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
A number of aspects of magnetic force microscopy (MFM) specific to the imaging of hard magnetic films have been studied. Firstly, we show that topographic images made in tapping mode with probes characterized by the moderate cantilever stiffness usual for MFM (1−4 N/m), contain artifacts due to strong probe-sample interactions which lead to probe retraction. As a result, stiffer cantilevers (e.g. 40 N/m) are better adapted to characterizing such hard magnetic films. Secondly, imaging with probes coated by a hard magnetic film leads to phase maps which show a two-fold symmetry, with paired dark/light contrast on opposite domain edges along the direction of the cantilever. We analyze quantitatively this effect, due to the tilt of the direction of tip magnetization and direction of oscillation, with respect to the sample normal. Thirdly, due to the long-range nature of the stray field produced by hard magnetic films containing micron-sized domains, MFM phase contrast reflects the stray field itself, as opposed to that of its spatial derivatives, as is generally the case in MFM.
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.
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