Magneto-Structural Correlations in Fe60Al40 Thin Films

Ferromagnetism in certain alloys with a crystalline B2-structure, such as Fe60Al40, can be switched on, and tuned, via antisite disordering of the atomic arrangement. This disorderinduced B2→A2 phase transition is accompanied by a ∼1% increase in the lattice parameter. The induced ferromagnetism can be switched off as well via atomic rearrangements causing the A2→B2 transition. In this thesis, the B2↔A2 phase transition will be manipulated by ion- as well as laser-irradiation. Ion-irradiation allows for a sensitive control of the degree of antisite disorder and thus can be applied to understand the correlation between gradual disorder and magnetic properties in the Fe60Al40 alloy. The reversibility of the laser-driven B2↔A2 transition will be shown in this work. B2-Fe60Al40 thin films have been disordered systematically by ion-irradiation and correlations between the chemical disorder (1-S), lattice parameter (a0), and the induced saturation magnetization (Ms) have been obtained. As the lattice is gradually disordered,a critical point occurs at 1-S=0.6 and a0=2.91Å, where a sharp increase of the Ms is observed. The regimes below and above the critical regime are characterized by a different, but nearly stable Ms behaving paramagnetic and ferromagnetic, respectively. Density functional theory (DFT) calculations suggest that below the critical point the system magnetically behaves as it would still be fully ordered, i.e. paramagnetic, whereas above, it is largely the increase of a0 in the disordered state that determines the Ms. Furthermore, disordered thin films possessing various open-volume defect types have been ordered via thermal annealing. The A2→B2 ordering process occurs by the vacancy diffusion mechanism and the ordering rate shows a strong dependence on the defect types, as obtained from ab-initio DFT calculations: The ordering rate is increased by mono-vacancies and decreased by triple defects and vacancy clusters. The defects can be engineered by a thermal pre-annealing and/or ion-irradiation offering a control of the subsequent ordering process. Additionally, the reversible disordering and subsequent reordering implying an on and off switching of ferromagnetism, respectively, is demonstrated by applying femtosecond laser pulse irradiation. The irradiation with a single laser pulse above the threshold fluence induces chemical disorder and hence ferromagnetism. A subsequent laser-irradiation below the threshold ŕuence causes a reordering at the surface erasing the ferromagnetism. The laser-irradiation leads to a melting and subsequent solidification of the material; if the solidification temperature is lower than the melting temperature, the liquid is supercooled. Simulations reveal the crucial role of the extent of supercooling: A single laser pulse above the threshold fluence causes a strong undercooling of the liquid phase before solidification limiting the vacancy diffusion and hence ordering. Laser pulsing below the threshold forms a limited supercooled surface region that solidifies at sufficiently high temperatures, enabling vacancy diffusion-assisted reordering.:1 Introduction and Fundamentals 1.1 Magnetism and Structure in Chemically Ordered Materials 1.1.1 Effects Induced by Chemical Disorder 1.1.2 Properties of Fe-Al Alloys 1.2 Modiőcation of B2 Materials 1.2.1 Interaction of Ions with Solids 1.2.2 Laser-Solid Interaction 1.3 Motivation 2 Experimental and Theoretical Methods 2.1 Sample Preparation 2.1.1 Magnetron Sputtering 2.1.2 Annealing Process 2.1.3 Ion-Irradiation 2.1.4 Laser-Irradiation 2.2 Structural Characterization 2.2.1 X-Ray Diffraction 2.2.2 Rutherford Backscattering Spectrometry 2.2.3 Transmission Electron Microscopy 2.3 Magnetic Characterization 2.3.1 Vibrating Sample Magnetometry 2.3.2 Spin-Resolved Photoemission Electron Microscopy 2.4 Defect Analysis by Positron Annihilation Spectroscopy 2.5 Theoretical Approaches 2.5.1 DFT Calculations on the Properties of Fe60Al40 2.5.2 Ab-initio Calculations of Positron Lifetimes in Fe60Al40 2.5.3 Simulations on the Laser-Irradiation of Fe60Al40 3 Unraveling Magneto-Structural Correlations 3.1 Characterization of Ordered B2 and Disordered A2 Films 3.1.1 Experiments 3.1.2 Structural Properties 3.1.3 Magnetic Properties 3.1.4 Summarizing Remarks 3.2 Systematic Disordering by Ion-Irradiation 3.2.1 Experiments 3.2.2 Structural Characterization 3.2.3 Analysis of Magnetic Properties 3.2.4 Correlation of Structural and Magnetic Properties 3.2.5 Comparison to Previously Reported Data 3.2.6 Theoretical Calculations 3.2.7 Discussion and Summarizing Remarks 4 Defect-Mediated Atomic Rearrangements 4.1 Experiments 4.2 Analysis of Magnetic Properties 4.3 Defect Characterization 4.4 Ab-initio Calculations of Positron Lifetimes 4.5 Discussion 5 Laser Pulse Induced Reversible Order-Disorder Transition 5.1 Experiments 5.2 Results 5.3 Simulations 5.4 Discussion 6 ConclusionsIn bestimmten Legierungen mit einer kristallinen B2-Struktur, wie beispielsweise Fe60Al40, kann durch eine chemische Unordnung Ferromagnetismus erzeugt und modifiziert werden. Dieser durch Unordnung hervorgerufene B2→A2 Phasenübergang geht mit einer Vergrößerung des Gitterparameters von ungefähr 1% einher. Der erzeugte Ferromagnetismus kann durch eine atomare Neuordnung, d.h. durch den A2→B2 Phasenübergang, wieder abgeschaltet werden. In der vorliegenden Arbeit wird der B2↔A2 Phasenübergang mittels Ionen- und Laserbestrahlung hervorgerufen und kontrolliert. Ionenbestrahlung ermöglicht eine präzise Kontrolle des Unordnungsgrades und kann daher eingesetzt werden, um den Zusammenhang zwischen gradueller Unordnung und magnetischen Eigenschaften in der Fe60Al40 Legierung zu untersuchen. Die Reversibilität des laserinduzierten B2↔A2 Phasenübergangs wird in der vorliegenden Arbeit gezeigt. In B2-Fe60Al40 Dünnschichten ist mittels Ionenbestrahlung systematisch Unordnung erzeugt worden, wodurch die Zusammenhänge von atomarer Unordnung (1-S), dem Gitterparameter (a0) und der erzeugten Magnetisierung (Ms) offengelegt worden. Während der schrittweisen Unordnung des Kristallgitters tritt ein kritischer Punkt bei 1-S=0.6 und a0=2.91Å auf, an welchem Ms stark ansteigt. Die Bereiche unter- und oberhalb des kritischen Bereiches sind durch ein unterschiedliches, aber nahezu gleichbleibendes Ms charakterisiert. Das Verhalten ist para- bzw. ferromagnetisch. Berechnungen mittels Dichtefunktionaltheorie (DFT) deuten an, dass sich das System unterhalb des kritischen Punktes verhält, als wäre es noch vollständig geordnet, d.h. paramagnetisch; wohingegen Ms oberhalb des kritischen Bereiches größtenteils durch den Anstieg des Gitterparameters bestimmt wird. Darüber hinaus sind ungeordnete Dünnschichten mit verschiedenen Typen leerstellenartiger Defekte mittels Wärmebehandlung neu geordnet worden. Der A2→B2 Ordnungsprozess geschieht auf der Basis des Leerstellendiffusionsmechanismus. Die starke Abhängigkeit der Ordnungsrate von den Defekttypen ist mittels ab-initio DFT Berechnungen bestätigt worden: Die Ordnungsrate wird durch Einzel-Leerstellen erhöht und sinkt durch Dreifach-Defekte und Leerstellencluster. Die Defekte können durch eine vorherige Wärmebehandlung und/oder Ionenbestrahlung manipuliert werden, wodurch der darauffolgende Ordnungsprozess kontrolliert werden kann. Des Weiteren wird die reversible Unordnung und anschließende Neuordnung, d.h. ein Erzeugen bzw. Abschalten von Ferromagnetismus, durch die Bestrahlung mit Femtosekunden- Laserpulsen demonstriert. Die Bestrahlung mit einem einzelnen Laserpuls mit einer Fluenz über dem Schwellenwert erzeugt atomare Unordnung und damit Ferromagnetismus. Eine anschließende Laserbestrahlung unterhalb der Schwellenŕuenz bewirkt eine Neuanordnung der Atome an der Oberfläche und damit einem Auslöschen des Ferromagnetismus. Die Laserbestrahlung führt zu einem Aufschmelzen und dem anschließenden Erstarren des Materials; liegt die Erstarrungstemperatur unterhalb der Schmelztemperatur, so ist die Schmelze unterkühlt. Die ausschlaggebende Rolle des Grades der Unterkühlung wird durch Simulationen aufgezeigt: Ein einzelner Laserpuls über der Schwellenfluenz führt zu einer starken Unterkühlung der flüssigen Phase vor der Erstarrung, wodurch die Leerstellendiffusion und damit die atomare Neuordnung eingeschränkt werden. Durch Laserpulse unterhalb der Schwellenfluenz wird der Oberŕächenbereich kaum unterkühlt und erstarrt anschließend bei hinreichend hohen Temperaturen, um eine leerstellendiffusionsunterstützte Neuanordnung der Atome zu ermöglichen.:1 Introduction and Fundamentals 1.1 Magnetism and Structure in Chemically Ordered Materials 1.1.1 Effects Induced by Chemical Disorder 1.1.2 Properties of Fe-Al Alloys 1.2 Modiőcation of B2 Materials 1.2.1 Interaction of Ions with Solids 1.2.2 Laser-Solid Interaction 1.3 Motivation 2 Experimental and Theoretical Methods 2.1 Sample Preparation 2.1.1 Magnetron Sputtering 2.1.2 Annealing Process 2.1.3 Ion-Irradiation 2.1.4 Laser-Irradiation 2.2 Structural Characterization 2.2.1 X-Ray Diffraction 2.2.2 Rutherford Backscattering Spectrometry 2.2.3 Transmission Electron Microscopy 2.3 Magnetic Characterization 2.3.1 Vibrating Sample Magnetometry 2.3.2 Spin-Resolved Photoemission Electron Microscopy 2.4 Defect Analysis by Positron Annihilation Spectroscopy 2.5 Theoretical Approaches 2.5.1 DFT Calculations on the Properties of Fe60Al40 2.5.2 Ab-initio Calculations of Positron Lifetimes in Fe60Al40 2.5.3 Simulations on the Laser-Irradiation of Fe60Al40 3 Unraveling Magneto-Structural Correlations 3.1 Characterization of Ordered B2 and Disordered A2 Films 3.1.1 Experiments 3.1.2 Structural Properties 3.1.3 Magnetic Properties 3.1.4 Summarizing Remarks 3.2 Systematic Disordering by Ion-Irradiation 3.2.1 Experiments 3.2.2 Structural Characterization 3.2.3 Analysis of Magnetic Properties 3.2.4 Correlation of Structural and Magnetic Properties 3.2.5 Comparison to Previously Reported Data 3.2.6 Theoretical Calculations 3.2.7 Discussion and Summarizing Remarks 4 Defect-Mediated Atomic Rearrangements 4.1 Experiments 4.2 Analysis of Magnetic Properties 4.3 Defect Characterization 4.4 Ab-initio Calculations of Positron Lifetimes 4.5 Discussion 5 Laser Pulse Induced Reversible Order-Disorder Transition 5.1 Experiments 5.2 Results 5.3 Simulations 5.4 Discussion 6 Conclusion

Pervasive Gaming: Testing Future Context Aware Applications

Over the last few years, many discussions have centred around the issue of interconnection rates and their economic impact on the market. Interconnection charging in Europe is still based mainly on the calling party pays (CPP) principle combined with element based charging (EBC). Due to the convergence of the classical PSTN/ISDN and the IP world to next generation networks (NGN), the different charging principles and systems are being reviewed to determine the optimal solution for the future. In its working program for the year 2008, the Austrian Regulatory Authority (RTR) launched an industry working group on charging principles and systems for wholesale services. This paper highlights some of the central issues of the discussions that have taken place and contains the authors’ views and conclusions .1 Further, the paper identifies possible charging systems, as well as economic assessment criteria for these systems and how the different charging systems may be evaluated with respect to those criteria. Regarding the usefulness of industry working groups, the work has shown that these lead to a higher degree of transparency between regulator and market players as well as a better understanding between the market players themselves. The main drawback is that working groups are time consuming and that it is almost impossible to agree on meaningful outcomes. Regarding the assessment of the charging models it was possible to derive a set of 10 criteria according to which charging systems can be evaluated. There was a rather broad consensus on the delineation of charging models as well as the economic criteria. When it comes to the results of the evaluation, the discussions brought forward very controversial views amongst the participants. No common views could be achieved on which the charging model fulfills the defined criteria in the best manner.Interconnection, NGN, charging principles, CPP, Bill&Keep.

Data-poor categorization and passage retrieval for Gene Ontology Annotation in Swiss-Prot

<p>Abstract</p> <p>Background</p> <p>In the context of the BioCreative competition, where training data were very sparse, we investigated two complementary tasks: 1) given a Swiss-Prot triplet, containing a protein, a GO (Gene Ontology) term and a relevant article, extraction of a short passage that justifies the GO category assignement; 2) given a Swiss-Prot pair, containing a protein and a relevant article, automatic assignement of a set of categories.</p> <p>Methods</p> <p>Sentence is the basic retrieval unit. Our classifier computes a distance between each sentence and the GO category provided with the Swiss-Prot entry. The Text Categorizer computes a distance between each GO term and the text of the article. Evaluations are reported both based on annotator judgements as established by the competition and based on mean average precision measures computed using a curated sample of Swiss-Prot.</p> <p>Results</p> <p>Our system achieved the best recall and precision combination both for passage retrieval and text categorization as evaluated by official evaluators. However, text categorization results were far below those in other data-poor text categorization experiments The top proposed term is relevant in less that 20% of cases, while categorization with other biomedical controlled vocabulary, such as the Medical Subject Headings, we achieved more than 90% precision. We also observe that the scoring methods used in our experiments, based on the retrieval status value of our engines, exhibits effective confidence estimation capabilities.</p> <p>Conclusion</p> <p>From a comparative perspective, the combination of retrieval and natural language processing methods we designed, achieved very competitive performances. Largely data-independent, our systems were no less effective that data-intensive approaches. These results suggests that the overall strategy could benefit a large class of information extraction tasks, especially when training data are missing. However, from a user perspective, results were disappointing. Further investigations are needed to design applicable end-user text mining tools for biologists.</p