Spatially controlled formation of superparamagnetic (Mn,Ga)As nanocrystals in high temperature annealed (Ga,Mn)As/GaAs superlattices

The annealing-induced formation of (Mn,Ga)As nanocrystals in (Ga,Mn)As/GaAs superlattices was studied by X-ray diffraction, transmission electron microscopy and magnetometry. The superlattice structures with 50 A thick (Ga,Mn)As layers separated by 25, 50 and 100 A thick GaAs spacers were grown by molecular beam epitaxy at low temperature (250 C), and then annealed at high temperatures of: 400, 560 and 630 C. The high temperature annealing causes decomposition to GaMnAs ternary alloy and formation of (Mn,Ga)As nanocrystals inside the GaAs matrix. The nanocrystals are confined in the planes that were formerly occupied by (Ga,Mn)As layers for up to the 560 C of annealing and diffuse throughout the GaAs spacer layers at 630 C annealing. The corresponding magnetization measurements show the evolution of the magnetic properties of as-grown and annealed samples from ferromagnetic, through superparamagnetic to the combination of both.Comment: 14 pages, 3 figure

Off-axis electron holography of magnetic nanostructures: magnetic behavior of Mn rich nanoprecipitates in (Mn,Ga)As system

The Lorentz off-axis electron holography technique is applied to study the magnetic nature of Mn rich nanoprecipitates in (Mn,Ga)As system. The effectiveness of this technique is demonstrated in detection of the magnetic field even for small nanocrystals having an average size down to 20 nm.Comment: 11 pages, 3 figure

Superconductivity and magnetism in RbxFe2-ySe2: Impact of thermal treatment on mesoscopic phase separation

An extended study of the superconducting and normal-state properties of various as-grown and post-annealed RbxFe2-ySe2 single crystals is presented. Magnetization experiments evidence that annealing of RbxFe2-ySe2 at 413 K, well below the onset of phase separation Tp=489 K, neither changes the magnetic nor the superconducting properties of the crystals. In addition, annealing at 563 K, well above Tp, suppresses the superconducting transition temperature Tc and leads to an increase of the antiferromagnetic susceptibility accompanied by the creation of ferromagnetic impurity phases, which are developing with annealing time. However, annealing at T=488K=Tp increases Tc up to 33.3 K, sharpens the superconducting transition, increases the lower critical field, and strengthens the screening efficiency of the applied magnetic field. Resistivity measurements of the as-grown and optimally annealed samples reveal an increase of the upper critical field along both crystallographic directions as well as its anisotropy. Muon spin rotation and scanning transmission electron microscopy experiments suggest the coexistence of two phases below Tp: a magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens just at Tp does not affect the volume fraction of the two phases, although the magnetic field distribution in the samples changes substantially. This suggests that the microstructure of the sample, caused by mesoscopic phase separation, is modified by annealing just at Tp, leading to an improvement of the superconducting properties of RbxFe2-ySe2 and an enhancement of Tc.Comment: 13 pages, 12 figure

Differential Adhesive Properties of Sequestered Asexual and Sexual Stages of Plasmodium falciparum on Human Endothelial Cells Are Tissue Independent

The protozoan parasite Plasmodium falciparum, responsible for the most severe form of malaria, is able to sequester from peripheral circulation during infection. The asexual stage parasites sequester by binding to endothelial cell receptors in the microvasculature of various organs. P. falciparum gametocytes, the developmental stages responsible for parasite transmission from humans to Anopheles mosquitoes, also spend the almost ten days necessary for their maturation sequestered away from the peripheral circulation before they are released in blood mainstream. In contrast to those of asexual parasites, the mechanisms and cellular interactions responsible for immature gametocyte sequestration are largely unexplored, and controversial evidence has been produced so far on this matter. Here we present a systematic comparison of cell binding properties of asexual stages and immature and mature gametocytes from the reference P. falciparum clone 3D7 and from a patient parasite isolate on a panel of human endothelial cells from different tissues. This analysis includes assays on human bone marrow derived endothelial cell lines (HBMEC), as this tissue has been proposed as a major site of gametocyte maturation. Our results clearly demonstrate that cell adhesion of asexual stage parasites is consistently more efficient than that, virtually undetectable of immature gametocytes, irrespectively of the endothelial cell lines used and of parasite genotypes. Importantly, immature gametocytes of both lines tested here do not show a higher binding efficiency compared to asexual stages on bone marrow derived endothelial cells, unlike previously reported in the only study on this issue. This indicates that gametocyte-host interactions in this tissue are unlikely to be mediated by the same adhesion processes to specific endothelial receptors as seen with asexual forms

Effects of heavy metals on the activity of dehydrogenases, phosphatases and urease in naturally and artificially contaminated soils

Most of the processes occurring in soil are catalysed by enzymes. As a result of their sensitivity towards heavy metals, enzymes in contaminated soils are usually less active. The purpose of this paper was to assess the influence of bioavailable forms of Cd, Cu, Pb and Zn on the activity of dehydrogenases, urease, acid and alkaline phosphatase, and to compare the results obtained from naturally and artificially contaminated soils. A pot experiment was carried out on two loamy sand soils, naturally and artificially contaminated with Cd, Cu, Pb and Zn. The total content of heavy metals classified these soils as very heavily contaminated with Cu, heavily contaminated with Pb and contaminated with Cd and Zn, all according to the IUNG system (1995). One of the following organic materials: swine manure or triticale straw, was added to the soil batches. The experiment was carried out in three replications, in two pH ranges: slightly acid and acid. Soil samples for analyses were taken after 14, 28, 165 and 450 days of incubation. The results of the experiment showed that the activity of soil enzymes depended on the content of bioavailable heavy metals; the total concentration of trace elements and H+ were less important. However, considerable differences were found in enzyme activity between naturally and artificially contaminated soils. This indicates that results obtained from other research conducted on freshly contaminated soils cannot be easily transferred to field conditions. The analysed enzymes responded differently to the concentration of bioavailable forms of heavy metals. Alkaline phosphatase was the least tolerant to bioavailable forms of heavy metals, unlike urease, which was the most tolerant soil enzyme. A similar pattern of sensitivity toward trace elements, which could be ordered as Zn > Cd > Cu > Pb, was noticed for dehydrogenases, acid and alkaline phosphatases. Urease was found to be more tolerant to Zn

Fabrication and Ferroelectric Properties of BiFeO₃/BaTiO₃ Heterostructures

We report on preparation and electrical characterization of the epitaxial BaTiO₃ (BTO), BiFeO₃ (BFO) thin films and BFO/BTO bi- and multilayers, grown on (001) SrTiO₃ (STO) and $(LaAlO_3)_{0.3}(Sr_2TaAlO_6)_{0.7}$ (LSAT) substrates. The ferroelectric properties were characterized using the electric force microscopy method to image and switch the electric domains. This fabrication process opens the routes towards wide study of magnetoelectric effect in complex oxide heterostructures