Magnetisation dynamics in magnetostrictive nanostructures

Spin torque oscillator devices have presented themselves as an energy efficient method of generating microwave frequencies in recent years. These are devices which rely on giant magnetoresistance to create a device resistance which oscillates at microwave frequencies due to the microwave oscillation of a magnetically free layer in the device. A spin torque oscillator can be improved by using a vortex core oscillator as the magnetically free layer offering a narrower linewidth and greater synchronisation possibilities, at the expense of a lower power output. The desire to tune the frequency of oscillation of these devices has been the focus of a great deal of research in recent years and one promising avenue of investigation is to alter the frequency of oscillation by inducing a strain anisotropy in such a device by the use of a piezoelectric transducer. In order for the induced anisotropy to be large a material must be used which exhibits a strong magnetostrictive effect such as Fe1−xGax, a material which exhibits strong magnetostrictive properties without the need for rare earth elements. This thesis describes investigations into the magnetisation dynamics of nanostructures fabricated from magnetostrictive thin films of Fe1−xGax under conditions of in-plane uniaxial anisotropy induced by an applied stress. Chapter 4 describes investigations into the effects of altering the thickness of sputter grown Fe1-xGax films on the crystalline anisotropy of the films. It was found that the intrinsic magnetocrystalline uniaxial anisotropy within the films increased with film thickness. The cubic anisotropy was shown to be roughly constant with respect to film thickness except when the film was 20nm thick when the cubic anisotropy of the sample was anomalously high. Investigations of the magnetostrictive properties of these materials revealed sputter grown thin films to exhibit similar magnetostrictive properties as bulk material and thin films grown by molecular beam epitaxy. X-ray analysis performed by Dr. P. Wadley and Prof.V. Holy failed to explain the relationships between film thickness and magnetocrystaline anisotropy observed in the samples, but suggested that the average grain size increases as the thickness of the film increases. Chapter 5 describes the results of time resolved XMCD PEEM measurements performed at the Diamond Light Source synchrotron facility performed in order to investigate the magnetisation dynamics within a series of Fe1−xGax squares. It was found that these squares demonstrated no significant response to an applied stress, probably due to strong shape anisotropy. Preliminary work to investigate Ni squares revealed that they do exhibit a strong response to stress. The dynamic response of the Ni squares was notsuccessfully measured however. Chapter 6 presents results of micromagnetic simulations performed to predict the effects of strain-induced anisotropy on magnetic square nanostructures fabricated from Fe1−xGax. Time resolved simulations demonstrated the ability of a strain induced anisotropy to modify the frequency of oscillation of the vortex core oscillations and the confined spin wave modes as well as the amplitude of the magnetic field pulse required to induce switching of the polarisation of the vortex core. The effects of size and uniaxial anisotropy on the spin wave modes within square devices was studied and an s shaped spin wave mode was shown to form in the presence of a uniaxial anisotropy

Using cascading Bloom filters to improve the memory usage for de Brujin graphs

De Brujin graphs are widely used in bioinformatics for processing next-generation sequencing data. Due to a very large size of NGS datasets, it is essential to represent de Bruijn graphs compactly, and several approaches to this problem have been proposed recently. In this work, we show how to reduce the memory required by the algorithm of [3] that represents de Brujin graphs using Bloom filters. Our method requires 30% to 40% less memory with respect to the method of [3], with insignificant impact to construction time. At the same time, our experiments showed a better query time compared to [3]. This is, to our knowledge, the best practical representation for de Bruijn graphs.Comment: 12 pages, submitte

Magnetisation dynamics in magnetostrictive nanostructures

Spin torque oscillator devices have presented themselves as an energy efficient method of generating microwave frequencies in recent years. These are devices which rely on giant magnetoresistance to create a device resistance which oscillates at microwave frequencies due to the microwave oscillation of a magnetically free layer in the device. A spin torque oscillator can be improved by using a vortex core oscillator as the magnetically free layer offering a narrower linewidth and greater synchronisation possibilities, at the expense of a lower power output. The desire to tune the frequency of oscillation of these devices has been the focus of a great deal of research in recent years and one promising avenue of investigation is to alter the frequency of oscillation by inducing a strain anisotropy in such a device by the use of a piezoelectric transducer. In order for the induced anisotropy to be large a material must be used which exhibits a strong magnetostrictive effect such as Fe1−xGax, a material which exhibits strong magnetostrictive properties without the need for rare earth elements. This thesis describes investigations into the magnetisation dynamics of nanostructures fabricated from magnetostrictive thin films of Fe1−xGax under conditions of in-plane uniaxial anisotropy induced by an applied stress. Chapter 4 describes investigations into the effects of altering the thickness of sputter grown Fe1-xGax films on the crystalline anisotropy of the films. It was found that the intrinsic magnetocrystalline uniaxial anisotropy within the films increased with film thickness. The cubic anisotropy was shown to be roughly constant with respect to film thickness except when the film was 20nm thick when the cubic anisotropy of the sample was anomalously high. Investigations of the magnetostrictive properties of these materials revealed sputter grown thin films to exhibit similar magnetostrictive properties as bulk material and thin films grown by molecular beam epitaxy. X-ray analysis performed by Dr. P. Wadley and Prof.V. Holy failed to explain the relationships between film thickness and magnetocrystaline anisotropy observed in the samples, but suggested that the average grain size increases as the thickness of the film increases. Chapter 5 describes the results of time resolved XMCD PEEM measurements performed at the Diamond Light Source synchrotron facility performed in order to investigate the magnetisation dynamics within a series of Fe1−xGax squares. It was found that these squares demonstrated no significant response to an applied stress, probably due to strong shape anisotropy. Preliminary work to investigate Ni squares revealed that they do exhibit a strong response to stress. The dynamic response of the Ni squares was notsuccessfully measured however. Chapter 6 presents results of micromagnetic simulations performed to predict the effects of strain-induced anisotropy on magnetic square nanostructures fabricated from Fe1−xGax. Time resolved simulations demonstrated the ability of a strain induced anisotropy to modify the frequency of oscillation of the vortex core oscillations and the confined spin wave modes as well as the amplitude of the magnetic field pulse required to induce switching of the polarisation of the vortex core. The effects of size and uniaxial anisotropy on the spin wave modes within square devices was studied and an s shaped spin wave mode was shown to form in the presence of a uniaxial anisotropy

Investigations To Characterize Multi-Junction Solar Cells In The Stratosphere Using Low-Cost Balloon And Communication Technologies

The use of current balloon, control and communication technologies to test multi-junction solar sell in the stratosphere to achieve near AMO conditions have been investigated. The design criteria for the technologies are that they be reliable, low cost and readily available. Progress is reported on a program to design, launch, fly and retrieve payloads dedicated to testing multi-junction solar cells

Mediation, translation and local ecologies: understanding the impact of policy levers on FE colleges

This article reports the views of managers and tutors on the role of policy ‘levers’ on teaching, learning, and inclusion in colleges of Further Education (FE) in our research project, ‘The impact of policy on learning and inclusion in the Learning and Skills Sector (LSS)’.i Using data from five research visits conducted over two years in eight FE learning sites, we explore the processes by which colleges ‘mediate’ and ‘translate’ national policy levers and how this affects their ability to respond to local need. The paper tentatively develops three related concepts/metaphors to explain the complexity of the policy/college interface – ‘the process of mediation’, ‘acts of translation’ and ‘local ecologies’. We found that policy levers interacted with a complex set of national, local and institutional factors as colleges responded to pressures from the external environment and turned these into internal plans, systems and practices. We conclude by suggesting that national policy-makers, who design national policy levers, may not be fully aware of these complexities and we make the case for the benefits of greater local control over policy levers, where these interactions are better understood

Instability Heating of Sympathetically-Cooled Ions in a Linear Paul Trap

Sympathetic laser cooling of ions stored within a linear-geometry, radio frequency, electric-quadrupole trap has been investigated using computational and theoretical techniques. The simulation, which allows 5 sample ions to interact with 35 laser-cooled atomic ions, revealed an instability heating mechanism, which can prevent ions below a certain critical mass from being sympathetically cooled. This critical mass can however be varied by changing the trapping field parameters thus allowing ions with a very large range of masses to be sympathetically cooled using a single ion species. A theoretical explanation of this instability heating mechanism is presented which predicts that the cooling-heating boundary in trapping parameter space is a line of constant $q_u$ (ion trap stability coefficient), a result supported by the computational results. The threshold value of $q_u$ depends on the masses of the interacting ions. A functional form of this dependence is given

Effects of study design and allocation on participant behaviour-ESDA: study protocol for a randomized controlled trial

Background: What study participants think about the nature of a study has been hypothesised to affect subsequent behaviour and to potentially bias study findings. In this trial we examine the impact of awareness of study design and allocation on participant drinking behaviour. Methods/Design: A three-arm parallel group randomised controlled trial design will be used. All recruitment, screening, randomisation, and follow-up will be conducted on-line among university students. Participants who indicate a hazardous level of alcohol consumption will be randomly assigned to one of three groups. Group A will be informed their drinking will be assessed at baseline and again in one month (as in a cohort study design). Group B will be told the study is an intervention trial and they are in the control group. Group C will be told the study is an intervention trial and they are in the intervention group. All will receive exactly the same brief educational material to read. After one month, alcohol intake for the past 4 weeks will be assessed. Discussion: The experimental manipulations address subtle and previously unexplored ways in which participant behaviour may be unwittingly influenced by standard practice in trials. Given the necessity of relying on self-reported outcome, it will not be possible to distinguish true behaviour change from reporting artefact. This does not matter in the present study, as any effects of awareness of study design or allocation involve bias that is not well understood. There has been little research on awareness effects, and our outcomes will provide an indication of the possible value of further studies of this type and inform hypothesis generation

Production of antihydrogen at reduced magnetic field for anti-atom trapping

We have demonstrated production of antihydrogen in a 1$,$T solenoidal magnetic field. This field strength is significantly smaller than that used in the first generation experiments ATHENA (3$,$T) and ATRAP (5$,$T). The motivation for using a smaller magnetic field is to facilitate trapping of antihydrogen atoms in a neutral atom trap surrounding the production region. We report the results of measurements with the ALPHA (Antihydrogen Laser PHysics Apparatus) device, which can capture and cool antiprotons at 3$,$T, and then mix the antiprotons with positrons at 1$,$T. We infer antihydrogen production from the time structure of antiproton annihilations during mixing, using mixing with heated positrons as the null experiment, as demonstrated in ATHENA. Implications for antihydrogen trapping are discussed