Magnetisation dynamics and tuneable GHz properties of unsaturated magnetic nanostructures

In this Thesis, investigations on spin wave propagation in sufficiently thick, magnetic unsaturated nanostructures of various shapes have been carried out. Analytical, numerical and experimental techniques in the time and the frequency domains have been used throughout this thesis. Unsaturated magnetic states are of high interest, more specifically the magnetic vortex configuration since the remanent state is typically dominated and stabilised by the shape anisotropy of the structure, with no need of strong or any bias fields. Due to the diversity of magnetic inhomogeneities, a high degree of reconfigurability by applying external low bias fields can be obtained. On the other hand, the inhomogeneous magnetic landscape becomes, very often, difficult to model analytically and therefore, propagating spin waves can turn out to be complicate to control in practice. The aim of this work has been twofold. Firstly, to explore the most prominent magnetisation dynamics found in thick enough magnetic patches, or ‘2.5-dimensional’ nanostructures. The explored propagating modes are exchange-dominated spin waves in the range of GHz, which allows us to obtain very short wavelength spin waves that can propagate along different ‘paths’ in the unsaturated landscape. The ‘thickness and shape-induced’ enhancing of these spin waves in structures in a flux closure configuration, suggests their use as highly tuneable spin wave emitters. Secondly, analytical and mathematical models are proposed for controlling the spin wave propagation in multidomain structures of various shapes and in domain walls. Their magnetic configuration is dominated by the shape anisotropy of the patch, which allows us to design particularized shapes to control the spin wave wavenumber (or equivalently, wavelength) while it propagates. Also, the modes are shown to be sensitive to bias fields, which further enhances their tuneability and reconfigurability. All this previous work is joint in the last part of the Thesis, more focused on potential applications for Magnonics, where spin waves are not necessarily seen as undesirable energy loss mechanisms in magnetic structures but as information carriers or the base of novel computing paradigms. Single, or interconnected unsaturated elements of various shapes are proposed to be the base of interesting highly tuneable spin wave devices.Engineering and Physical Sciences Research Council (EPSRC

A TEM investigation of controlled magnetic behaviour in thin ferromagnetic films

Understanding the magnetic behaviour of thin film elements is of major importance for the magnetic sensor and storage industries, but also for fundamental micromagnetics. To store digital information, each memory element must support two distinct remanent magnetisation configurations that can be switched between using an applied field. In magnetoresistive random access memory (MRAM), a low switching field and reproducible reversal behaviour are desirable properties. The low field keeps the power consumption to a minimum and the reproducility enables efficient writing and read back of data. However, simple geometric structures are able to support a variety of metastable remanent configurations which can be problematic for device applications. For example, with rectangular elements, the switching fields are history dependent, and there is the possibility of flux-closure formation on repeated switching. This means different field strengths may be required to reverse the magnetisation of the same bit (binary digit) during different field cycles, and the information stored in a cell could be accidentally lost. In addition, the miniaturisation of these elements faces the problem that the coercivity is inversely proportional to element width for a given thickness; a factor which limits their use in high density arrays. The optimum geometry for supporting the stored information is therefore an important issue. In this thesis, different element shapes designed to tackle these problems have been investigated using transmission electron microscopy (TEM) backed by micromagnetic simulations. It has been found that variations in element geometry and symmetry can lead to a greater control of the states that can be formed. Alongside this work on patterned elements, continuous film multilayer samples in the form of magnetic tunnel junctions (MTJs) have also been studied. These multilayer structures serve as storage cells in MRAM devices so their successful operation is of the utmost importance to the development of this technology. At the most basic level, MTJs comprise two ferromagnetic layers separated by a layer of electrical insulator. Whilst one magnetic layer is fixed (pinned layer), the other is free to switch direction when an external field is applied (free layer). Ideally the free layer hysteresis loop would be centred at zero field, but because of magnetostatic interactions caused by layer roughness, the ferromagnets couple to one another and the hysteresis loop is offset. This shift means that the fields required to switch the cell in opposite directions are different. In collaboration with Philips Research in Eindhoven, the magnetic and physical structure of new MTJ stacks incorporating an artifical antiferromagnet (AAF) in the free layer were studied using TEM. An AAF consists of two ferromagnetic layers coupled anti-parallel through a thin layer of non-magnetic metal, typically Ru. These samples were found to reduce the offset field by up to 36% when compared to the basic MTJ stack. Whilst this research is valuable to the magnetic storage industry, the information it provides on these complicated magnetic systems is equally beneficial for solid state physics

Statistics of gradient directions in natural images.

Interest in finding statistical regularities in natural images has been growing since the advent of information theory and the advancement of the efficient coding hypothesis that the human visual system is optimised to encode natural visual stimuli. In this thesis, a statistical analysis of gradient directions in an ensemble of natural images is reported. Information-theoretic measures have been used to compute the amount of dependency which exists between triples of gradient directions at separate image locations. Control experiments are performed on other image classes: phase randomized natural images, whitened natural images, and Gaussian noise images. The main results show that for an ensemble of natural images the average amount of de pendency between two and three gradient directions is the same as for an ensemble of phase randomized natural images. This result does not extend to i) the amount dependency between gradient magnitudes, ii) gradient directions at high gradient magnitude locations, or iii) individual natural images. Furthermore, no significant synergetic dependencies are found between triples of gradient directions in an ensemble natural images a synergetic dependency is an increase in dependency between a pair of gradient directions given the interaction of a third gradient direction. Additional experiments are performed to establish both the generality and specificity of the main results by studying the gradient direction dependencies of ensembles of noise (random phases) images with varying power law power spectra. The results of the additional experiments indicate that, for ensembles of images with varying power law power spectra, the amount of dependency between two and three gradient directions is determined by the ensemble's mean power spectrum rather than the phase spectrum. A framework is also presented for future work and preliminary results are provided for the dependency between second order derivative measurements (shape index) for up to 9-point configurations

Macroscopic and microscopic investigation of spin reorientation of iron tin

The work reported in this thesis examines the spin reorientation transition in Fe3Sn2 using a combination of microscopic and macroscopic techniques. Macroscopic measurement of single crystal Fe3Sn2 was undertaken using a SQUID (superconducting quantum interference device) magnetometer to measure the change in magnetisation along the Kagome plane. The domain structure on the surface of the crystal was measured using magnetic force microscopy (MFM) over a range of temperatures. The combination of techniques used for measuring the bulk and microscopic properties increases the understanding of first order spin reorientation transitions. The bulk magnetisation measurement of single crystal Fe3Sn2 displayed features that are typical of a first order phase transition. The magnetisation measured along the Kagome plane on warming and cooling differed in magnitude and a first order jump in magnetisation was also observed on cooling. This highlighted the hysteretic nature of the phase transition. The hysteresis and first order jump arises from the mechanism of the spin reorientation. On cooling through the spin reorientation temperature the sample is supercooled until the low temperature phase nucleates, after the nucleation the low temperature phase grew rapidly leading to a cascade effect and a first order jump in magnetisation. No evidence of a first order jump on warming was observed and therefore no significant superheating occurred. The lack of superheating is due to remanent high temperature phase at 2 K from an incomplete phase transition. The remanent phase provided ‘seeds’ for the growth of the high temperature phase, reducing the activation energy. The variable temperature magnetic force microscopy (MFM) measurements allowed the changes in the domain structure to be examined as the crystal underwent a spin reorientation transition. The evolution of the domain structure during the phase transition allowed the mechanism of the phase transition to be observed. On cooling from the high temperature phase, a fine structure associated with the low temperature phase formed within the branches of the domains associated with the high temperature structure. The fine structure grew with further cooling as a greater portion of the sample underwent the spin reorientation transition. Evidence of a thermal hysteresis was also observed in the domain images on warming and cooling, with images at a given temperature having different domain structures. The development of a fine structure was only observed on cooling, the absence of a fine structure on warming confirms that remanent high temperature phase prevents superheating on warming. The combination of macroscopic and microscopic results has allowed the phase diagram for the spin reorientation transition to be determined and the phase coexistence region to be determined.Open Acces

Videos in Context for Telecommunication and Spatial Browsing

The research presented in this thesis explores the use of videos embedded in panoramic imagery to transmit spatial and temporal information describing remote environments and their dynamics. Virtual environments (VEs) through which users can explore remote locations are rapidly emerging as a popular medium of presence and remote collaboration. However, capturing visual representation of locations to be used in VEs is usually a tedious process that requires either manual modelling of environments or the employment of specific hardware. Capturing environment dynamics is not straightforward either, and it is usually performed through specific tracking hardware. Similarly, browsing large unstructured video-collections with available tools is difficult, as the abundance of spatial and temporal information makes them hard to comprehend. At the same time, on a spectrum between 3D VEs and 2D images, panoramas lie in between, as they offer the same 2D images accessibility while preserving 3D virtual environments surrounding representation. For this reason, panoramas are an attractive basis for videoconferencing and browsing tools as they can relate several videos temporally and spatially. This research explores methods to acquire, fuse, render and stream data coming from heterogeneous cameras, with the help of panoramic imagery. Three distinct but interrelated questions are addressed. First, the thesis considers how spatially localised video can be used to increase the spatial information transmitted during video mediated communication, and if this improves quality of communication. Second, the research asks whether videos in panoramic context can be used to convey spatial and temporal information of a remote place and the dynamics within, and if this improves users' performance in tasks that require spatio-temporal thinking. Finally, the thesis considers whether there is an impact of display type on reasoning about events within videos in panoramic context. These research questions were investigated over three experiments, covering scenarios common to computer-supported cooperative work and video browsing. To support the investigation, two distinct video+context systems were developed. The first telecommunication experiment compared our videos in context interface with fully-panoramic video and conventional webcam video conferencing in an object placement scenario. The second experiment investigated the impact of videos in panoramic context on quality of spatio-temporal thinking during localization tasks. To support the experiment, a novel interface to video-collection in panoramic context was developed and compared with common video-browsing tools. The final experimental study investigated the impact of display type on reasoning about events. The study explored three adaptations of our video-collection interface to three display types. The overall conclusion is that videos in panoramic context offer a valid solution to spatio-temporal exploration of remote locations. Our approach presents a richer visual representation in terms of space and time than standard tools, showing that providing panoramic contexts to video collections makes spatio-temporal tasks easier. To this end, videos in context are suitable alternative to more difficult, and often expensive solutions. These findings are beneficial to many applications, including teleconferencing, virtual tourism and remote assistance

Ferromagnetism in highly perfect metallic platelets

Imperial Users onl