M. Yu. Presnyakov

Tunneling anomalous Hall effect in nanogranular CoFe-B-Al-O films near the metal-insulator transition

We present results of experimental studies of structural, magneto-transport and magnetic properties of CoFe-B-Al-O films deposited onto a glass ceramic substrate by the ion-beam sputtering of the target composed of Co40Fe40B20 and Al2O3 plates. The system consists on the strained crystalline CoFe metallic nanogranules with the size 2-5 nm which are embedded into the B-Al-O oxide insulating matrix. Our investigations are focused on the anomalous Hall effect (AHE) resistivity Rh and longitudinal resistivity R at T=5-200 K on the metallic side of metal-insulator transition in samples with the metal content x=49-56 at.%, that nominally corresponds to (Co40Fe40B20)x(Al2O3)100-x in the formula approximation. The conductivity at T > 15 K follows the lnT behavior that matches a strong tunnel coupling between nanogranules. It is shown that the scaling power-laws between AHE resistivity and longitudinal resistivity strongly differ, if temperature T or metal content x are variable parameters: Rh(T)~R(T)^0.4-0.5 obtained from the temperature variation of R and Rh at fixed x, while Rh(x)/x~R(x)^0.24, obtained from measurements at the fixed low temperature region (10-40 K) for samples with different x. We qualitatively describe our experimental data in the frame of phenomenological model of two sources of AHE e.m.f. arising from metallic nanogranules and insulating tunneling regions, respectively, at that the tunneling AHE (TAHE) source is strongly shunted due to generation of local circular Hall currents. We consider our experimental results as the first experimental proof of the TAHE manifestation.

The interaction between nerve cells and carbon nanotube networks made by CVD process investigation

In this research we investigate neuroblastoma cells cultivated on single-walled carbon nanotubes networks made by CVD method on silicon substrates. The complex analysis of grown cells made by atomic force, electron microscopy and Raman spectroscopy was carried out and the effect of nanotube growth process on proliferation factor was investigated. It is shown that despite of a weak decrease in proliferation, cell morphology remains unchanged and no physical or chemical interaction between carbon nanotubes and cells is observed. The results of the research can be used to investigate the interaction between conductive nano- materials and cells for the development of neural replacement implants. Also they can be useful in bio-electronic interface investigation of signal propagation in neurons.

Single-crystal structure study of iron chalcogenides Fe1 + δTe1 − xSx

Single crystals of iron chalcogenides Fe1 + δTe1 − xSx (x = 0, 0.09, and 0.1) were studied by scanning, transmission, and scanning transmission electron microscopy. In addition, a sample with x = 0 was studied by X-ray diffraction. The Te-site disorder observed in Fe1 + δ samples is, apparently, assigned to the incorporation of superstoichiometric iron Fe2 into the structure. The Fe1 atoms were found to be displaced from their ideal positions, which can only partially be attributed to electro-optical effects. The replacement of Te atoms by S in Fe1 + δTe1 − xSx crystals gives rise to domains with ordered S atoms in some regions of the single crystal, resulting in the formation of a 2 × 1 or 2 × 2 superstructure.

Study of the structural quality of heteroepitaxial silicon-on-sapphire structures by high-resolution X-ray diffraction, X-ray reflectivity, and electron microscopy

Heteroepitaxial silicon-on-sapphire (SOS) structures have been studied by high-resolution X-ray diffraction, X-ray reflectivity, electron microscopy, and electron diffraction. These methods yielded a large amount of complementary data on the defect structure of both the sapphire substrate and the silicon film, including integral and local (at the atomic-resolution level) information about the substrate, film, and sapphire-silicon interface.

Tribological and mechanical properties of composites based on ethylene-tetrafluoroethylene and quasicrystalline Al−Cu−Fe filler

Samples of composites, in which ethylene-tetrafluoroethylene copolymer is used as a matrix and quasicrystalline Al−Cu−Fe powder as a filler with 0, 1, 2, 4 and 8 vol % concentrations, are prepared. Electron microscopy studies of the sample structure are carried out. The influence of the filler on the crystallinity and temperatures of sample melting and destruction is investigated. The mechanical and tribological properties of the samples are tested. It is found that an increase in the filler content changes neither the mechanical nor thermodynamic characteristics of the material but significantly improves the tribological characteristics. The friction coefficient decreases twice at 1 vol % of the filler and the wear resistance increases by 40 times at 8 vol %. Experimental data indicate the probability of good adhesion of the filler particles to the fluoropolymer matrix. The composites under investigation may be of interest as promising materials for polymer friction bearings.

Evolution of morphology and structure with growth of thickness of condensed films of Pd–Cu on a surface with open porosity

Cross sections obtained by the method of a focused ion beam and fractures of the PdCu/Al2O3 nanocomposite synthesized by magnetron sputtering (MS) of alloy-type target PdCu and condensation in vacuum on the surface of nanoporous Al2O3 produced by anodic oxidation of aluminum foil are investigated using the methods of high-resolution scanning electron microscopy implemented on the basis of a Helios 600i setup (FEI, United States). Regularities of the formation of the structure and morphology of crystallites of vacuum condensate of Pd–Cu solid solution with a thickness of 0.1–4 μm on a surface with open porosity are disclosed. Approaches to the formation of the gradient structures near a free surface are discussed, and the conditions of the initiation of MS mechanisms of the formation of discrete, porous, and anisotropic condensates are found. Approaches to nonselective filling of nanopores in a dielectric by metal clusters and forming of the gradient structure of the nanocomposite PdCu/Al2O3 are carried out.

Study of one-dimensional crystal@single wall carbon nanotube nanocomposites using atomic resolution scanning transmission electron microscopy

Abstract1DCuI@SWCNT nanocomposites have been studied by scanning transmission electron microscopy (STEM) using the registration modes of large-angle scattered electrons and bright-field images in combination with the spectroscopy of electron energy losses. A new structure of a 1DCuI@SWCNT nanocomposite has been found. The results of the studies indicate that by using STEM modes it is necessary to obtain simultaneously both the images in a bright field and in high-angle annular dark field (HAADF) imaging. Using the spectroscopy of electron energy losses, the formation of a chemical bond between Cu3d and C2pz states of an intercalated nanocrystal and nanotube with the corresponding transfer of the electron density ∼0.09 e/carbon atom has been shown.

Effect of a quasicrystalline filler on the tribological properties of a composite based on ultrahigh-molecular-weight polyethylene

We prepare composites in which ultrahigh-molecular-weight polyethylene (UHMWPE) is used as a matrix and quasicrystalline Al–Cu–Fe powder is used as a filler with concentrations of 1, 10, 30, and 50 vol %. The mechanical and tribological properties of the samples are measured, and electron microscopic studies of fractured regions are performed. It is shown that with increasing concentrations of the filler, the nature of fracturing of the samples changes. The minimum value of the friction coefficient is 0.07. It is assumed that the limiting factor in improving the tribological properties of UHMWPE–quasicrystal composites is the chipping of filler particles from the polymer matrix.

Transport, Magnetic, and Memristive Properties of a Nanogranular (CoFeB) x (LiNbO y )100–x Composite Material

The properties of (CoFeB)x(LiNbOy)100–x nanocomposite films with a ferromagnetic alloy content x = 6–48 at % are comprehensively studied. The films are shown to consist of ensembles of CoFe granules 2–4 nm in size, which are strongly elongated (up to 10–15 nm) in the nanocomposite growth direction and are located in an LiNbOy matrix with a high content of Fe2+ and Co2+ magnetic ions (up to 3 × 1022 cm–3). At T ≤ 25 K, a paramagnetic component of the magnetization of nanocomposites is detected along with a ferromagnetic component, and the contribution of the former component is threefold that of the latter. A hysteresis of the magnetization is observed below the percolation threshold up to x ≈ 33 at %, which indicates the appearance of a superferromagnetic order in the nanocomposites. The temperature dependence of the electrical conductivity of the nanocomposites in the range T ≈ 10–200 K on the metallic side of the metal–insulator transition (44 at % < x < 48 at %) is described by a logarithmic law σ(T) ∝ lnT. This law changes into the law of “1/2” at x ≤ 40 at %. The tunneling anomalous Hall effect is strongly suppressed and the longitudinal conductivity turns out to be lower than in a (CoFeB)x(AlOy)100–x composite material by an order of magnitude. The capacitor structures based on (CoFeB)x(LiNbOy)100–x films exhibit resistive switching effects. They are related to (i) the formation of isolated chains of elongated granules and an anomalously strong decrease in the resistance in fields E > 104 V/cm because of the suppression of Coulomb blockage effects and the generation of oxygen vacancies VO and (ii) the injection (or extraction) of VO vacancies (depending on the sign of voltage) into a strongly oxidized layer in the nanocomposites, which is located near an electrode of the structure and controls its resistance. The number of stable resistive switchings exceeds 105 at a resistance ratio Roff/Ron ~ 50.

Electrocatalytic layers based on reduced graphene oxide for fabrication of low-temperature fuel cells

Reduced graphene oxide (rGO) with a specific surface area of ~600 m2/g has been synthesized and characterized. A series of membrane-electrode assemblies has been fabricated and tested as an element of a fuel cell. In the fabrication of these assemblies, rGO (0–10% of the weight of the Pt/Vulcan XC-72 electrocatalyst) was added to the electrocatalytic composition. The optimum rGO concentration in the active layer of a fuel cell is 5 wt %. At this rGO concentration, the specific power of the fuel cell is at least 20% higher than in the case of the rGO-free electrocatalytic layers.