Strain-Control of the magnetic anisotropy in (Ga,Mn)(As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10 %) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects stongly affect the domain wall propagation

MARINE-EXPRESS: taking advantage of high throughput cloning and expression strategies for the post-genomic analysis of marine organisms

Background: The production of stable and soluble proteins is one of the most important steps prior to structural and functional studies of biological importance. We investigated the parallel production in a medium throughput strategy of genes coding for proteins from various marine organisms, using protocols that involved recombinatorial cloning, protein expression screening and batch purification. This strategy was applied in order to respond to the need for post-genomic validation of the recent success of a large number of marine genomic projects. Indeed, the upcoming challenge is to go beyond the bioinformatic data, since the bias introduced through the genomes of the so called model organisms leads to numerous proteins of unknown function in the still unexplored world of the oceanic organisms. Results: We present here the results of expression tests for 192 targets using a 96-well plate format. Genes were PCR amplified and cloned in parallel into expression vectors pFO4 and pGEX-4T-1, in order to express proteins N-terminally fused to a six-histidine-tag and to a GST-tag, respectively. Small-scale expression and purification permitted isolation of 84 soluble proteins and 34 insoluble proteins, which could also be used in refolding assays. Selected examples of proteins expressed and purified to a larger scale are presented. Conclusions: The objective of this program was to get around the bottlenecks of soluble, active protein expression and crystallization for post-genomic validation of a number of proteins that come from various marine organisms. Multiplying the constructions, vectors and targets treated in parallel is important for the success of a medium throughput strategy and considerably increases the chances to get rapid access to pure and soluble protein samples, needed for the subsequent biochemical characterizations. Our set up of a medium throughput strategy applied to genes from marine organisms had a mean success rate of 44% soluble protein expression from marine bacteria, archaea as well as eukaryotic organisms. This success rate compares favorably with other protein screening projects, particularly for eukaryotic proteins. Several purified targets have already formed the base for experiments aimed at post-genomic validation

Normal-state bubbles and lamellae in type-I superconductors

International audienceWe report an extensive study of the formation of normal-state domains in type-I superconductors. Domain patterns are first considered theoretically. The magnetic interaction between domains is described in the framework of the ``current-loop'' model: the intermediate state is modeled by a set of loops of screening current encircling the domains and interacting as in the free space. This system is shown to be formally equivalent to a set of uniformly magnetized domains. An extension of the current-loop model is proposed to take into account the constraint of the magnetic shielding by the superconducting regions. We determine the free energy of a hexagonal array of cylindrical domains (bubbles) and of a lattice of infinitely long and parallel stripes. The equilibrium values of both the volume fraction of the normal phase and the domain size are calculated as functions of the magnetic field. A bubble-to-stripe transition is predicted to occur for a volume fraction of the normal phase about 0.3. Experimentally, normal-state domains are studied with the high-resolution magneto-optical imaging technique. The observed patterns consist in coexisting bubbles and disordered labyrinthine lamellae structures. We show evidence of the contribution of pinning on the position of domain interfaces. The average width of the lamellae is then analyzed as a function of the applied magnetic field and found to increase in good agreement with the predictions. In contrast, the average diameter of bubbles remains constant: it is almost independent of the magnetic interaction between domains. A very good agreement, over three decades of the magnetic Bond number, is found with the equilibrium diameter of an isolated bubble. The proposed constrained current-loop model is shown to provide significantly more accurate predictions than the current-loop model, in particular for small magnetic Bond numbers. Additionally, increasing the volume fraction of the normal phase results in a bubble-to-lamella transition, as predicted theoretically

Novel magneto-optic layers based on semiconductor nanostructures for Kerr microscopy.

Symposium on Magneto-Optical Materials for Photonics and Recording held at the 2004 MRS Fall Meeting, Boston, MA, NOV 29-DEC 29, 2004International audienceA novel type of magneto-optic layers based on CdMnTe quantum wells is used to image the magnetic flux pattern at the surface of type I superconductors. The magneto-optic layer is designed as an anti-reflecting optical cavity. The quantum wells are arranged in a Bragg structure and placed at maxima of the electric field in the cavity in order to enhance Faraday rotation

Unusual domain-wall motion in ferromagnetic semiconductor films with tetragonal anisotropy

International audienceMagnetic field-driven domain-wall propagation in the flow regime is investigated in (Ga, Mn) As ferromagnetic semiconductor layers. Square-shape magnetic domains with an unexpected orientation of their edges, at pi/8 with respect to the anisotropy axes, are found. This is shown to arise from the effect of tetragonal magnetic anisotropy on domain-wall dynamics. Using a one-dimensional model for domain-wall motion and modeling domain growth by contour dynamics the shape and orientation of domains and their field range for existence are well reproduced. These results point to the key role of the vectorial nature of the order parameter in the dynamics of ferromagnetic domains

(Ga,Mn)As layers with perpendicular anisotropy: a study of magnetic domain patterns

International audienceThe magnetic domain pattern in (Ga,Mn)As layers with perpendicular magnetic easy-axis is studied by Kerr microscopy at various temperatures. We investigate the effect of post-growth annealing. As-grown samples exhibit a strongly anisotropic domain growth with dendritic-like expansion of domains along the directions. For annealed samples, the density of nucleation centers is decreased and domain wall propagation is more isotropic. Two kinds of pinning centers are observed in annealed samples: line-like defects that strongly hinder domain wall propagation and point-like defects with a density of a few 10(5) cm(-2)

Magnetic domain patterns in as-grown and annealed (Ga,Mn)As epilayers with perpendicular easy axis

International audienceMagnetic properties of ferromagnetic (GaMn)As epilayers grown with tensile strain are investigated using combined experimental techniques including Kerr microscopy, transport measurements, and SQUID magnetometry. We show that post-growth annealing substantially increases the Curie temperature, strengthens the perpendicular anisotropy, weakens the in-plane anisotropy and drastically changes the magnetic domain pattern

X-ray Diffraction and Density Functional Theory Provide Insight into Vanadate Binding to Homohexameric Bromoperoxidase II and the Mechanism of Bromide Oxidation

International audienceX-ray diffraction of native bromoperoxidase II (EC 1.11.1.18) from the brown alga Ascophyllum nodosum reveals at a resolution of 2.26 Å details of orthovanadate binding and homohexameric protein organization. Three dimers interwoven in contact regions and tightened by hydrogen-bond-clamped guanidinium stacks along with regularly aligned water molecules form the basic structure of the enyzme. Intra- and intermolecular disulfide bridges further stabilize the enzyme preventing altogether the protein from denaturing up to a temperature of 90 °C, as evident from dynamic light scattering and the on-gel ortho-dianisidine assay. Every monomer binds one equivalent of orthovanadate in a cavity formed from side chains of three histidines, two arginines, one lysine, serine, and tryptophan. Protein binding occurs primarily through hydrogen bridges and superimposed by Coulomb attraction according to thermochemical model on density functional level of theory (B3LYP/6-311++G**). The strongest attractor is the arginine side chain mimic N-methylguanidinium, enhancing in positive cooperative manner hydrogen bridges toward weaker acceptors, such as residues from lysine and serine. Activating hydrogen peroxide occurs in the thermochemical model by side-on binding in orthovanadium peroxoic acid, oxidizing bromide with virtually no activation energy to hydrogen bonded hypobromous acid

Magnetic domain pattern asymmetry in (Ga, Mn)As/(Ga, In)As with in-plane anisotropy

International audienceAppropriate adjustment of the tensile strain in (Ga, Mn)As/(Ga,In)As films allows for the coexistence of in-plane magnetic anisotropy, typical of compressively strained (Ga, Mn)As/GaAs films, and the so-called cross-hatch dislocation pattern seeded at the (Ga, In) As/GaAs interface. Kerr microscopy reveals a close correlation between the in-plane magnetic domain and dislocation patterns, absent in compressively strained materials. Moreover, the magnetic domain pattern presents a strong asymmetry in the size and number of domains for applied fields along the easy [1 (1) over bar0] and hard [110] directions which is attributed to different domain wall nucleation/propagation energies. This strong influence of the dislocation lines in the domain wall propagation/nucleation provides a lithography-free route to the effective trapping of domain walls in magneto-transport devices based on (Ga, Mn) As with in-plane anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4704385