetoelectric Effect 5.3 Analysing the Magnetic Phase Transitions with SPM 5.3.1 Motivation from Theory 5.3.2 Distinguishing the Helical, Conical and Field-Polarised Phases 5.3.3 The Helical–Conical Phase Transition 5.3.4 Passing through the Conical Phase 6 Investigations on GaV4S8 6.1 Observing the Structural Phase Transition 6.1.1 Results from nc-AFM 6.1.2 Results from ct-AFM and PFM 6.2 Observing the Magnetic Phases 6.3 Analysing the Magnetic SDM Images 6.3.1 Theoretical Considerations 6.3.2 Rescaling from the Measured to the Magnetic Hard Plane 6.3.3 Influence of the Surface on the Patterns Observed 6.4 Influence of Structural Domain Walls on Magnetic Patterns 7 Further Investigation on Lacunar Spinels 7.1 Investigations on GaMo4S8 7.1.1 Experimental Results 7.1.2 Theoretical Considerations 7.1.3 Evaluation of the Experimental Data 7.2 Magnetoelectric Effect of Lacunar Spinels 8 Remarks About Magnetic Non-Contact Dissipation 9 Summary and Outlook 9.1 Synopsis 9.2 Outlook – Probing the Future A Permissions For Usage of Content B Some Additional Information on Non-Contact Dissipation C Bonus Images Bibliography Publications Acknowledgements Erklärung