Том 58, № 1 (2016)

Electromagnetic emission accompanying the serrated deformation of the aluminum‒magnesium alloy Al-6Mg has been revealed and studied experimentally. By means of high-speed video recording and a complex of methods for measuring the strain, load, and electric potential, it has been found that there is a relation between the electromagnetic emission signals and the dynamics of deformation bands. Possible mechanisms of the generation of electromagnetic signals have been discussed.

Partial (ionic, proton, and hole) conductivities of oxides CaZr_1–xSc_xO_3–α(x = 0.03–0.20) with the perovskite structure in air atmosphere have been studied as functions of temperature in the range of 600–900°C and partial water-vapor pressure in the range of \({P_{{H_2}O}}\)= 40–2500 Pa. The influence of the humidity of the atmosphere on the relative change in the concentration of oxygen vacancies as a function of temperature has been estimated.

Vibrational spectra of TlInS₂ crystals in the terahertz frequency range (0.1–3 THz) have been obtained and analyzed. The observed low-frequency band is attributed to the libration oscillations of nanolayers (so-called boson peak). A correlation between the coherent low-frequency oscillation and a disordered character of the crystal structure is shown. A conclusion is made on the presence of a topological disorder that manifests itself on the layered subsystem level.

Iron disulfide (FeS₂) films with a wide range of electrical resistivities 100 Ω cm ⩽ ρ ⩽ 800 kΩ cm, a high adhesion to the substrate, and a resistance to aggressive media have been prepared by the spray pyrolysis of aqueous solutions of the salts FeCl₃ · 6H₂O and (NH₂)₂CS at low temperatures in the range 250°C ⩽ T_S ⩽ 400°C. It has been found that the FeS₂ films have a high transmittance T ≈ 60–70% and are characterized by a sharp transmission edge. It has been shown that the optical band gap for direct (E_g^op = 2.19–2.78 eV) and indirect (E_g^′op = 1.26–1.36 eV) optical transitions depends on the conditions of film preparation.

A number of solid solutions Bi₇Ti_{4 + x}W_xTa_1–2xO₂₁ (x = 0–0.5) have been synthesized from oxides by solid-phase reaction. The crystal structure, the electrophysical characteristics, and the microstructure of the prepared ceramic samples have been studied. According to X-ray powder diffraction, all the compounds are single-phase with the structure of mixed-layer Aurivillius phases (m = 2.5) with the orthorhombic crystal lattice (space group I2cm, Z = 2). Temperature dependences of the relative permittivity ε(T) of the compound have been measured, from which it has been found that the Curie temperature T_C of perovskite-like oxides Bi₇Ti_{4 + x}W_xTa_1–2xO₂₁ (x = 0–0.5) decreases linearly as substitution parameter x decreases. The activation energies of charge carriers have been found in different temperature ranges.

Components of the fields scattered by a periodic planar strip structure of thin magnetic films possessing a uniaxial magnetic anisotropy in the plane have been calculated using the phenomenological model. Regularities in the dependence of these fields on the design parameters of the structure have been studied. The results obtained agree with the numerical analysis of the micromagnetic model of this structure. It has been shown that, near the edges of strips magnetized orthogonally to the major axis, the components of the scattered field can exceed the external magnetizing field by a few orders of magnitude. This fact makes it possible to design highly efficient magnetoresistive elements on the basis of a strip structure of magnetic films and thin semiconductor films.

The La_2/3Ca_1/3MnO₃ films 25-nm thick biaxially compressed in the substrate plane, grown quasi-coherently on the (001) surface of the LaAlO₃ single crystal, were studied using laser vaporization. It was found that mechanical stresses acting during nucleation and growth promoted calcium enrichment of the cation sublattice of the manganite layer, which caused a decrease in its unit cell volume. Crystalline grains in manganite films were distinctly oriented along the normal to the substrate plane; the grain size in the substrate plane was within 20–40 nm, and the relative grain misorientation in the substrate plane did not exceed 0.2°. In zero magnetic field, the maximum in the temperature dependence of the resistivity ρ of La_2/3Ca_1/3MnO₃ films was observed at temperatures close to 210 K. At T < 100 K and μ₀H = 2 T, the magnetoresistance of manganite films was negative, weakly depended on temperature, its value was about–0.45. The magnetic field caused transformation of nonferromagnetic phase inclusions to ferromagnetic ones, which resulted in a decrease in the La_2/3Ca_1/3MnO₃ film resistivity with increasing magnetic field. At low temperatures (T < 100 K), a hysteresis was observed in the dependences of the film resistivity on the magnetic field.

The complete magnetic refrigeration cycle has been simulated on a sample of gadolinium in magnetic fields of a Bitter coil magnet up to 12 T. The total change of temperature of the sample during the cycle is a consequence of magnetic refrigeration, and the dependence of the magnetization of the sample on the magnetic field exhibits a hysteretic behavior. This makes it possible to determine the work done by the magnetic field on the sample during the magnetic refrigeration cycle and to calculate the coefficient of performance of the process. In a magnetic field of 2 T near the Curie temperature of gadolinium, the coefficient of performance of the magnetic refrigeration is found to be 92. With an increase in the magnetic field, the coefficient of performance of the process decreases sharply down to 15 in a magnetic field of 12 T. The reasons, for which the coefficient of performance of the magnetic refrigeration is significantly below the fundamental limitations imposed by the reversed Carnot theorem, have been discussed.

The form of phase diagrams has been determined for epitaxial magnetic films of the composition Y_3–x–yBi_xPr_yFe₅O₁₂ on the plane (H_‖H_⊥), where H_‖ and H_⊥ are the external magnetic field components parallel and orthogonal, respectively, to the direction of the normal n to the surface of the film. The specific features have been revealed in the magnetization reversal of the objects under investigation, which are associated with their structural stratification.

The results of the Mössbauer studies on ⁵⁷Fe nuclei in multiferroics BiFe_1–xT_xO₃ (T = Sc, Mn; x = 0, 0.05) in the temperature range of 5.2–300 K have been presented. The Mössbauer spectra have been analyzed in terms of the model of an incommensurate spatial spin-modulated structure of cycloid type. Information has been obtained about the effect of the substitution of Sc and Mn atoms for Fe atoms on the hyperfine parameters of the spectrum: the shift and the quadrupole shift of the Mössbauer line, the isotropic and anisotropic contributions to the hyperfine magnetic field, and also the parameter of anharmonicity of the spatial spin-modulated structure.

The valence state of transition metal ions in the Co_1–xFe_xCr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.

The effect of a constant magnetic field on the dielectric properties of triglycine sulfate crystals has been investigated. It has been shown that, after the magnetic treatment of the crystal (2 T, 20 min), the hysteresis loop becomes narrower; i.e., the coercive field decreases, and the dielectric permittivity changes in the region of the phase transition. It has been found that the observed effect is anisotropic with respect to the orientation of the crystal in a magnetic field and occurs when the vector of magnetic induction is perpendicular to the polar axis of the crystal. The relative orientation of the magnetic field and the domain structure determines its sign. The doping of the crystal with chromium makes the effect more pronounced and leads to a change in the kinetics of the magnetically stimulated increase in the dielectric permittivity.

The results of the investigation of the quadratic electro-optic effect in Sr_1–xCa_xTiO₃ with x = 0.014 (SCT) and in nominally pure SrTiO₃ (STO) at room temperature in applied direct-current (dc) and alternating-current (ac) electric fields have been presented. It has been shown that the quadratic (in polarization) electro-optic coefficients of STO and SCT crystals coincide within the accuracy of the determination (±5%). It has been found that, in nominally pure STO measured in a dc electric field, there is a relaxation of the electro-optic effect with a relaxation time τ ≈ 30 s due to the formation of a space charge in the sample. No similar effect in SCT has been observed. A possible mechanism for the formation of a space charge in STO and SCT has been discussed.

The effect of ultrasonic deformation on the inelastic behavior of potassium hydrogen phthalate single-crystal samples with different crystallographic orientations at frequencies of ∼10⁵ Hz has been investigated. It has been shown that the inelastic behavior of potassium hydrogen phthalate single crystals during the ultrasonic deformation is determined not by the dislocation mechanisms. It has been found that the main mechanism responsible for energy losses of mechanical vibrations in these crystals is the mechanism of ionicrelaxation polarization. The characteristic relaxation times of this process have been determined and varied in the range from 10^–8 to 10^–7 s.

Infrared absorption spectra of a Nd_0.5Ho_0.5Fe₃(BO₃)₄ crystal in the spectral range of 30–1700 cm^–1 have been measured at temperatures from 6 to 300 K. The experimental spectra have been analyzed based on the semiempirical calculation of the lattice dynamics and the analysis of correlation diagrams of borate complexes. No changes associated with structural phase transitions have been detected in the temperature range of measurements; the effect of magnetic ordering on the infrared absorption spectra has not been observed.

The results of the investigation of the optical properties and calculations of the density of states N(E) of the Heusler alloys Co₂Cr_1–xFe_xAl (x = 0, 0.4, 0.6, 1.0) have been presented. It has been found that the band spectrum substantially changes, especially in the vicinity of the Fermi level, due to the concentration transition from Co₂FeAl to Co₂CrAl. These changes are accompanied by significant changes in the optical properties. The optical properties of the Co₂FeAl alloy are typical of metals. The anomalous behavior of the optical properties of the alloys with x = 0, 0.4, and 0.6 is determined by the electronic states at the Fermi level.

The results of the atomistic simulation of a superionic transition and melting of uranium dioxide are presented. The temperature dependences of the concentration of defects in the oxygen sublattice and changes in the heat capacity and isothermal compressibility upon the superionic transition are calculated. It is shown that the curve of the superionic transition in the PT diagram can be described by the Ehrenfest’s equation. The possibility of describing the superionic transition within the framework of the theory of second- order phase transitions is discussed. Based on the results obtained, it is considered that this structural transformation can occur in other materials.

A theoretical description of the process of reorientation of the c-director and the tangential component of the velocity vector in a hybrid-oriented SmC film stretched between the rotating inner and rest outer frames has been proposed. Within the framework of the nonlinear generalization of the classical Ericksen–Leslie theory for the case of smectic phases, it has been shown that the magnitude and character of the reorientation of the c-director field and the formation of hydrodynamic flows in SmC films are strongly affected by the curvature and direction of rotation of the inner frame.

The spatial distribution of light emission has been studied in planar field electron emitters with long and sparse carbon nanofilaments/nanotubes. The photographic recording of light emission of the emitting nanofilaments/nanotubes is shown to be efficient to determine the position of individual nanofilaments/ nanotubes in different emitter surface areas, as well as to highlight the nanofilaments/nanotube agglomerate distribution over the emitter surface, which mainly contributes to its emission.

The thermal conductivity k and resistivity ρ of biocarbon matrices, prepared by carbonizing medium-density fiberboard at T_carb = 850 and 1500°C in the presence of a Ni-based catalyst (samples MDF-C( Ni)) and without a catalyst (samples MDF-C), have been measured for the first time in the temperature range of 5–300 K. X-ray diffraction analysis has revealed that the bulk graphite phase arises only at T_carb = 1500°C. It has been shown that the temperature dependences of the thermal conductivity of samples MDFC- 850 and MDF-C-850(Ni) in the range of 80–300 K are to each other and follow the law of k(T) ∼ T^1.65, but the use of the Ni-catalyst leads to an increase in the thermal conductivity by a factor of approximately 1.5, due to the formation of a greater fraction of the nanocrystalline phase in the presence of the Ni-catalyst at T_carb = 850°C. In biocarbon MDF-C-1500 prepared without a catalyst, the dependence is k(T) ∼ T^1.65, and it is controlled by the nanocrystalline phase. In MDF-C-1500(Ni), the bulk graphite phase formed increases the thermal conductivity by a factor of 1.5–2 compared to the thermal conductivity of MDF-C-1500 in the entire temperature range of 5–300 K; k(T = 300 K) reaches the values of ∼10 W m^–1 K^–1, characteristic of biocarbon obtained without a catalyst only at high temperatures of T_carb = 2400°C. It has been shown that MDF-C-1500(Ni) in the temperature range of 40‒300 K is characterized by the dependence, k(T) ∼ T^1.3, which can be described in terms of the model of partially graphitized biocarbon as a composite of an amorphous matrix with spherical inclusions of the graphite phase.