Vol 61, No 2 (2019)

The influence of relative content of a metal component on the phase composition and substructure of Co_x(MgF₂)_100 – x in a wide range of x = 16–63 at % is studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy. The layers of nanocomposite with a micron thickness have been obtained by the ion-beam sputtering of the target in the argon environment. The results reveal that the relative metal cobalt content in the MgF₂ dielectric matrix strongly affects the phase composition and substructure of nanocomposites. With a lower content of cobalt, it is in the amorphous state in the form of clusters in the MgF₂ nanocrystalline matrix. An increase of cobalt content to x = 29 at % on a sitall substrate and to x = 42 at % on a glass substrate in the X-ray amorphous MgF₂ dielectric matrix leads to the formation of cobalt nanocrystals with a hexagonal crystallographic system, whose sizes are on the order of 10 nm. These are predominately oriented in the basis plane of the (001) hexagonal lattice of α-Со. A further increase of cobalt content to c = 59 at % enlarges the α-Со nanocrystals to ~20 nm with retaining the same orientation. In accordance with a fine structure analysis of XPS spectra of Со 2p and О 1s, cobalt is strongly oxidized on the surface of all the composites; only on the surface of samples with a low content of Co is X‑ray amorphous cobalt found in a metallic state along with cobalt oxide. The IR spectra of these samples with the lowest metal phase content exhibit the pronounced modes from the nanocrystalline MgF₂ dielectric phase.

Within the density functional method, a simple method for determining the dependence of the work function of electrons and specific surface energy of the metal on the relative density of internal vacancies \({{c}_{{v}}}\) is proposed. Preserving the style of the stabilized jellium model, the preliminarily calculated volume shift of the conductivity zone bottom ε⁽⁰⁾ ∝ \({{c}_{{v}}}\) in a specific homogeneous metal is introduced into a one-dimensional functional as the zero-point energy. Using the quantity \({{c}_{{v}}}\) as a small parameter, linear corrections to the abovementioned quantities are found. The expansion coefficients are expressed in terms of characteristics of a defectless metal. Calculations for Na and Al are carried out by the Kohn–Sham method. Temperature dependences of Al characteristics have been constructed in the thermodynamic limit.

A method for calculating the critical current of an inhomogeneous superconducting film (plate) in different external magnetic fields has been proposed. The superconducting properties are changed by varying the coherence length and London penetration depth over the thickness. The coherence length is maximum at the center of the plate and decreases upon approaching its boundaries and the London depth, on the contrary, is minimum at the center of the plate and increases toward its boundaries. Using the proposed approach, the magnetic field dependences of the critical current on the external magnetic field for the cases of nonuniform and uniform distributions of the superconducting properties over the film thickness have been calculated and compared. It has been established that at the nonuniform distribution of the superconducting properties the critical current of the plate is higher than at the uniform distribution at the same external magnetic fields. This is due to the fact that the order parameter is redistributed over the inhomogeneous plate thickness in such a way that the superconducting state becomes more stable against the current and magnetic field. It has been shown that the vortex-free Meissner state in inhomogeneous films is maintained at higher fields than in homogeneous films. The greater the film nonuniformity, the higher the stability of the Meissner state against an external magnetic field.

In this paper, we studied the effect of the magnetic field and temperature on conductivity of Bi_2 ‒ xSb_xTe_3 – ySe_y topological insulators of different composition to identify electronic surface states in these films. The experimental data can be explained based on the model with two types of charge carriers. Thermoactivation conductivity, electron or hole, is observed in the film bulk depending on the sample composition while metal-like electronic states are found on the surface. The noticeable contribution of surface topological states to the total conductivity opens up the possibility of studying them.

Alterations in the ground state of thin (001) films of a BiFeO₃ (BFO)-type multiferroic in a magnetic field are studied theoretically, with changes in the energy of induced anisotropy taken into account. The anisotropy vs. field phase diagrams identifying the stability regions of homogeneous antiferromagnetic states and the regions of emergence of spatially modulated antiferromagnetic states are constructed for three mutually orthogonal orientations of applied magnetic field. We show that, as the magnetic field decreases, the transformation of a homogeneous phase into a spatially modulated state occurs at the instability point of the homogeneous state via gradual emergence of the conical phase that transforms into a planar cycloid with the decreasing magnetic field. A multiferroic film grown on a (001) substrate develops considerable anisotropy of the energy of spatially modulated state, depending on the modulation orientation. Meanwhile, cycloids with different orientations undergo the transitions from incommensurate phase into the homogeneous state differently: either the conical cycloid is formed followed by its collapse into the homogeneous state or an unlimitedly growing domain of the homogeneous phase is formed within the flat cycloid. Examples of field-induced changes in magnetization, with changes in spin states taken into account, are provided. These results are of value in practical applications of multiferroic strain engineering.

Magnetic anisotropy values are obtained for [(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100 – x/Bi₂Te₃]₄₇ heterostructures consisting of SiO₂ alternating layers, CoFeB nanoparticles distributed in them, and Bi₂Te₃ layers with ferromagnetic resonance and magnetometry. The heterostructures have anisotropy of ~10⁶ erg/cm³, which orients the magnetic moment in films plane. The films are not solid, but they disintegrate into CoFeB nanoparticles with an average diameter of 5 nm during deposition, which corresponds to the blocking magnetization temperature of ~30 K during their saturation magnetization of M_S = 720 emu/cm³. The relationship between anisotropy constant and thickness of the layers of the heterostructures is nonmonotonic due to competition between surface and bulk anisotropies of the ferromagnetic granules, which the films are made of.

We present the results of studies of the dielectric properties of nanocomposites based on Al₂O₃ oxide films with a pore size of 330 and 60 nm with particles of an organic ferroelectric diisopropylammonium bromide (C₆H₁₆BrN, DIPAB) introduced into the pores, aimed at determining the size dependences of phase transition parameters. A shift in the phase transition to low temperatures and diffusion of the transition are found, which become more significant for smaller pores. A broadening of the temperature hysteresis of the dielectric constant of nanocomposites during the phase transition was also observed. The decrease in the phase transition temperature in nanocomposites with DIPAB nanoparticles is consistent with theoretical models of the size effects on the structural phase transition.

In the Ge/LTGe/GeSi/Si(001) heterostructures, the GeSi buffer layer remains pseudomorphic in a certain range of the heterostructure parameters and growth regimes, while the Ge film is completely relaxed owing to the edge dislocation network at the Ge/GeSi interface. It has been experimentally shown that, along with edge dislocations, dislocations with the Burgers vectors of the a₀〈100〉-type form. Their formation is caused by the reaction of 60° dislocations with a unidirectional screw component. In this case, if during the buffer layer relaxation the edge dislocations split with the formation of an edge-type dislocation complex, in which the 60° dislocations remained bound, the dislocations with the Burgers vectors a₀〈001〉 split into two independent 60° dislocations.

It has been established that the Portevin–Le Chatelier effect in the AlMg6 aluminum–magnesium alloy deformed in an aqueous medium is accompanied by an intermittent electrochemical response, such as the negative jumps of the electrode potential of the sample, which occur simultaneously with the jumps of the mechanical stress on the deformation curve. Statistical and fractal analyses of the stepwise component of the electrode potential indicate the presence of long-term correlations in the structure of the electrochemical response, which are typical for the state of self-organized criticality. A possible mechanism of the appearance of jumps of the electrode potential, which is associated with the dynamics of deformation bands, is discussed.

The electron paramagnetic resonance spectra of impurity trivalent erbium ions in synthetic forsterite single crystals (Mg₂SiO₄) have been studied by the electron paramagnetic resonance method in the X and Q frequency bands. It is found that erbium ions predominantly substitute for magnesium ions in crystallographic position M1 that is characterized by the inversion symmetry of the crystal field. In this case, a pronounced effect of dimer self-organization of erbium ions during the crystal growth takes place; the effect is manifested in the fact that the concentration of erbium dimer associates consisting of two closely spaced ions bound by the spin– spin interaction is several orders of magnitude higher than the concentration of dimer associates that form randomly at a statistical distribution of impurity ions in the forsterite crystal lattice. The directions of the main magnetic axes and the parameters of the effective spin Hamiltonian describing the magnetic characteristics of impurity erbium centers have been determined.

Raman spectra of Hg₂Br₂ model improper ferroelastic crystals have been investigated in a wide range of high hydrostatic pressures. Baric dependences of the phonon frequencies have been obtained; of greatest interest are the observed soft mode originating from the slowest TA₁ acoustic branch at the Brillouin zone boundary (X point) of the tetragonal phase and the anomalous behavior of this mode. In the ferroelastic phase spectra, the ignition of the second acoustic TA₂ from the same point has also been detected and its baric behavior has been studied. Under sufficiently high pressures, splitting of doubly degenerate phonons with the E_g symmetry has been observed and explained. The parameters of the Grüneisen constants have been determined from the baric dependences of the phonon frequencies and discussed.

The T–P phase diagrams of the halogenomethane compounds (CCl_4 – nBr_n, n = 0, 1, 2, 4) are calculated using a mean field model. By expanding the free energy in terms of the order parameters for the transitions of the liquid (L), rhombohedral (R), face-centered cubic (FCC) and monoclinic (M) phases in those compounds, the phase line equations are derived and they are fitted to the experimental data from the literature. This method of calculating the T–P phase diagram is satisfactory to explain the T–P measurements for the halogenomethane compounds and it can also be applied to two-component systems.

The main scenarios of nonequilibrium diffusional transformations induced by moving defects (dislocations, grain boundaries) in alloys under severe plastic deformation are considered. It has been shown that the phase state locally changes in the area of a defect where thermodynamic properties of alloy are locally changed, and the attained state is frozen after the displacement of a defect due to the difference between the rates of bulk diffusion and diffusion on a defect. For this reason, an alloy shifts from the state of its thermodynamic equilibrium under treatment, thus different nonequilibrium states, such as the disordering of alloy, the dissolution of equilibrium phase precipitates, the appearance of nonequilibrium phases, and the formation of regular structures, are possible depending on the type of the system. These effects may take place if the treatment of an alloy is performed at moderate temperatures, when diffusion is frozen in the bulk and rather active on defects. The phenomena of phase and structural instability developing under severe plastic deformation at moderate temperatures are considered within the framework of the proposed model.

The molecular dynamics method is applied to study structural and mechanical effects appearing during the lithium ion motion in a dc electric field along a planar channel formed by perfect silicene sheets and sheets containing vacancy-type defects. Mono-, di-, tri-, and hexavacancies of rather densely and uniformly filled silicene sheets are arranged one above the other on a graphite substrate. The times of Li⁺ ion passage through silicene channels with various gaps are determined. The construction of Voronoi polyhedra and truncated polyhedrons, whose centers coincide with the moving ion position allowed revealing the structural features inherent to the two-dimensional layered structure. The nature of stresses appearing in silicene sheets most critical to ion motion over the channel is determined.

The structure, lattice dynamics, and dielectric characteristics of Sr_0.61Ba_0.39Nb₂O₆ (SBN-61) single crystals and SBN-61/(001)MgO thin films have been studied. The analysis of temperature and frequency dependences of permittivity and dielectric loss tangents in a range of 10–500 K for a SBN-61 crystal has allowed one to establish the phase transition temperatures. The SBN-61/(001)MgO films are found to possess the tetragonal symmetry, characteristic of a SBN-61 crystal, but their lattice parameters are different from the latter.

Proton magnetic resonance (PMR) spectra and terahertz IR spectra of Vectra A950 fibers and granules and Armos fibers before and after heat treatment were obtained and analyzed in order to understand the molecular mobility mechanisms that ensure the self-organization and restructurization of these rigid-chain liquid-crystal polymers, as well as their similarity and difference. It is shown that large-scale thermal (quasi-segmental) mobility in such LC polymers is due to the reptation of macromolecules with respect to each other and the conformational transitions necessary for the motion of chains in them are the result of random accumulation of displacements that occur during local bending and torsional-vibrational movements in the links of polymer chains according to the Bresler–Frenkel mechanism.

The quantum chemical simulation of adsorption of atomic hydrogen on pristine and nitrogen-doped graphdienes has been performed. The preferential sites, adsorption on which is most energetically beneficial, are indicated. The nitrogen presence is shown to substantially increase the adsorption capacity of the sheet. A capacity of the nitrogen-doped graphdiene to be reversibly stretched by 4% under action of external mechanical stress is demonstrated. A mechanical stretching is found to enable the control of the adsorption properties of pristine and also doped graphdienes.