In der vorliegenden Arbeit wurden die mikromagnetische Struktur sowie das Ummagnetisierungsverhalten epitaktisch gewachsener MnAs Filme auf dem Substrat GaAs untersucht. Im Mittelpunkt steht die mikromagnetischen Struktur von anisotrop erspannten MnAs Filmen auf GaAs(001). Die Verspannung führt zur selbstorganisierten Anordnung ferromagnetischer Streifen. Ihre Domänenstruktur wurde mittels MFM (magnetischer Kraftmikroskopie) bestimmt und mit den Resultaten der XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy) verglichen. Um eine vollständige Charakterisierung der mikromagnetischen Eigenschaften der Streifenstruktur zu erreichen, wurden die MFM Experimente in einem äusseren Magnetfeld durchgeführt. Die Beantwortung der zentralen Frage nach der Domänenstruktur ist mit der Entwicklung eines mikromagnetischen Simulators für dreidimensionale magnetische Strukturen auf mesoskopischer Skala gelungen. Die Stabilität der dreidimensionalen mikromagnetischen Struktur hängt von den Eigenschaften der selbstorganisierten Streifenstruktur ab, d.h. sowohl von der Filmdicke als auch vom Verhältnis ihrer Breite zur Filmdicke - und damit der Temperatur. Durch die Erkenntnis, dass eine magnetische Struktur in der Tiefe des Streifens vorhanden ist, können die verbleibenden Unterschiede in den XMCDPEEM- und MFM-Resultaten erklärt werden. Durch die Simulationsergebnisse in Kombination mit den Experimenten wird eine widerspruchsfreie Deutung der mikromagnetischen Struktur sowie deren Ummagnetisierungsverhalten ermöglicht. Zudem wird die mikromagnetische Struktur von MnAs auf GaAs(111) simuliert und damit das Verständnis der mikromagnetischen Strukturen auf alle vorhandenen Substratorientierungen vervollständigt. / In the present thesis the micromagnetic structure, as well as the magnetization reversal, of epitaxial MnAs films on GaAs substrates are studied. The investigation is focused on the micromagnetic structure of anisotropically strained MnAs films on GaAs(001). The strain originates a selforganized array of ferromagnetic stripes. The magnetic domains were investigated using MFM (magnetic force microscopy) and the results were compared with XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy). To completely characterize the micromagnetic properties of the stripe structure, MFM experiments were performed in the presence of an external field. To unambiguously determine the domain structure a three-dimensional micromagnetic simulator was developed capable to calculate magnetic structures with mesoscopic dimensions. The stability of the three-dimensional micromagnetic structure depends on the properties of the selforganized stripe structure, i.e., on the film thickness as well as on the ratio of the stipe width to thickness - and thus the temperature. Taking into account the magnetization distribution in-depth, the remaining differences between the XMCDPEEM and the MFM results can be explained by the disturbing effect of the MFM tip. The results of the micromagnetic simulations, in combination with the experimental results, allow for a determination of the micromagnetic structure in an applied field throughout the phase coexistence regime. Moreover, the micromagnetic structure of MnAs films on GaAs(111) is simulated and thus the understanding of the micromagnetic properties have been extended on all substrate orientations.

In the current study, we give a report when oxalic acid was used as an electrolyte to synthesize an AAO template with hexagonal pore array. Optimum parameters were observed as 0.4 M of oxalic acid, anodizing voltage of 45 V, temperature of approximately 8 °C and the period of 120 minutes. Atomic force microscope (AFM) and High resolution scanning electron microscope (HRSEM) showed that template has an average pore diameter of 103 nm. Co and MnOx (x = 1,2) nanostructures were selectively deposited in the pores of the template using a novel atomic layer deposition (ALD) technique. The diameter sizes and the array of the nanostructures and the template were corresponding. Energy dispersive xrays (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co and MnOx (x =1,2) on the samples while x-ray diffraction (XRD) provided an indication of their orientations. Magnetic force microscopy as main characterization tool showed the existence of multi-domains on both Co and MnOx (x =1,2) nanostructures.

Local studies of magnetic flux line (vortex) distribution in superconducting thin films and their pinning by natural and artificial defects have been performed using low-temperature magnetic force microscopy (LT-MFM). Taken a 100 nm thin NbN film as an example, the depinning of vortices from natural defects under the influence of the force that the MFM tip exerts on the individual vortex was visualized and the local pinning force was estimated. The good agreement of these results with global transport measurements demonstrates that MFM is a powerful and reliable method to probe the local variation of the pinning landscape. Furthermore, it was demonstrated that the presence of an ordered array of 1-μm-sized ferromagnetic permalloy dots being in a magneticvortex state underneath the Nb film significantly influences the natural pinning landscape of the superconductor leading to commensurate pinning effects. This strong pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located at each dot. Additionally, for industrially applicable YBa$_2$Cu$_3$O$_{7-\delta} thin films the main question discussed was the possibility of a direct correlation between vortices and artificial defects as well as vortex imaging on rough as-prepared thin films. Since the surface roughness (droplets, precipitates) causes a severe problem to the scanning MFM tip, a nanoscale wedge polishing technique that allows to overcome this problem was developed. Mounting the sample under a defined small angle results in a smooth surface and a monotonic thickness reduction of the film along the length of the sample. It provides a continuous insight from the film surface down to the substrate with surface sensitive scanning techniques.

In the current study, we give a report when oxalic acid was used as an electrolyte to synthesize an AAO template with hexagonal pore array. Optimum parameters were observed as 0.4 M of oxalic acid, anodizing voltage of 45 V, temperature of approximately 8 °C and the period of 120 minutes. Atomic force microscope (AFM) and High resolution scanning electron microscope (HRSEM) showed that template has an average pore diameter of 103 nm. Co and MnOx (x = 1,2) nanostructures were selectively deposited in the pores of the template using a novel atomic layer deposition (ALD) technique. The diameter sizes and the array of the nanostructures and the template were corresponding. Energy dispersive xrays (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co and MnOx (x =1,2) on the samples while x-ray diffraction (XRD) provided an indication of their orientations. Magnetic force microscopy as main characterization tool showed the existence of multi-domains on both Co and MnOx (x =1,2) nanostructures.

>Magister Scientiae - MSc / In the current study, we give a report when oxalic acid was used as an electrolyte to synthesize an AAO template with hexagonal pore array. Optimum parameters were observed as 0.4 M of oxalic acid, anodizing voltage of 45 V, temperature of approximately 8 °C and the period of 120 minutes. Atomic force microscope (AFM) and High resolution scanning electron microscope (HRSEM) showed that template has an average pore diameter of 103 nm. Co and MnOx (x = 1,2) nanostructures were selectively deposited in the pores of the template using a novel atomic layer deposition (ALD) technique. The diameter sizes and the array of the nanostructures and the template were corresponding. Energy dispersive xrays (EDX) and X-ray photoelectron spectroscopy (XPS) confirmed the presence of Co and MnOx (x =1,2) on the samples while x-ray diffraction (XRD) provided an indication of their orientations. Magnetic force microscopy as main characterization tool showed the existence of multi-domains on both Co and MnOx (x =1,2) nanostructures. / South Africa

Local studies of magnetic flux line (vortex) distribution in superconducting thin films and their pinning by natural and artificial defects have been performed using low-temperature magnetic force microscopy (LT-MFM). Taken a 100 nm thin NbN film as an example, the depinning of vortices from natural defects under the influence of the force that the MFM tip exerts on the individual vortex was visualized and the local pinning force was estimated. The good agreement of these results with global transport measurements demonstrates that MFM is a powerful and reliable method to probe the local variation of the pinning landscape. Furthermore, it was demonstrated that the presence of an ordered array of 1-μm-sized ferromagnetic permalloy dots being in a magneticvortex state underneath the Nb film significantly influences the natural pinning landscape of the superconductor leading to commensurate pinning effects. This strong pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located at each dot. Additionally, for industrially applicable YBa$_2$Cu$_3$O$_{7-\delta} thin films the main question discussed was the possibility of a direct correlation between vortices and artificial defects as well as vortex imaging on rough as-prepared thin films. Since the surface roughness (droplets, precipitates) causes a severe problem to the scanning MFM tip, a nanoscale wedge polishing technique that allows to overcome this problem was developed. Mounting the sample under a defined small angle results in a smooth surface and a monotonic thickness reduction of the film along the length of the sample. It provides a continuous insight from the film surface down to the substrate with surface sensitive scanning techniques.

Magnetoelectricity (ME) is a physical property that results from an exchange between polar (electric dipole) and spin (magnetic dipole) subsystem: i.e., a change in polarization (P) with application of magnetic field (H), or a change in magnetization (M) with applied electric field (E). Magnetoelectricity can be found both in single phase and composite materials. Compared with single phase multiferroic materials, composite multiferroics have higher ME effects. Through a strictive interaction between the piezoelectricity of the ferroelectric phase and the magnetostriction of the ferromagnetic phase, said multiferroic composites are capable of producing relatively large ME coefficients. This Dissertation focused on the deposition and characterization of two-phase composite magnetoelectric thin films. First, single phase ferroelectric thin films were studied to improve the multiferroic properties of the composite thin films. Then structural, ferroelectric, ferromagnetic, and magnetoelectric properties of composite thin films were researched. Finally, regular nano-array composite films were deposited and characterized. First, for single phase ferroelectric thin films, the phase stability was controlled by epitaxial engineering. Because ferroelectric properties are strongly related to their crystal structure, it is necessary to study the crystal structures in single phase ferroelectric thin films. Through constraint of the substrates, the phase stability of the ferroelectric thin films were able to be altered. Epitaxial thin-layers of Pb(Fe1/2Nb1/2)O3 (or PFN) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, orthorhombic, and rhombohedral respectively. The larger constraint stress induces higher piezoelectric constants in tetragonal PFN thin film. Epitaxial thin-layers of Pb(Zr0.52Ti0.48)O3 (or PZT) grown on (001), (110), and (111) SrTiO3 substrates are tetragonal, monoclinic C, and rhombohedral respectively. Enhanced ferroelectric properties were found in the low symmetry monoclinic phase. A triclinic phase in BFO was observed when it was deposited on tilted (001) STO substrates by selecting low symmetry (or interim) orientations of single crystal substrates. Then, in two phase composite magnetoelectric thin films, the morphology stability was controlled by epitaxial engineering. Because multiferroic properties are strongly related to the nano-structures of the composite thin films, it is necessary to research the nano-structures in composite thin films. Nano-belt structures were observed in both BaTiO3-CoFe2O4 and BiFeO3-CoFe2O4 systems: by changing the orientation of substrates or annealing condition, the nano-pillar structure could be changed into nano-belts structure. By doing so, the anisotropy of ferromagnetic properties changes accordingly. The multi-ferroic properties and magnetoelectric properties or (001), (110) and (111) self-assembled BiFeO3-CoFe2O4 nano-composite thin film were also measured. Finally, the regular CoFe2O4-BiFeO3 nano-array composite was deposited by pulsed laser deposition patterned using a focused ion beam. Top and cross-section views of the composite thin film showed an ordered CoFe2O4 nano-array embedded in a BiFeO3 matrix. Multiferroic and magnetoelectric properties were measured by piezoresponse force microscopy and magnetic force microscopy. Results show (i) switching of the magnetization in ferromagnetic CoFe2O4 and of the polarization in ferroelectric BiFeO3 phases under external magnetic and electric field respectively, and (ii) changes of the magnetization of CoFe2O4 by applying an electric field to the BiFeO3 phase. / Ph. D.

Iron filled carbon nanotubes (FeCNT) can be described as carbon nanotubes which contain an iron nanowire of several micrometers length and a diameter of approximately 10-100 nm. The carbon shells protect the iron core from oxidation and mechanical damage thus enabling a wide range of applications that require a long-term stability. The magnetic properties of the enclosed nanowire are in part determined by its small size and elongated shape. Magnetic force microscopy (MFM) measurements show that the iron wire exhibits a single domain behavior. Due to the large shape anisotropy it is magnetized along the long wire axis in the remanent state. Two magnetic monopoles of opposing polarity are located at the wire extremities. Depending on the structure and geometry of the individual nanowire, switching fields in the range of 100-400 mT can be found when the external field is applied along the FeCNT’s easy axis. Cantilever magnetometry shows that the switching can be attributed to a thermally assisted magnetization reversal mechanism with the nucleation and propagation of a domain wall. The defined magnetic properties of individual FeCNT combined with their mechanical strength make them ideal candidates for an application as high resolution high stability MFM probes. The fabrication of such probes can be achieved with the help of a micromanipulation setup in a scanning electron microscope. FeCNT MFM probes achieve a sub 25 nm lateral magnetic resolution. MFM measurements with FeCNT MFM probes in external fields show that the magnetization of these probes is exceptionally stable compared to conventional coated MFM probes. This greatly simplifies the data evaluation of such applied field MFM measurements. The emphasis of this work was put on the calibration of FeCNT probes to enable straightforward quantitative MFM measurements. The defined shape of the magnetically active iron nanowire allows an application of a point monopole description. Microscale parallel current carrying lines that produce a defined magnetic field are used as calibration structures to determine the effective magnetic moment of different MFM probes. The line geometry is varied in order to produce multiple magnetic field decay lengths and investigate the influence on the effective probe moment. The results show that while the effective magnetic monopole moment of a conventional MFM probe increases with an increasing sample stray field decay length, the effective moment of a FeCNT MFM probe remains constant. This enables a MFM probe calibration that stays valid for a large variety of magnetic samples. Furthermore, the fitted monopole moment of a FeCNT probe (in the order of 10E-9 Am) is consistent with the moment calculated from the nanowire geometry and the saturation magnetization of iron.

Die vorliegende Arbeit befasst sich mit der Untersuchung der mikromagnetischen Domänenstruktur und des gekoppelten magneto-strukturellen Phasenübergangs dünner epitaktischer MnAs-Filme auf GaAs. Im Besonderen wird der Einfluss der Substratorientierung, der Filmdicke und eines externen magnetischen Feldes auf die magnetischen und strukturellen Eigenschaften untersucht. Dabei kommen die komplementären Untersuchungsmethoden AFM (atomic force microscopy) / MFM (magnetic force microscopy) und LEEM (low energy electron microscopy) / XMCDPEEM (X-ray magnetic circular dichroism photoemission electron microscopy) zum Einsatz. Im Zuge des Phasenübergangs erster Ordnung zeigen MnAs Filme auf GaAs (001) und (311)A eine regelmäßige Anordnung ferromagnetischer alpha-MnAs und paramagnetischer beta-MnAs Streifen. Die Breite der Streifen ist eine Funktion der Temperatur, während die Periodizität eine lineare Funktion der Filmdicke ist. Die Domänenstruktur hängt stark von der Breite bzw. dem Abstand der ferromagnetischen Streifen ab, da diese direkt die Formanisotropie bzw. die magnetische Kopplung beeinflussen. Die Domänenstrukturen wird, abhängig von der Zahl der Subdomänen entlang der leichten Magnetisierungsrichtung, klassifiziert, wobei bis zu drei elementare Domänentypen beobachtet werden. Bei MnAs-Filmen die auf der GaAs (111)B Oberfläche gewachsen wurden, führt die Epitaxie zu einem geänderten Spannungszustands des Films, wobei eine erhöhte Phasenübergangstemperatur beobachtet wird. Durch temperaturabhängige XMCDPEEM-, AFM- und MFM-Messungen kann gezeigt werden, daß durch den lokalen Abbau der Verspannung in der Nähe eines Risses die Phasenübergangstemperatur lokal erhöht ist. Um Ummagnetisierungsprozesse auf einer mikroskopischen Skala untersuchen zu können und um den Einfluß eines magnetischen Feldes auf die Domänenstruktur sichtbar zu machen, wurde das temperaturvariable Rastersondenmikroskop um einen variablen Magnetfeldaufbau ergänzt. / This work presents the study of the micromagnetic domain structure and the coupled magneto-structural phase transition of epitaxial MnAs thin films on GaAs. In particular, the influence of substrate orientation, film thickness and external magnetic field on the magnetic and structural properties are investigated, employing the complementary measurement techniques atomic force microscopy (AFM) / magnetic force microscopy (MFM) and low energy electron microscopy (LEEM) / X-ray magnetic circular dichroism photoemission electron microscopy (XMCDPEEM. In the course of the first-order phase transition MnAs films on GaAs (001) and (311)A substrates show a regular array of ferromagnetic alpha- and paramagnetic beta-MnAs stripes. The width of the ferromagnetic stripes are a function of the temperature, whereas the periodicity of the stripe pattern is a function of the film thickness. The domain structure strongly depends on the width and the distance of the ferromagnetic stripes, as it directly affects the shape anisotropy and magnetic coupling, respectively. The domain patterns are classified depending on the number of subdomains along the easy axis direction. Up to three basic domain types can be distinguished. For MnAs films grown on GaAs (111)B, the epitaxy leads to a different strain state of the film, resulting in polygonal ferromagnetic structures embedded in a honeycomb-like paramagnetic network, and a higher phase transition temperature. Using temperature-dependent AFM, MFM and XMCDPEEM it is shown that the local strain relaxation in the vicinity of cracks in the MnAs film results in a locally increased phase transition temperature. In order to study magnetization reversal processes on a microscopic scale, as well as the influence of the magnetic field on the domain structure, a variable-magnetic field set-up is employed.

Welding of duplex stainless steel pipeline material for the oil and gas industry is now common practice. To date, research has been conducted primarily on the parent material and heat affected zones in terms of its susceptibility to various forms of corrosion. However, there has been little research conducted on the degree of sensitisation of the various successive weld layers, namely the root, fill and cap layers. The focus of this research study was to: (i) provide an in-depth microstructural analysis of the various weld passes, (ii) study the mechanical properties of the weld regions; (iii) determine degree of sensitisation of the various weld passes; and (iv) investigate the residual stress levels within the various regions/ phases of the welds. Four test conditions were prepared using manual Gas Tungsten Arc Welding with 'V' and 'U' bevel configuration. Structural analysis consisted of (i) optical microscopy, scanning electron microscopy and magnetic force microscopy; (ii) ferrite determination using Magna-Gauge, Fischer Ferrite-scope and Point Count method. Mechanical testing consisted of Vickers hardness measurements, Charpy impact studies and transverse tensile testing. The degree of sensitisation was determined by three test methods: a modified ASTM A262, ASTM A923 and a modified Double Loop Electrochemical Potentiodynamic Reactivation (DL-EPR) test. Residual stress levels were determined using two neutron diffraction techniques: a reactor source and a time of flight spallation source. Microstructure observed by optical microscopy and magnetic force microscopy shows the formation of both fine and coarse structures within the weld metal. There was no evidence of secondary austenite, being present in any of the weld metal conditions examined. In addition, no detrimental intermetallic phases or carbides were present. The DL-EPR test results revealed that the fill layer regions for all four conditions and the base material showed the highest values for Ir/Ia and Qr/Qa. All four test conditions passed the ASTM A262 and A923 qualitative type tests, even under restricted and modified conditions. Residual stress measurements by neutron diffraction conducted at Lucas Heights Hi-Flux Reactor revealed that the ferrite phase stress was tensile in the heat affected zones and weld, and appeared to be balanced by a local compressive austenite phase stresses in the normal and transverse directions. Residual stress measurements by neutron diffraction conducted at Los Alamos Nuclear Science Centre revealed that in the hoop direction, ferrite (211) and austenite (311) exhibit tensile strains in the weld. In the axial and radial direction, the strains for both phases were more compressive. Correlations between the degree of sensitization and microstructural changes / ferrite content were observed. Higher degrees of sensitization (Ir/Ia and Qr/Qa) were associated with reduced ferrite (increased austenite) content. Correlations between the stresses generated, the evolved microstructures and degree of sensitization were evident. Stresses within the cap region were generally shown to be of a tensile nature in the transverse and longitudinal direction. In summary, the study has shown that correlations exist between the weld microstructure, susceptibility to sensitisation and levels / distribution of internal stresses within the weld regions.