Vol 59, No 3 (2017)

The electronic structure and optical properties of the hexagonal intermetallic compound Gd₅Si₃ are investigated. The spin-polarization calculation of the band spectrum is performed in the local spin density approximation, taking account for the strong electron correlations in the 4 f shell of a Gd ion (LSDA + U method). Optical constants of the compound in the wavelength range of 0.22–15 μm are determined by the ellipsometry technique and some spectral characteristics are calculated. The frequency dependence of optical conductivity in the light quantum absorption region is analyzed on the basis of the calculated electron density of states.

Quasicrystalline coatings prepared under various thermal conditions of spraying have been studied. Initial quasicrystalline powders with dispersion of 10–50 μm were prepared in a low-pressure arc discharge plasma. The coatings have been sprayed on copper rings using a swinging plasmatron. It is found that the increase in the quenching rate of melt droplets increases the chemical homogeneity and leads to formation of nanostructured formations. The precipitation of nanostructured grains (d < 100 nm) in the sprayed alloy leads to an increase in the mechanical characteristics (hardness, deformation, and ductility) and can be considered as an additional factor of hardening of the material.

This paper presents a comparative study of the complex permeability μ* of single crystals of hightemperature superconductors RBa₂Cu₃Oy (R = Y, Tm) as a function of the magnetic field applied along the crystallographic ab-plane. Contributions to μ* from the oscillatory motion of vortices perpendicular to layers of the crystal lattice, μ_v, and realization of the critical state along the layers are obtained. It is found that, as the temperature approaches T_c, the behavior of the field dependences of μ_v and the critical current is substantially different for the given samples. This effect is related to the manifestation of an additional unusual mechanism of intrinsic pinning, which arises when Y₃₊ is replaced with the Tm³⁺ magnetic ion. The revealed specificities of the interaction of vortices with the magnetic ion layer suggest that, most probably, they have a magnetic nature and are not related to the variation in the condensation energy in the core of the vortex.

The results of the phenomenological study of the abnormal photoresponse kinetics in layered TlGaSe₂ ferroelectric semiconductor have been discussed over the temperature range T of ~170–280 K corresponding to the paraelectric phase of crystal. Taking into account the alterations in the photoresponse kinetics temperature, the main mechanisms of anomalies caused by the spatial inhomogeneity of localized and nonlocalized charges in the bulk of the crystal have been assumed. The mechanism of parametric resonance is suggested to be favored by the photogalvanic currents in the crystal.

The magnetic properties of an oxygen-deficient nanocrystalline undoped titanium dioxide synthesized by the gas-phase, electric-explosion, and chemical method have been studied. The defect state was controlled using reduction treatments in vacuum or in a hydrogen atmosphere. It is shown that the defect state of the surface of nanocrystalline oxides (for example, the existence of vacancies in the anion sublattice and other defects) has a dominant influence on the formation of the magnetic properties of the samples under study. The main contributions to the magnetism of TiO₂ nanoparticles after the reduction treatments are the paramagnetic contribution of the matrix, the paramagnetic Curie–Weiss contribution, and the contribution of the spontaneous magnetic moment provided by the existence of regions with different spin ordering. A heterogeneous magnetic state is found to exist in the TiO₂ nanopowders; for example, at low temperatures, shifted hysteresis loops are observed as a result of a possible set of magnetic states with different spin orders. It is shown that a soft compaction or grinding of nanopowders in an agate mortar lead to substantial increase in the magnetization, sometimes, by a factor of more than two, regardless of the nanopowder synthesis method and the initial phase state of TiO₂ (anatase or rutile structures). This experimental fact proves the key role of the surface defects and the magnetic moment carriers with different spin configurations localized mainly on the nanoparticle surface. The compaction changes the magnetization only in the case when the initial magnetic state has a nonlinear “quasi-superparamagnetic” character of the magnetization curve. As a result of predominant exchange interaction between the nanoparticles with a frustrated character of spin ordering on the nanoparticles surface, the ferromagnetic contribution increases as nanoparticles contact.

Luminescence of yttrium-aluminum garnet Y₃Al₅O₁₂: Eu,Si samples is studied. It is found that luminescence is associated with Eu²⁺ ions, but does not correspond to intracenter 4f⁶5d¹–4f⁷ transitions. Measurements of the excited state lifetime point to the luminescence mechanism involving charge-transfer states.

Using ab initio calculations, the data have been obtained on the structural, electronic, and optical properties of bismuth titanates with the pyrochlore type structure and compounds with the substitution of scandium or indium atoms for bismuth and titanium atoms. The results of the theoretical calculations agree with the experimentally obtained structural and optical characteristics of the synthesized compounds doped with scandium or indium. It has been shown that the substitution of scandium or indium atoms for bismuth atoms in the pyrochlore structure is energetically favorable. The energies corresponding to the direct and indirect electronic transitions in scandium- and indium-doped bismuth titanates have been determined based on the optical spectroscopy data obtained for the studied samples. These energies are in agreement with the theoretically calculated values.

The influence of the anisotropic exchange interaction on the phase states of a non-Heisenberg ferromagnet with a magnetic-ion spin S = 1 is studied. It is shown that, depending on the relationships between the parameters of anisotropic exchange interaction and their signs, either a biaxial non-Heisenberg magnet or an Ising magnet are realized in the system. Dynamic properties of the system near orientational phase transitions and transitions in the absolute value of the magnetic moment are studied. Phase diagrams of the system for different relationships between constitutive parameters are plotted.

A three-dimensional computer simulation of static magnetization configurations and dynamic processes occurring in a domain wall moving in a uniaxial magnetic film with perpendicular anisotropy has been performed based on the numerical solution of Landau–Lifshitz–Gilbert equations. The calculated static states correspond to a domain wall containing Bloch lines with a surface magnetization distribution that depends on the thickness of the film. It has been shown that these structures can be characterized by particular values of the homotopy index. It has been found that the vortex and antivortex structures existing in the bulk of the film form vortex filaments. A method has been proposed for visualization of the joint motion of vortex filaments and Bloch points, which is based on the numerical calculation of the homotopy index and the winding number.

Regularities of hole transport and its correlation with percolation magnetism caused by localized carriers simultaneously involved in the formation of the magnetic and electrical properties of Ge: Mn thin films are investigated. It is established that at temperatures of T > 22 K the activationless band carrier transport occurs in the Ge: Mn samples (2 at % Mn). At low temperatures, the hopping mechanism with a variable hopping length works.

Single crystals of the Tb_0.75Ho_0.25Fe₃(BO₃)₄ ferroborate have been grown by the group method from a solution–melt based on bismuth trimolybdate. The magnetic and magnetoelectric properties of the ferroborate single crystals have been investigated in the temperature range from 4.2 to 300 K and in magnetic fields up to 9 T. Magnetically, this material is an antiferromagnet with the Néel temperature T_N = 38.8 K and easy-axis anisotropy. The magnitude of the magnetoelectric polarization has been found to be more than 1.5–2.0 times greater than the sum of the polarizations induced by the magnetic field for the ferroborates TbFe₃(BO₃)₄ and HoFe₃(BO₃)₄ taken in the corresponding shares.

Electron spin resonance spectra of Gd³⁺ ions forming a small (~0.0001 at %) impurity in a LiYF₄ single crystal have been investigated in a wide temperature range from liquid helium to room temperature. A number of the fine-structure components of the spectrum exhibit a pronounced superhyperfine structure depending on the orientation of the external magnetic field with respect to the crystallographic axes. The superhyperfine structure was not observed in earlier ESR studies of double fluorides with a Gd³⁺ impurity.

For the first time, fatigue processes in triglycine sulfate crystals were studied using a combination of electrical and scanning probe microscopy methods. Long-term (>100 h) influence of a sinusoidal field with a frequency of 50 Hz and an amplitude of 1 kV/cm lead to a sharp decrease of the permittivity in the phase transition region and degradations of P–E hysteresis loops (a decrease of spontaneous polarization, an increase of coercive and bias fields). Changes in dielectric properties were accompanied by an increase of the defect nanoclusters density and broadening their size distribution spectrum on the (010) cleavage surface. A subsequent exposure of “fatigued” crystals in the static magnetic field of 2 T for 20 min led to hysteresis loop symmetrization, which indicates a magnetoinduced transformation in the structure of defects responsible for fatigue effects.

An electrical breakdown of the air near the surface of a compressed granite plate initiates a shock wave in it. Having reached the back side of the plate, the shock wave causes successive (with an interval of ~50 ns) emission of plasma jets presumably consisting of positively charged ions. The intensities of the jets are distributed exponentially. While the compression pressure P does not exceed ~0.9–0.95 of the failure pressure P_f, it does not affect the number and efficiency of the radiation sources. At P ≈ (0.9–0.95)P_f, the shock wave causes the emergence of a crack destructing the sample. Simultaneously, the number and efficiency of ion sources increase 3–4-fold. This phenomenon is explained by an increase in the concentration of clusters of dislocations upon the creep of the sample.

The electron paramagnetic resonance (EPR) spectra of Gd³⁺ impurity centers in CaMoO₄ single crystals have been investigated at temperatures T = 1.8, 4.2, and 99–300 K. The temperature dependences of the spin Hamiltonian parameters b_n^m(T) (n = 2, 4, m = 0, 4) have been determined, and their analysis has been carried out. The temperature contributions from the static lattice, b₂(L) and b_4m(L), to the spin Hamiltonian parameters b_n^m(T) have been separated. For this purpose, the changes of the static lattice contributions b₂(L) and b_4m(L) have been determined taking into account the temperature shifts of the oxygen ions nearest to the Gd³⁺ ions in CaMoO₄ single crystals. The differences b_nm(F) = b_n^m–b_nm(L) have been attributed to the spin–phonon contribution. The analysis of the results obtained has demonstrated that the parameters b₂₀(F) and b_4m(F) are positive, and the dependence of b₂₀(F) on the temperature T is well described in the model of local vibrations (G. Pfister). However, the temperature behavior of the parameter b_4m(F) could not be described in the framework of the well-known models of the spin–phonon interaction.

Oriented porous polypropylene films with lamellar crystal structure are studied by the laser radiation scattering method. The dependence of the scattering pattern on the degree of film orientation is determined. It is shown that the sizes of the central maximum of the scattering pattern depend linearly on the degree of film extension. The results obtained can be used in developing methods of optical nondestructive testing of polymeric materials.

The structure of a ~30 nm thick surface layer of a heterogeneous nanocrystalline solid body (sandstone) before and after the friction was investigated using photoluminescence and Raman spectroscopy. Before the friction, this layer contained nanocrystals of quartz, anatase, feldspar, and montmorillonite. The friction caused a sharp decrease in the concentration of nanocrystals of quartz and feldspar.

The electrical, optical, and structural properties of silver nanostructures at the percolation threshold, which were prepared from a conductive film by laser treatment, have been investigated experimentally. It has been found that the threshold voltage applied to the silver film leads to an abrupt change in its electrical resistance. At high voltages, there is a region with a negative differential resistance. These changes in the electrical conductivity under the influence of the applied voltage have been explained by small structural changes in the film.

The chemical composition, interatomic chemical bonds, and specific features of the atomic structure of nanosized carbon films on the surface of iron, which were prepared by magnetron sputtering before and after ion-beam mixing, have been investigated using X-ray photoelectron spectroscopy, X-ray diffraction analysis, extended electron energy loss fine structure spectroscopy, and Raman spectroscopy. It has been found that, in the ion-beam mixing region, there are structural inhomogeneities and nonstoichiometric iron carbides. The parameters of the local atomic structure of an ultrathin surface layer and the “carbon film–metal” transition region have been determined.

A carbon composite material based on fullerenes and exfoliated graphite with different ratios (from 16: 1 to 1: 16 by weight) is synthesized and investigated. Samples are obtained by the introduction of C₆₀ in a conductive matrix by means of heat treatment of initial disperse mixtures in a vacuum diffusion–adsorption process followed by cold pressing and annealing. It is shown that covalent bonds are formed between the fullerenes and the environment. The conductivity of the samples is quite high and ranges from a few to hundreds (Ω cm)^–1. The concentration of charge carriers (mainly holes) is ~10¹⁹ to 10²⁰ cm^–3. It is concluded that the material obtained using these ratios of the components can be attributed to metal systems with structural disorder.

The frequency shift of the Raman G peak of epitaxial graphene due to the interaction with a substrate described by effective bond force constant k is investigated using the model of two coupled oscillators. The relative G-peak shift is shown to be Δω(G)/ω(G) ∝ k/k_0g, where k0g is the bond-stretching force constant of single-layer graphene. Assuming k ∝ P and k ∝–T, where P and T are the pressure and temperature, and the k variation to be dominant, we qualitatively explain the experimental dependences of Δω(G) on P and T. The effect of the substrate on the G-peak broadening in epitaxial graphene is discussed.

Experimental studies of the thermal conductivity of single-crystal and polycrystalline gallium antimonide samples are performed by an absolute method under steady-state thermal conditions in the temperature range of 273–423 K under uniform compression at pressures from atmospheric to 0.35 GPa. Mechanisms responsible for heat transfer under these conditions are revealed. The Bridgman parameter characterizing the volume dependence of the thermal conductivity is determined. It is shown that the difference in magnitudes of the single crystal and polycrystals thermal conductivity is associated with phonon scattering by defects in boundary layers of crystallites. The correlation between the thermal conductivity under uniform compression and the change in the crystal phonon spectrum and elastic anisotropy is determined.