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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Towards autonomous sample positioning for ultra high vacuum chambers

Barreto, Suzana Maria January 2018 (has links)
Materials Science has in recent years become a high priority research area, having been identified as a growth sector for the UK economy over the next decade. Breakthroughs in this field are likely to have a significant impact on every area of our lives. There has recently been a trend toward automation at beamlines which is driven by rapid technology advancement. This technology advancement has improved the quality of experiment data and has allowed data collection times to improve exponentially. The Materials Science Research Group in the Institute of Mathematics, Physics and Computer Science, at Aberystwyth University have achieved international recognition for their research on materials under extreme conditions. They have a rich history in the development and use of specialist instruments to conduct real time surface analysis. Their custom made instrumentation has allowed them to greatly improve experiment throughput. Automation of the group's ultra high vacuum chambers is therefore a further enhancement that is advantageous, important, necessary and inevitable. This thesis presents the research undertaken to study what is required to provide automated sample positioning inside vacuum chambers that are operated under ultra high vacuum conditions, as the first step towards automation. As part of the research, a prototype automated positioning system that employs state of the art model based visual tracking techniques was developed to gain an understanding of the challenges the ultra high vacuum environment presents. Experimentation was carried out to assess the effects of different lighting conditions on tracking, evaluate the tracking library, extract suitable extrinsic parameters for tracker initialisation, and evaluate both monocular and stereo mode tracking. Key findings were that the model based tracking is a suitable approach for an automated positioning system but that performance depends on having suitable port placement for the cameras. Stereo tracking provided the best performance but was still prone to divergence at certain relative positions of the manipulator. On linear runs the average error was 0.06mm. On rotational runs, anti-clockwise runs proved better with an average error of 2<sup>o</sup> to 3<sup>o</sup>. The high errors of mixed rotational and linear tracking runs did not match the visual outputs indicating that there were inherent errors in the data evaluation. Tracking output video footage is available at [8]. More work is needed to take the system forward and close the tracking loop. Recommendations for improvements were provided.
2

Retournement de l’aimantation dans des jonctions tunnels magnétiques par effet de transfert de spin / Spin transfer torque driven magnetization switching in magnetic tunnel junctions

Lavanant, Marion 08 September 2017 (has links)
Les mémoires non-volatiles magnétiques à effet de couple de transfert de spin - STT-MRAM sont un nouveau type de mémoire pouvant remplacer les mémoires DRAM ou SRAM. Chaque point de mémoire STT-MRAM est une jonction tunnel magnétique sous forme d’un pilier de taille nanométrique, composée de deux couches magnétiques séparées par une barrière d'oxide. L'empilement multicouche doit être élaboré sous ultravide par épitaxie par faisceau moléculaire (M.B.E.) ou par pulvérisation cathodique (P.V.D.). Ces méthodes d’élaboration sont développées par la société Vinci Technologies (finançant ce travail de thèse par une bourse CIFRE). L’amplitude de la magnétorésistance tunnel, utilisée pour lire les informations stockées dans la mémoire, dépend de l'orientation relative des aimantations des deux couches magnétiques. Par ailleurs, l'écriture de l’information dans le dispositif est obtenue grâce à l'effet de couple de transfert de spin, qui permet la manipulation de l’aimantation en utilisant un courant polarisé. Enfin, la stabilité thermique du dispositif est donnée par la barrière en énergie séparant les deux orientations d'aimantation (vers le haut et vers le bas dans le cas d'un dispositif perpendiculaire). Pour que les STT-MRAM soient une technologie compétitive, la tension critique nécessaire au retournement de l’aimantation (tension d'écriture) ainsi que le temps de retournement doivent être réduits, tandis que la stabilité thermique doit rester suffisamment élevée pour assurer la conservation de l'information. Au cours de ma thèse, en collaboration avec Vinci Technologies, les équipements nécessaires à la croissance des couches minces composant les jonctions tunnels (M.B.E. et P.V.D.) ont été optimisées. Grâce à cela, nous avons pu obtenir des couches minces avec une anisotropie perpendiculaire (hors du plan) bien caractérisée. J'ai ensuite concentré mon étude sur les dispositifs STT-MRAM industriels (IBM et STT) présentant une aimantation perpendiculaire pour comprendre le mécanisme de retournement de l’aimantation induite par le courant. J'ai alors pu identifier les paramètres pertinents influençant la valeur de la tension de retournement et proposer des solutions pour l'abaisser tout en préservant la stabilité thermique. Grâce à une étude concernant la probabilité de retournement d'aimantation, comparée à une modélisation macrospin et micromagnétique, j'ai mis en évidence un mécanisme de retournement variable en fonction de la configuration magnétique initiale. En effet, le champ rayonné par une couche magnétique sur une autre et la forme de la jonction tunnel ont un impact important sur la manipulation de l'aimantation / Spin Transfer Torque - Magnetic Random Access Memories – STT-MRAM – are developed as a new type of memory which could replace DRAM or SRAM. In the case of STT- MRAM, each memory point is a nanopillar magnetic tunnel junction composed of two magnetic layers separated by an oxide barrier. The multilayer stack can be grown under ultra-high vacuum using Molecular Beam Epitaxy (MBE) or Physical Vapor Deposition (PVD). Those systems are developed by the company Vinci Technologies (sponsoring this PhD work). The tunnel magnetoresistance signal which depends on the relative orientation of the two magnetizations is used to read the information stored in the device. The writing of the information in the device is realized thanks to the spin transfer torque effect, which allows magnetization manipulation using a spin current. The thermal stability of the device is given by the energy barrier separating the two magnetization orientations (up and down in the case of a perpendicular device). For STT-MRAM to be a competitive technology, the critical voltage needed for magnetization switching (writing voltage) as well as the switching time have to be reduced while the thermal stability remains high enough to ensure the retention of information. During my thesis, in collaboration with Vinci-Technologies several tools to grow thin films have been optimized. With such equipment, we were able to grow thin films with well characterized perpendicular (out-of-plane) anisotropy. I have then focused my study on industrial STT-MRAM devices (from two companies: IBM and STT) with an out-of-plane magnetization direction so as to understand the mechanism of current induced magnetization switching. By doing so, I could identify the relevant parameters influencing the switching voltage value and propose solutions to lower it while preserving thermal stability. Through a probabilistic study of magnetization reversal, coupled with macrospin and micromagnetic modeling studies, I have evidenced different switching mechanisms depending on the initial magnetic configuration. Indeed both the stray field from one magnetic layer to the other and the shape of the nanopillar have a large impact on magnetization manipulation

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