Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

This is the final version of the article. Available from the American Institute of Physics via the DOI in this record.The coercive field and angular dependence of the coercive field of single-grain Nd$_{2}$Fe$_{14}$B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where a Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour.We acknowledge the financial support from the Technology Research Association of Magnetic Materials for High Efficient Motors (MagHEM)

Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets

International audienceThe coercive field and angular dependence of the coercive field of single-grain Nd$_{2}$Fe$_{14}$B permanent magnets are computed using finite element micromagnetics. It is shown that the thickness of surface defects plays a critical role in determining the reversal process. For small defect thicknesses reversal is heavily driven by nucleation, whereas with increasing defect thickness domain wall de-pinning becomes more important. This change results in an observable shift between two well-known behavioral models. A similar trend is observed in experimental measurements of bulk samples, where a Nd-Cu infiltration process has been used to enhance coercivity by modifying the grain boundaries. When account is taken of the imperfect grain alignment of real magnets, the single-grain computed results appears to closely match experimental behaviour

New AMS 14C dates track the arrival and spread of broomcorn millet cultivation and agricultural change in prehistoric Europe

Broomcorn millet (Panicum miliaceum L.) is not one of the founder crops domesticated in Southwest Asia in the early Holocene, but was domesticated in northeast China by 6000 bc. In Europe, millet was reported in Early Neolithic contexts formed by 6000 bc, but recent radiocarbon dating of a dozen 'early' grains cast doubt on these claims. Archaeobotanical evidence reveals that millet was common in Europe from the 2nd millennium bc, when major societal and economic transformations took place in the Bronze Age. We conducted an extensive programme of AMS-dating of charred broomcorn millet grains from 75 prehistoric sites in Europe. Our Bayesian model reveals that millet cultivation began in Europe at the earliest during the sixteenth century bc, and spread rapidly during the fifteenth/fourteenth centuries bc. Broomcorn millet succeeds in exceptionally wide range of growing conditions and completes its lifecycle in less than three summer months. Offering an additional harvest and thus surplus food/fodder, it likely was a transformative innovation in European prehistoric agriculture previously based mainly on (winter) cropping of wheat and barley. We provide a new, high-resolution chronological framework for this key agricultural development that likely contributed to far-reaching changes in lifestyle in late 2nd millennium bc Europe

OVERHEATED SECURITY? The Securitisation of Climate Change and the Governmentalisation of Security

Since the mid-2000s, climate change has become one of the defining security issues in political as well as academic debates and amongst others has repeatedly been discussed in the UN Security Council and countless high level government reports in various countries. Beyond the question whether the characterisation as ‘security issue’ is backed up by any robust empirical findings, this begs the question whether the ‘securitisation’ of climate change itself has had tangible political consequences. Moreover, within this research area there is still a lively discussion about which security conceptions apply, how to conceptualise (successful) securitisation and whether it is a (politically and normatively) desirable approach to deal with climate change. The aim of this dissertation is to shed light on these issues and particularly to contribute to a more thorough understanding of different forms or ‘discourses’ of securitisation and their political effects on a theoretical and empirical level. Theoretically, it conceptualises securitisation as resting on different forms of power, which are derived from Michel Foucault’s governmentality lectures. The main argument is that this framework allows me to better capture the ambiguous and diverse variants of securitisation and the ever-changing concept of security as well as to come to a more thorough understanding of the political consequences and powerful effects of constructing issues in terms of security. Empirically, the thesis looks at three country cases, namely the United States, Germany and Mexico. This comparative angle allows me to go beyond the existing literature on the securitisation of climate change that mostly looks at the global level, and to come to a more comprehensive and detailed understanding of different climate security discourses and their political consequences. Concerning the main results, the thesis finds that climate change has indeed been securitised very differently in the three countries and thus has facilitated diverse political consequences. These range from an incorporation of climate change into the defence sector in the US, the legitimisation of far-reaching climate policies in Germany, to the integration of climate change into several civil protection and agricultural insurance schemes in Mexico. Moreover, resting on different forms of power, the securitisation of climate change has played a key role in constructing specific actors and forms of knowledge as legitimate as well as in shaping certain identities in the face of the dangers of climate change. From a normative perspective, neither of these political consequences is purely good or bad but highly ambiguous and necessitates a careful, contextual assessment

Development and applications of a DNA labeling method with magnetic nanoparticles to study the role of horizontal gene transfer events between bacteria in soil pollutant bioremediation processes

International audienceHorizontal gene transfers are critical mechanisms of bacterial evolution and adaptation that are involved to a significant level in the degradation of toxic molecules such as xenobiotic pesticides. However, understanding how these mechanisms are regulated in situ and how they could be used by man to increase the degradation potential of soil microbes is compromised by conceptual and technical limitations. This includes the physical and chemical complexity and heterogeneity in such environments leading to an extreme bacterial taxonomical diversity and a strong redundancy of genes and functions. In addition, more than 99 % of soil bacteria fail to develop colonies in vitro, and even new DNA-based investigation methods (metagenomics) are not specific and sensitive enough to consider lysis recalcitrant bacteria and those belonging to the rare biosphere. The objective of the ANR funded project “Emergent” was to develop a new culture independent approach to monitor gene transfer among soil bacteria by labeling plasmid DNA with magnetic nanoparticles in order to specifically capture and isolate recombinant cells using magnetic microfluidic devices. We showed the feasibility of the approach by using electrotransformation to transform a suspension of Escherichia coli cells with biotin-functionalized plasmid DNA molecules linked to streptavidin-coated superparamagnetic nanoparticles. Our results have demonstrated that magnetically labeled cells could be specifically retained on micromagnets integrated in a microfluidic channel and that an efficient selective separation can be achieved with the microfluidic device. Altogether, the project offers a promising alternative to traditional culture-based approaches for deciphering the extent of horizontal gene transfer events mediated by electro or natural genetic transformation mechanisms in complex environments such as soil

Development and applications of a DNA labeling method with magnetic nanoparticles to study the role of horizontal gene transfer events between bacteria in soil pollutant bioremediation processes

Horizontal gene transfers are critical mechanisms of bacterial evolution and adaptation that are involved to a significant level in the degradation of toxic molecules such as xenobiotic pesticides. However, understanding how these mechanisms are regulated in situ and how they could be used by man to increase the degradation potential of soil microbes is compromised by conceptual and technical limitations. This includes the physical and chemical complexity and heterogeneity in such environments leading to an extreme bacterial taxonomical diversity and a strong redundancy of genes and functions. In addition, more than 99 % of soil bacteria fail to develop colonies in vitro, and even new DNA-based investigation methods (metagenomics) are not specific and sensitive enough to consider lysis recalcitrant bacteria and those belonging to the rare biosphere. The objective of the ANR funded project “Emergent” was to develop a new culture independent approach to monitor gene transfer among soil bacteria by labeling plasmid DNA with magnetic nanoparticles in order to specifically capture and isolate recombinant cells using magnetic microfluidic devices. We showed the feasibility of the approach by using electrotransformation to transform a suspension of Escherichia coli cells with biotin-functionalized plasmid DNA molecules linked to streptavidin-coated superparamagnetic nanoparticles. Our results have demonstrated that magnetically labeled cells could be specifically retained on micromagnets integrated in a microfluidic channel and that an efficient selective separation can be achieved with the microfluidic device. Altogether, the project offers a promising alternative to traditional culture-based approaches for deciphering the extent of horizontal gene transfer events mediated by electro or natural genetic transformation mechanisms in complex environments such as soil