SQUID-Magnetometrie an Fe Monolagen auf GaAs(001) im UHV

Die vorliegende Arbeit beschäftigt sich mit der Charakterisierung des Wachstums und der magnetischen Eigenschaften von ultradünnen Fe Filmen auf GaAs(001). Insbesondere kam hierbei ein im Rahmen dieser Arbeit weiterentwickeltes Raster-SQUID (superconducting interference device) Magnetometer im Ultrahochvakuum (UHV) zum Einsatz. Aus dem mit diesem Gerät gemessenen magnetischen Streufeld eines magnetisierten Films kann die remanente Magnetisierung absolut und mit Submonolagen-Nachweisempfindlichkeit bestimmt werden. Hierzu wurde im Rahmen dieser Arbeit das magnetische Streufeld analytisch berechnet. Die Kombination von SQUID- und Ferromagnetische-Resonanz-Messungen (FMR) am gleichen Film im UHV erlaubt die unabhängige Bestimmung von magnetischen Anisotropien und der Magnetisierung als Funktion der Temperatur, Schichtdicke, Substrattopographie und Sauerstoffangebot. Die Ergebnisse sind im Einzelnen: 1. Die schichtdickenabhängige remanente Magnetisierung von 2 bis 20 Monolagen Fe auf GaAs(001) wurde als Funktion der Temperatur ohne Deckschichten bestimmt. 2. Eine kontinuierliche Reorientierung der Magnetisierung in der Ebene (von [1 1 0] nach [1 0 0]) von Fe Filmen mit zunehmender Schichtdicke wurde mit der Raster-SQUID-Technik beobachtet und zeigt gute Übereinstimmung mit FMR-Messungen. 3. Die Änderung der Magnetisierung und der magnetischen Anisotropie wurde als Funktion von Sauerstoffangebot quantitativ untersucht. Es stellt sich heraus, dass bezogen auf den sich bildenden Eisenoxidanteil die Änderung der Magnetisierung in dünneren Filmen (5 und 8 ML) weit größer ist als für dickere Filme (16 ML). Bei geringem Sauerstoffangebot (<10 Langmuir) wird die senkrechte uniaxiale Anisotropiekonstante K2s um 40% reduziert wohingegen die anderen Anistropien nur geringfügig beeinflusst werden. Diese Untersuchungen wurden durch strukturelle IV-LEED Messungen ergänzt. 4. Ein 8.6 ML Fe/GaAs(001) Film, der bei 300 K einem Sauerstoffangebot von 25000 L ausgesetzt wurde, zeigte eine spontane Magnetisierungrichtung senkrecht zur Filmebene bei tiefen Temperaturen. Bei Erhöhung der Temperatur dreht sich die Magnetisierung zwischen 175 K< T <250 K in die Ebene hinein. Die Reorientierung wird auf die unterschiedliche Temperaturabhängigkeit der Formanisotropie und K2s zurückgeführt.This thesis deals with the characterization of the growth and of the magnetic properties of ultrathin Fe films on GaAs(001). In particular, a scanning SQUID (superconducting quantum interference device) magnetometer was used in ultrahigh vacuum (UHV), whose performance has been improved within the scope of this thesis. By probing the magnetic stray field of a magnetized film, the absolute remanent magnetization can be determined with submonolayer sensitivity. In the context of this thesis the magnetic stray field has been calculated analytically. The combined use of SQUID and ferromagnetic resonance (FMR) on the same film in UHV allows for the independent determination of the magnetization and the magnetic anisotropy constants as a function of temperature, film thickness, topography of the substrate and oxygen exposure. The results of this thesis are: 1. The thickness dependent remanent magnetization from 2 to 20 monolayer (ML) Fe on GaAs(001) without cap layer was measured as a function of temperature. 2. The continuous in-plane reorientation of the magnetization (from [1 1 0] to [1 0 0]) of Fe films with increasing film thickness was observed using the scanning SQUID technique and showed good agreement with FMR measurements. 3. The influence of controlled oxygen exposure on the remanent magnetization and the magnetic anisotropy constants of 5 to 16 ML Fe was investigated. A faster reduction of the magnetization is found for the thinner Fe films when the volume of the Fe oxide is taken into consideration. At low oxygen exposure (<10 Langmuir), the perpendicular uniaxial anisotropy constant K2s is reduced by about 40% whereas other anisotropy contributions remain virtually unchanged. In addition, structural investigations using IV-LEED during the oxygen exposure were carried out. 4. An 8.6 ML Fe/GaAs(001) film which was exposed to 25000 L O2 exhibits a spontaneous magnetization perpendicular to the film plane at low temperature. As the temperature is increased a continuous reorientation of the magnetization back to the plane of the film was observed from 175 to 250 K. The reorientation can be ascribed to the different temperature dependencies of the shape anisotropy (due to the temperature dependence of the magnetization) and K2s

Results of the European collaborative project “Product Uniformity Control“ to improve the inline sensing of mechanical properties and microstructure of automotive steels

A European consortium consisting of four major steel manufacturers and ten academic technology institutes has conducted a research and development project, called “Product Uniformity Control“ (PUC) in the period 2013 to 2017. This project aimed to develop and improve non-destructive (inline) measurement techniques to characterise the (uniformity of the) microstructure of steel strip products.In this project, a multitude of strip steel samples from various stages of production have been collected from the four participating steel manufacturers. The samples have been characterised in various ways, namely on their (1) non-destructive measurement parameters using different techniques suited for inline evaluation, (2) fundamental ultrasonic and electromagnetic properties (wave speed, ultrasonic attenuation, magnetisation loops, coercive field), (3) tensile properties (stress-strain curves) and (4) microstructure (by optical micrographs and EBSD images). The correlations between these different characterisations will be addressed.Besides the experimental characterisation, a strong accent has been on modelling activities: during the project, fundamental models have been developed to describe, starting from 2D and 3D microstructures, the ultrasonic and magnetic properties, which are next used as input to sensor models that predict the output of the inline measurement systems.This contribution will present the recent results of both the experimental and the modelling work, and underline their mutual importance in the interpretation of the measured data for the benefit of inline characterisation of the mechanical properties complementary to traditional destructive tensile testing

Product Uniformity Control (PUC): How 15 European research institutes contribute to improve the in-line characterisation of microstructure and mechanical properties in the manufacturing of steel strip

International audienc

In-line characterisation of microstructure and mechanical properties in the manufacturing of steel strip for the purpose of product uniformity control

International audienceThe uniformity of the microstructure of steel strip over the entire coil length and between different coils of the same grade is key to stable and consistent material behaviour in steel manufacturers’ proprietary processes, like rolling and levelling, and customers’ processes, like pressing and deep-drawing. In particular for high-strength steels, like dual phase and complex phase steels, the microstructure is very sensitive to processing variations resulting in a potentially larger spread in the mechanical properties of the product. In July 2013, a large European consortium consisting of 15 institutes started an RFCS (1) –funded project called “Product Uniformity Control” (PUC) with the primary objective to achieve enhanced and sustained product uniformity of steel strip by improved interpretation of data from inline measurement methods that aim for real-time and non-destructive characterisation of microstructure and techno-mechanical parameters. Commonly, these techniques are based on electromagnetic (EM) or ultrasonic (US) measurement principles, which are favoured because of their non-destructive and potentially contact-free nature. To improve the techniques for in-line materials characterisation, the PUC consortium takes a systematic approach to investigate the interrelations between mechanical properties -- microstructural parameters -- EM & US properties -- inline measurement thereof. The studies involve dedicated laboratory experiments, modelling of the EM and US properties of steel, modelling of inline measurement setups and statistical analysis of data from inline measurement systems. The synthesis of these activities should result in improved, model-based, calibrations and finally in a broader deployment and integration of the inline material characterisation techniques in steel manufacturing, adding value to the product and enhancing the process efficiency throughout the production chain from hot-rolling to finishing. This paper outlines the project approach, highlights interconnecting modelling and experimental research work, and demonstrates first results. Various contributions being presented at this WCNDT conference originate from the collaborative activities of this PUC project. (1). (European) Research Fund for Coal and Stee

The genome of Theobroma cacao.

We sequenced and assembled the draft genome of Theobroma cacao, an economically important tropical-fruit tree crop that is the source of chocolate. This assembly corresponds to 76% of the estimated genome size and contains almost all previously described genes, with 82% of these genes anchored on the 10 T. cacao chromosomes. Analysis of this sequence information highlighted specific expansion of some gene families during evolution, for example, flavonoid-related genes. It also provides a major source of candidate genes for T. cacao improvement. Based on the inferred paleohistory of the T. cacao genome, we propose an evolutionary scenario whereby the ten T. cacao chromosomes were shaped from an ancestor through eleven chromosome fusions

Biological control of cacao diseases

This chapter discusses the advances in biological control of cacao diseases over the last 15 years. Most attention has been focused on biological control of frosty pod rot (Moniliophthora roreri), witches' broom (Moniliophthora perniciosa) and black pod disease (Phytophthora spp.). Research on biocontrol of other diseases in the cacao phyllosphere or rhizosphere is scarce or in its infancy. There is, however, a steady increase in information regarding the factors influencing and the mechanisms underlying biological control of cacao diseases as well as practical aspects such as inoculum production, formulation and application. There has been a clear shift away from inundative approaches using epiphytic BCAs towards more classical biocontrol approaches using bacterial and fungal endophytes as well as vesicular arbuscular mycorrhiza. These have the advantage that they can permanently establish themselves in the cacao tree. Moreover, besides direct competition for space and nutrients, antibiosis and mycoparasitism, through induced resistance and growth promotion, endophytes have a larger arsenal of mechanisms through which they can help protect their host. Endophytic BCAs could thus provide more effective and sustainable disease control. Recent advances in our understanding of the mechanisms through which endophytic biocontrol agents can reduce pest and disease impact provide possibilities for innovative disease control strategies, including combination therapies together with natural or chemical products. Continued work on production, formulation and application is also necessary in order for biocontrol to become economically interesting. However, biological control will not become a stand-alone solution for disease control but should become part of integrated pest management strategies, with cultural management as a central and reinforcing pillar. (Résumé d'auteur