Georgeta Ciuta

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

The coercive field and angular dependence of the coercive field of single-grain Nd2Fe14B 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 an 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.

Magnetic nanoparticle DNA labeling for individual bacterial cell detection and recovery

A culture independent approach was developed for recovering individual bacterial cells out of communities from complex environments including soils and sediments where autofluorescent contaminants hinder the use of fluorescence based techniques. For that purpose fifty nanometer sized streptavidin-coated superparamagnetic nanoparticles were used to chemically bond biotin-functionalized plasmid DNA molecules. We show that micromagnets can efficiently trap magnetically labeled transformed Escherichia coli cells after these bacteria were subjected to electro-transformation by these nanoparticle-labeled plasmids. Among other applications, this method could extend the range of approaches developed to study DNA dissemination among environmental bacteria without requiring cultivability of recombinant strains or expression of heterologous genes in the new hosts.

Some Aspects of Magnetic Force Microscopy of Hard Magnetic Films

A number of aspects of magnetic force microscopy (MFM) specific to the imaging of hard magnetic films have been studied. First, we show that topographic images made in the 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. Second, imaging with probes coated by a hard magnetic film leads to phase maps, which show a twofold symmetry, with paired dark/light contrast on the 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. Third, due to the long-range nature of the stray field produced by hard magnetic films containing micrometer-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.

Micrometer-size Nd-Fe-B dots as model systems for the study of intergranular phase engineering in Nd-Fe-B permanent magnets

Nd-Fe-B micrometer-size dots were prepared by optical lithography and sputtering. It is proposed to use such structures as model systems to study intergranular phase engineering in Nd-Fe-B permanent magnets. The influence of Ta, Nd, Dy, and Gd coatings on the magnetization reversal of such Nd-Fe-B dot arrays are compared, after different heat treatments. A very thin layer of Dy (tNdFeB/tDy = 120) was found to lead to a significant increase of the coercive field, up to 80% for a total equivalent Dy content of less than 5 at. % of all the Nd. A coercivity increase of up to 20% was found with Gd coating which is attributed to the so-called superferrimagnetic coupling phenomenon. Nd and Ta coating are neutral or detrimental to the magnetic hardness.