Domain walls may be treated as single entities that can be used to convey bits of information in potential magnetic memory devices such as magnetic logic and racetrack memory, which use controlled domain wall movement in complex magnetic nanowire networks. Greater understanding and control over certain aspects of their behaviour is required, however, before such devices can be realised. The structure of magnetic domain walls in ferromagnetic nanowires is both material and geometry dependent, thus providing an extremely large parameter space to explore. The structure and control of magnetic domain walls in permalloy nanostructures is investigated throughout this thesis using static and pulsed magnetic fields in combination with deliberately fabricated pinning features using Lorentz microscopy. The effect of an abrupt corner on the structure of domain walls is investigated with the use of a castellated wire geometry. Two different domain wall structures are observed at the corners of the wire, and a completely different domain wall is observed in a straight segment of the wire. Additionally, the reversal behaviour observed is entirely different depending on the direction of the applied field. Reproducibility experiments are also performed to asses the suitability of this geometry for potential use in magnetic memory applications. The ability to control the behaviour of both vortex and transverse domain walls in nanowires with deliberately fabricated defects is also explored, using a range of domain wall nucleation techniques. The magnetic spin structure of a vortex domain wall is completely different from a transverse domain wall and as a result their interaction with deliberately fabricated pinning sites differs greatly. Both domain wall types possess a chirality or sense of rotation. In wider wires however, transverse domain walls also possess an asymmetry which acts as an additional degree of freedom in the interaction with a trap. The result of both increasing and decreasing the wire width on a domain wall is investigated by patterning a single or double notch or anti-notch along a permalloy wire. The propagation of a magnetic domain wall along both a gently tapered and straight wire under the application of a pulsed magnetic field is also investigated, where a distorted vortex domain wall structure is observed to form. The distance a domain wall travels following the application of a short field pulse is measured and a lower limit of the velocity is calculated. Additionally, a domain wall is observed to undergo a number of changes in structure and chirality as it is moved along a wire under pulsed fields. The wire edge roughness also has a significant effect on the domain wall propagation velocity and focused ion beam irradiation is utilised to smooth the wire edges.

This thesis is comprised of two separate physics themes, both of which involve the ATLAS detector situated at the LHC at CERN. The first constituent is a study of the top quark signal in the fully-leptonic channel for proton-proton collisions at a centre-of-mass energy of 10 TeV. Here an event counting analysis is performed based on Monte Carlo simulation. This is supplemented by a study into one of the sources of systematic error. The second component is forward-backward correlations in minimum bias events. For this, there is a Monte Carlo hadron-level comparison of the correlation for 900 GeV centre-of-mass collisions, followed by a comparison of Monte Carlo predictions to data for 900 GeV and 7 TeV collisions. Top Physics A measurement of the fully-leptonic ttbar cross-section in the three decay channels ee, mumu, and emu is performed on ATLAS produced fully simulated pseudo-event data-samples. Selection rates for signal and background events consistent with ATLAS results are found along with the kinematic distributions of selected events. A calculation of the non-hadronic ttbar cross-section, based on the measured cross-sections, will then return the theoretical value of 217:06pb used to generate the original samples, showing the closure of the pseudo-analysis process. A more detailed study is made of the systematic uncertainty arising from variations in the initial (ISR) and final (FSR) state showering models, based on the Pythia event generator. A fast simulation of the ATLAS detector is used with similar object and event selection to the fully simulated case. The effect of ISR variations on the signal is found to be negligible as it is washed out in the subsequent decays of the ttbar system. However, the effect of FSR is found to cause 5% uncertainty in the selected signal events. In addition, in the main background of each of the selection channels the effect of FSR is found to produce variations of up to 30% in well populated channels. The variations in signal and background measurements will then be used to calculate a new estimate of the systematics on the measured ttbar cross-section for each channel. Minimum Bias A detailed study of the forward-backward (FB) correlation and event shapes of a selection of Pythia tunes for pp collisions with CoM = 900 GeV is performed. This includes an investigation into the sources of particle production in generated minimum bias events as well as the component sub-processes in generated minimum bias events. The tunes are found to be practically degenerate (within 10 - 20% variation) for the 'standard' distributions. The inclusion of a new observable, namely the forward-backward correlation, to the standard set is recommended. The study finds that the FB-correlation and its pT and dependent variations are able to discern differences between the selected tunes to a greater degree than the usual inclusive distributions. Further, the FB-correlation is found to be sensitive to the particle production processes within the tunes, an invaluable property for the purposes of generator tuning. A measurement of the forward-backward correlation for pp collision of CoM = 900 GeV and 7 TeV at the LHC using the ATLAS detector is made. The measured correlation is compared to the predicted correlation of several ATLAS centrally produced generator tunes. A correction procedure is developed and validated on the generator samples to correct the generated correlation to the hadron-level correlation. This is then applied to the measured correlation and a comparison of corrected data to the hadron-level predictions of the generated tunes made. The corrected correlations at the two collision energies are compared as well as the calculation of a global correlation at both energies. The measured and corrected correlations are found to lie above the predicted distributions at both energies and across the eta-range. Further investigation of measured correlation using augmented FB-correlations is recommended.

Selected area and Kikuchi diraction patterns, traditional bright eld and dark eld imaging techniques in electron microscopy as well as high resolution TEM and STEM techniques, together with electron backscattered electron diraction technique have been used to study the domain structures, local crystallography and atomic structures in PZT-based materials. Reliable EBSD mapping of 90 degrees domains in a tetragonal Pb(Zrx; Ti1-x)O3 with x = 0.5 ferroelectric perovskite has been achieved for the rst time, together with reliable automated orientation determination from TEM-Kikuchi patterns. This has been used to assess the local crystallography of domains by determining misorientation angles at 90 degrees domain boundaries and thus local c/a ratios. In most cases, a good agreement is found between local c/a ratios and global measurements by X-ray diraction, but some clear discrepancies have also been found suggesting that real local variations are present, perhaps as a consequence of compositional inhomogeneities. The details of the domain structure of the incommensurate antiferroelectric struc- ture in La-doped zirconium-rich lead zirconate titanate have been revealed in detail for the rst time. The structure is dominated by 60 degrees domain boundaries close to {101} planes of the primitive perovskite cell; and tilts of the perovskite sublattice of about 0.5 degrees are also noted at such boundaries consistent with a small tetragonal distortion of the primitive cell. Within each domain a streaked nanostructure is revealed under weak diraction conditions perpendicular to the long b-axis of the incommensurate supercell, which appears to be a consequence of planar faulting perpendicular to this b-axis. 90 degrees domain boundaries are also observed but are less frequent than 60 degrees boundaries and in con- trast to previous reports, these often have rather curved and irregular boundary planes. The atomic arrangement of these 90 degrees boundaries was studied by aberration corrected HRSTEM. Dierent stackings and periodicities were identied.

Direct detection of gravitational radiation, predicted by Einstein’s general theory of relativity, remains one of the most exciting challenges in experimental physics. Due to their relatively weak interaction with matter, gravitational waves promise to allow exploration of hitherto inaccessible objects and epochs. Unfortunately, this weak coupling also hinders detection with strain amplitudes at the Earth estimated to be of order 10^−21. Due to their wide bandwidth and theoretical sensitivity, kilometre-scale Michelson style interferometers have become the preferred instrument with which to attempt ground based detection. A worldwide network of first generation instruments has been constructed and prodigious volumes of data recorded. Despite each instrument approaching or having reached its design sensitivity, a confirmed detection remains elusive. Planned upgrades to these instruments aim to increase strain sensitivity by an order of magnitude, commencing the era of second generation detectors. Entry into this regime will be accompanied by an entirely new set of challenges, two of which are addressed in this work. As advanced interferometers are commissioned, instrumental artifacts will give way to fundamental noise sources. In the region of peak sensitivity it is expected that thermal noise in the interferometers’ dielectric mirror coatings will be the principal source of displacement noise. Theory suggests that increasing the spot size of laser light incident on these mirrors will reduce the measured thermal noise. In the first part of this work we examine one method of realising larger spots. By adopting non-spherical mirrors in the interferometers’ arms it is possible to create resonators which support a wide, flat-topped field known as the mesa beam. This beam has been shown to theoretically reduce all forms of mirror thermal noise without being significantly more difficult to control. In this work we investigate these claims using a bespoke prototype mirror. The first results regarding a non-Gaussian beam created in a manner applicable to a gravitational wave interferometer are presented. A common theme among all second generation interferometer designs is the desire to maximise circulating power. This increased power is partnered by commensurately increased thermal perturbations. Since the attractive properties of the mesa beam are produced by the fine structure of its supporting mirrors, it is important that we understand the effects absorption of stored optical power could have on mesa fields. In the second part of this work we report on numerical evaluations of measured thermal noise and mesa beam intensity profile as a function of absorbed power. Increased optical power also has less obvious consequences. As a result of radiation pressure, there exists a pathway between optical energy stored in an interferometer’s arms and mechanical energy stored in the acoustic modes of its test masses. Under appropriate conditions, this coupling can excite one or more test masses to such a degree that interferometer operation becomes impossible. In the final part of this work we determine whether it is possible to mitigate these parametric instabilities using electrostatic actuators originally designed to control the position and orientation of the test masses.

This thesis presents measurements of the charm sector mixing and CP-violation parameters yCP and AGamma, made using data collected in 2010 by the LHCb experiment at the LHC at s 1/2 = 7 TeV. yCP is defined as the difference from unity of the ratio of the effective lifetime of the D0 meson decaying to a CP-undefined final state to its lifetime when decaying to a CP-eigenstate. AGamma is the CP-asymmetry of the effective lifetimes of the D0 and D0bar when decaying to a CP-eigenstate. In the absence of CP-violation yCP will be consistent with the mixing parameter y, and AGamma will be consistent with zero. CP-violation in the charm sector is predicted to be very small in the SM, though first evidence for direct CP-violation in D0 decays has recently been observed by LHCb. Observation of significantly more CP-violation than is allowed in the SM would be a strong indication of new physics. The current world best measurements of yCP and AGamma show no evidence of CP-violation. The methods used to measure the effective lifetime of the D0 are presented, together with a detailed study of the impact parameter resolutions achieved by Vertex Locator (VELO) sub-detector. A resolution of < 36 μm on the x and y components of impact parameter measurements is measured for particles with pT > 1 GeV. The final dataset on which yCP and AGamma are measured comprises 28.0 +/- 2.8 pb−1, from which 226,110 D0 to Kpi and 30,481 D0 to KK candidates are selected. The dominant sources of systematic uncertainty arise from combinatorial backgrounds and D0 produced in decays of B mesons. The final results are found to be AGamma = (−5.9 +/- 5.9 (stat.) +/- 2.1 (syst.)) x 10−3, yCP = (5.5 +/- 6.3 (stat.) +/- 4.1 (syst.)) x 10−3. yCP is consistent with the world average of y and with zero, and AGamma is consistent with zero. Thus, these results show no evidence for CP-violation or mixing in the D0 system.

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.

The use and optimisation of integration grid techniques to generate next-to-leading order predictions of jet cross-sections, independent of parton distribution functions, was investigated. Such methods were found to provide an accurate approximation to a standard Monte-Carlo simulation (within 1%) and enable collider data to be readily included in global PDF fitting procedures. However, the benefit of including inclusive-jet cross-section data from ATLAS in global fits is only significant if the jet energy scale (JES) can be constrained to ~1% at high pT. Uncertainties in the theoretical prediction of the inclusive-jet cross-section such as PDFs and fixed-order (scale) uncertainties were studied and compared with experimental errors arising from jet energy resolution and absolute scale. These uncertainties were then considered in the context of a quark compositeness search where a sensitivity to a compositeness scale of Lambda<10TeV can be achieved with 10 inverse femtobarns of data, if the jet energy scale can be constrained to ~1%. An analysis using dijet angular distributions found a similar sensitivity without the dependence on the jet energy scale. A potential method of evaluating the stability of the jet energy scale out to high pT by `bootstrapping' the calibration at low pT by the use of multi-jet events was also investigated. This suggests that a calorimeter non-linearity can be detected for jets with pT>500GeV at ~1.5%/500GeV (i.e. a 1.5% change in JES over 500GeV in pT). An investigation of inner-detector commissioning issues associated with the ATLAS Semiconductor Tracker (SCT), including a review of `noisy' modules on the SCT Barrel (from May 2007) was carried out. In addition a tool for DCS monitoring within the online monitoring framework was developed and tested during the M5 and M6 commissioning weeks. Finally, a method of assessing the track reconstruction efficiency by track-insertion was considered for the particular case of minimum bias events.

Gravitational waves were predicted by Einstein, in his General Theory of Relativity in 1916. These waves can be thought of as ripples in the curvature of space-time. They have not yet been directly detected but strong indirect evidence of their existence was provided by Hulse and Taylor when they measured the rate of decay of the inspiral motion of a binary system. Research towards direct detection of gravitational radiation from astrophysical sources has been carried out for many years through the design, construction and initial operation of a network of gravitational wave detectors. Direct detection of gravitational waves will provide insights into the astrophysical sources which produce them and will provide a new method of observing events in the Universe. Gravitational waves are quadrupole in nature and produce strains in spacetime, which have extremely small amplitudes. The largest most violent events in the Universe are expected to cause strains of approximately 10^-22 at the Earth in the frequency band of a few Hz to a few kHz. Long baseline interferometry between suspended test masses is currently used to search for the induced strains in space-time, and thus the direct effects of gravitational waves of astrophysical origin. There is currently a network of interferometer detectors running worldwide. A 600 m long detector, GEO600, was built near Hannover in Germany by a collaboration between the Institute for Gravitational Research at the University of Glasgow, the Albert-Einstein-Intitut in Hannover and Golm, the University of Hannover and Cardiff University. There are three detectors in the United States of America forming the LIGO project. Two detectors, one of 4 km arm length and one of 2 km arm length were constructed on a site near Hanford, Washington State, and one detector of 4 km arm length was constructed near Livingston Louisiana. A 3 km detector, Virgo, was built near Cascina, Italy, by a European collaboration involving France, Italy and more recently the Netherlands. Six data collecting science runs have taken place to date with different combinations of these detectors in operation during the various runs; no detections have yet been made. An important noise source in the current operating frequency band of ground-based detectors is the thermal noise of the test mass mirrors in the interferometers, and the mirror suspension elements. The research presented in this thesis focusses on studies of techniques for quantifying and reducing the mechanical loss associated with the suspended mirrors and thus reducing the associated thermal noise thereby increasing detector sensitivity. In particular, experiments were carried out to study the loss of fused silica and investigate aspects of the hydroxy-catalysis bonding process used to joint elements of the test mass suspensions. In addition, silicon was investigated as a potential candidate for use as a mirror substrate material for use in future gravitational wave detectors. In Chapter 1 the nature of gravitational waves is explained and some of the sources which are expected to produce the largest amount of gravitational wave radiation are described. The development of resonant bar detectors and interferometers is given along with the current status of detectors and that of planned future projects. Noise sources which cause limitations to the detector sensitivity are discussed and an important noise source, thermal noise, is described in Chapter 2. Thermal noise is an important noise source in the current frequency band of gravitational wave detectors. Reduction of thermal noise is a major challenge but is possible through careful design of the mirrors and their suspension systems. One technique aimed at minimising the thermal noise of a suspension system involves the creation of a quasi-monolithic suspension system by the use of hydroxy-catalysis bonding. This is a high precision, high strength method of adjoining suspension elements. In Chapter 3 investigations were made of the strengths of hydroxy-catalysis bonds and on the effect on strength of various parameters associated with the most commonly used version of the bonding procedure, and of putting bonds through different treatments. It is shown that the average strength of a hydroxy-catalysis bond between silica substrates is ~ 15 MPa and that somewhat elevated temperature treatment (similar to an airbake) can improve on this strength, but that thermal shock conditions can decrease the strength. These investigations provide information on processes which can be used in the suspension construction to produce the lowest loss, highest strength suspension system. Chapter 4 details mechanical loss measurements of bulk silica at room temperature. Different types of fused silica are studied and techniques to reduce their mechanical loss are discussed along with the effect which time and heat treatments can have on the mechanical loss of a hydroxy-catalysis bond. It is shown that Suprasil 3001 is an acceptable choice of material for the mirrors in gravitational wave detectors and that the mechanical loss of silica can be reduced through heat treatment. In Chapter 5 the mechanical loss of bulk silicon is studied, where silicon forms a potential candidate for future generation gravitational wave detectors. Silicon samples having two different crystal orientations, <100> and <111>, were studied. Both orientations were manufactured and polished by the same vendor and have equivalent doping levels. At room temperature it is seen that the crystal orientation <111> material yielded mechanical loss values which were slightly lower than the <100> material. It is shown that it is possible to further reduce the loss of the material through heat treatment. An upper limit of the mechanical loss of a hydroxy-catalysis bond between silicon substrates is determined and found to be within the range of 0.27 to 0.52. The results presented in this thesis indicate that the mechanical losses of silica suspensions in gravitational wave detectors can be reduced through methods such as heat treatments and, potentially, chemical etching. Silicon is seen to be an interesting candidate for the suspension material in future generation detectors run at cryogenic temperatures.

The ability to observe at tiny length scales has enabled key advances across the physical and life sciences. Much of what we know about the structure of cells and tissues comes from experiments on the micron length scale, enabled by new microscopy techniques. Modern manufacturing is increasingly concerned with materials that are structured on the nanometre scale, and devices which have ever-smaller features. Manipulating and measuring microscopic objects is a problem common to fields as diverse as microfabrication and cell biology, and it is these challenges that my doctoral studies have addressed. Tiny sizes mean tiny forces; so small that the light from a laser can be used to propel objects. Optical tweezers, a technique pioneered some two and a half decades ago, exploit light’s momentum to trap and manipulate objects. Now an established tool, single particles can be trapped and tracked to measure forces on a molecular scale, and this work is responsible for much of our current knowledge of motor proteins. This thesis describes advances in the holographic technology used to control multiple optical traps (and hence many trapped particles), and improved methods for monitoring the positions and forces involved. The speed with which multiple holographic optical traps can be moved has traditionally been limited by the time taken to calculate holograms, but by using consumer graphics cards and high speed Spatial Light Modulators (SLMs) I have implemented holographic systems fast enough to react to the Brownian motion of trapped particles. Brownian motion can, to some extent, be suppressed by this approach, and it also allows the trap's stiffness to be engineered to balance sensitivity against tight constraint of position. Feedback control using an SLM, rather than the other beam steering technologies that have been employed, is able to react to motion in three dimensions. This requires 3D position measurement, which is provided by the stereo microscopy technique described in Chapter 2. By illuminating and viewing the sample from two different angles it is possible to reconstruct the depth of objects. This is accomplished through a single high numerical aperture microscope objective, the same lens used to focus the trapping laser. In conjunction with a fast CMOS camera, it is possible to track particles with an accuracy of 2-3nm at several thousand frames per second. This allows measurement of forces and displacements within the control loop, that can be fed back to influence the position of the optical traps. This force information can also be relayed to the operator using a force-feedback joystick as detailed in Chapter 7. Interface design is an important part of making technology accessible to scientists from other disciplines; to this end I have also developed a multi-touch tablet application to control optical tweezers. By creating simple, reliable systems and coupling them to an intuitive interface, I have endeavoured to produce developments which are of use to the non specialist as well as to experts in optical tweezers-a number of which are now available commercially (Section 8.7). These technologies form the basis of a toolkit for working with multi-part probes in optical tweezers, and they should bear fruit in the coming years as a new form of scanning-probe microscopy emerges.

Gravitational waves, predicted by the theory of General Relativity, are fluctuations in the curvature of space-time which arise from the asymmetric acceleration of mass. While gravitational waves have yet to be detected directly, measurements of the inspiral rate of a binary pulsar system have provided strong evidence for their existence and a world-wide effort to develop more sensitive detectors is ongoing. In addition to testing predictions of General Relativity, observation and analysis of gravitational waves from astrophysical sources will provide new insights into a wide range of phenomena including black holes, neutron stars and supernovae. Gravitational waves are quadruple in nature, and therefore produce fluctuating tidal strains on space. Long baseline gravitational wave detectors aim to measure this effect using laser interferometry to measure fluctuations in the relative separation of free masses, coated to form highly reflective mirrors and suspended as pendulums at the ends of perpindicular arms up to 4 km in length. There are currently several long baseline gravitational wave detectors in operation around the world, including the three LIGO detectors in the US, the UK/German GEO600 detector near Hannover and the French/Italian Virgo detector near Pisa. The strain expected from gravitational waves is very small, of order [~10-[superscript 22. The magnitude of the resultant displacement is such that the thermal motion of the mirrors and their suspensions forms an important limit to detector sensitivity. The level of thermal noise is related to the mechanical dissipation of the materials used in the test mass and the mirror coatings.