


Том 59, № 3 (2017)
- Год: 2017
- Статей: 33
- URL: https://journal-vniispk.ru/1063-7834/issue/view/12462
Metals
Specific features of the electronic structure and spectral characteristics of the Gd5Si3 compound
Аннотация
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.



Electron paramagnetic resonance in YbNiAl2 single crystals with strong magnetic anisotropy
Аннотация
Anisotropy in the magnetic properties of YbNiAl2 intermetallide has been studied. Electron paramagnetic resonance (EPR) signals assigned to the localized magnetic moments of trivalent ytterbium have been detected at temperatures below 20 K. Spin–lattice relaxation processes like the Orbach–Aminov process with participation of the first excited Stark sublevel of the Yb3+ ion with an energy of 96 K govern electron–spin dynamics and the disappearance of spectrum lines with a further increase in temperature. Strong magnetic anisotropy effects are discussed as a main reason for the appearance of electron paramagnetic resonance.



Formation of the structure and physicomechanical properties of a quasicrystalline Al–Cu–Fe alloy upon plasma spraying
Аннотация
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.



Mössbauer studies of spatial spin-modulated structure and hyperfine interactions in multiferroic Bi57Fe0.10Fe0.85Cr0.05O3
Аннотация
Results of Mössbauer investigations on 57Fe nuclei in multiferroic material Bi57Fe0.10Fe0.85Cr0.05O3 in the temperature range from 5.2 to 300 K are presented. Bulk rhombohedral samples were obtained by solidstate synthesis at high pressure. Mössbauer spectra were analyzed using the model of spatial incommensurate spin-modulated structure of the cycloidal type. Information on the influence of substituting Cr cations for Fe cations on hyperfine spectral parameters was obtained: the shift and quadrupolar shift of a Mössbauer line, and isotropic and anisotropic contributions into the hyperfine magnetic field. The anharmonicity parameter m of the spatial spin-modulated structure increases almost 1.7 times at 5.2 K when BiFeO3 is doped with chromium. The data on m were used for calculation of the uniaxial magnetic anisotropy constants and their temperature dependences for pure and chromium-doped BiFeO3.



Superconductivity
Enhancement of the intrinsic pinning by a magnetic field in a single crystal of high-temperature superconductor TmBa2Cu3Oy
Аннотация
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, μv, and realization of the critical state along the layers are obtained. It is found that, as the temperature approaches Tc, the behavior of the field dependences of μv 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.



Semiconductors
Parametric resonance and photogalvanic currents in layered TlGaSe2 crystals
Аннотация
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.



Linearly and circular dichroism in a semiconductor with a complex valence band with allowance for four-photon absorption of light
Аннотация
This paper presents a theoretical study of the linear and circular dichroism of multiphoton absorption of light in semiconductors with a complex valence band. Matrix elements of optical transitions between subbands of the valence bands of a p-GaAs semiconductor are calculated. Transitions connected with both nonsimultaneous absorption of single photons and simultaneous absorption of two photons are taken into account. An expression for the temperature dependence of the coefficient of multiphoton absorption of polarized radiation with allowance for transitions between subbands of heavy and light holes is obtained.



Anomalous magnetism of the nanocrystalline oxide TiO2 surface
Аннотация
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.



Transport and magnetic properties of a Zn0.1Cd0.9GeAs2 + 10 wt % MnAs composite with magnetic clusters at high pressure
Аннотация
The temperature (T = 77–420 K) dependences of the electrical resistivity and the magnetization, the magnetic-field (H ≤ 5 kOe) and pressure (P ≤ 7 GPa) dependences of the resistivity, the Hall coefficient, and the magnetization have been measured in the Zn0.1Cd0.9GeAs2 + 10 wt % MnAs composite with the Curie temperature TC = 310 K. The magnetoresistive effect has been observed at high hydrostatic pressure to 7 GPa. At nearly room temperature, the pressure dependence of the magnetization demonstrated a transition from the ferromagnetic to paramagnetic state at P ~ 3.2 GPa that was accompanied by the semiconductor–metal phase transition.



Dielectrics
Luminescence of yttrium–aluminum garnet crystals with Eu2+ impurity ions
Аннотация
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.



Detection of defects binding free polarons in colored alkali halide crystals
Аннотация
The assumption has been made that defects binding free polarons in colored alkali halide crystals are F'-center, i.e., defects that slow down the motion of dislocations (photoplastic effect). This assumption has been confirmed by the experiments performed in this study. Thus, the anion vacancy in alkali halide crystals at a low temperature can capture three electrons: two electrons at a deep level (F'-center) and one electron in a bound polaron state. This electron is retained due to the energy gain in the interaction of a local deformation of the polaron and a local deformation surrounding the F'-center, despite the presence of the Coulomb repulsion.



Experimental investigation and ab initio calculation of the properties of Sc-, in-doped bismuth titanates with the pyrochlore type structure
Аннотация
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.



Magnetism
Dynamic and static properties of a non-Heisenberg magnet with complex interion anisotropy
Аннотация
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.



Crystal structure and magnetic properties of Al2O3 nanoparticles by 27Al NMR data
Аннотация
The magnetization of a series of Al2O3 with different particle sizes and their 27Al NMR spectra have been studied at room temperature. The field dependence of the magnetization demonstrated the existence of a long-range ferromagnetic order in a small part of the sample at room temperature; however, the relative volume of this contribution was very small (less than 1%), and this seems likely due to an impurity phase. The NMR spectra did not contain any lines of metallic aluminum the existence of which in these nanooxides was assumed before in a surface layer of the nanoparticles, according to the data of other techniques. The data on the phase composition and the charge distribution in different phases of the Al2O3 nanoparticles have been obtained. The change in the mean particle size (by a factor of almost three) only insignificantly changed their phase composition.



Three-dimensional simulation of nonlinear dynamics of domain walls in films with perpendicular anisotropy
Аннотация
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.



Spin state of Co3+ ions in EuBaCo2–xO5.5–δ in the paramagnetic temperature range
Аннотация
The magnetic studies of EuBaCo2O5.50, EuBaCo1.93O5.40, and EuBaCo1.90O5.36 have been performed. Layered cobaltites EuBaCo2–xO5.5–δ demonstrated the ferrimagnetic ordering in the temperature range 200 K < T < 650 K. The ferromagnetic Curie temperature TC almost did not change as the number of Co3+–O–Co3+ bonds decreased. The contributions of the ferro- and antiferromagnetic interactions were nearly the same and determine the relationship TC > TN and TC < TN. At T > TC, the antiferromagnetic order was retained in the EuBaCo1.90O5.36. As temperature increased, EuBaCo2–xO5.5–δ conserved the IS state in pyramids, and gradual LS → IS → HS transitions occurred in octahedron sites. The increase in the effective magnetic moment μeff in EuBaCo1.90O5.36 at T > 580 K demonstrated the transition of Co3+ ions to the HS state at lower temperatures as compared to the EuBaCo2O5.50 stoichiometric composition, and the structural distortions and the increase in the unit cell volume favored to this transition.



Competition between band and hopping carrier transport in Ge : Mn thin films
Аннотация
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.



Infrared diffuse reflectance spectra of micropowders of Ni1–xZnxFe2O4 ferrites
Аннотация
The results of experimental studies of the IR diffuse reflectance spectra in the range from 4000 to 50 cm–1 of Ni1–xZnxFe2O4 ferrite micropowders with different concentrations of nickel and zinc are presented. The dependence of the intensity of the IR spectra of these ferrites on the changing concentrations of Fe2+ and Fe3+ cations in ferrites of different compositions is found. The features of change of the reflectance spectra are studied depending on the ferrite compositions, and an interpretation of the observed spectral structures is proposed.



Magnetic and magnetoelectric properties of the Tb0.75Ho0.25Fe3(BO3)4 ferroborate
Аннотация
Single crystals of the Tb0.75Ho0.25Fe3(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 TN = 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.



Magnetic and resonance properties of ferrihydrite nanoparticles doped with cobalt
Аннотация
Powders of undoped ferrihydrite nanoparticles and ferrihydrite nanoparticles doped with cobalt in the ratio of 5: 1 have been prepared by hydrolysis of 3d-metal salts. It has been shown using Mössbauer spectroscopy that cobalt is uniformly distributed over characteristic crystal-chemical positions of iron ions. The blocking temperatures of ferrihydrite nanoparticles have been determined. The nanoparticle sizes, magnetizations, surface anisotropy constants, and bulk anisotropy constants have been estimated. The doping of ferrihydrite nanoparticles with cobalt leads to a significant increase in the anisotropy constant of a nanoparticle and to the formation of surface rotational anisotropy with the surface anisotropy constant Ku = 1.6 × 10–3 erg/cm2.



Superhyperfine structure of the ESR spectra of Gd3+ impurity ions in LiYF4 double fluoride
Аннотация
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.



Ferroelectricity
Fatigue processes in triglycine sulfate and the effect of a magnetic field on them
Аннотация
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.



Mechanical Properties, Physics of Strength, and Plasticity
The influence of stress on electron emission initiated by a shock wave from a heterogeneous material (granite)
Аннотация
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 Pf, it does not affect the number and efficiency of the radiation sources. At P ≈ (0.9–0.95)Pf, 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.



Impurity Centers
Temperature dependences of the parameters of the initial splitting of Gd3+ in CaMoO4 (lattice and spin–phonon contributions)
Аннотация
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 bnm(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 b4m(L), to the spin Hamiltonian parameters bnm(T) have been separated. For this purpose, the changes of the static lattice contributions b2(L) and b4m(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 bnm(F) = bnm–bnm(L) have been attributed to the spin–phonon contribution. The analysis of the results obtained has demonstrated that the parameters b20(F) and b4m(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 b4m(F) could not be described in the framework of the well-known models of the spin–phonon interaction.



Optical Properties
Light scattering by porous oriented polypropylene films
Аннотация
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.



Lattice Dynamics
Structural changes in the surface of a heterogeneous nanocrystalline body (sandstone) under the friction
Аннотация
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.



Specific features of tangential modes in Raman scattering spectra of semiconducting single-walled carbon nanotubes with a large diameter
Аннотация
The lattice dynamics and optical properties of individual semiconducting single-walled carbon nanotubes with diameters of greater than 2.0 nm have been investigated using resonant Raman spectroscopy over a wide range of laser excitation wavelengths. Individual suspended semiconducting single-walled carbon nanotubes have been synthesized by the chemical vapor deposition method. The chirality indices (n, m) of the carbon nanotubes have been determined using the electron diffraction method. The dependences of the ratios of absolute intensities of the tangential transverse optical and tangential longitudinal optical modes on the laser excitation energy and on structural parameters (diameter and chiral angle) of the nanotube have been established. The observed dependences are inconsistent with the spectral data obtained previously for structurally identified individual semiconducting single-walled carbon nanotubes with diameters of less than 1.1 nm.



Low-Dimensional Systems
Silver structures at the percolation threshold, prepared by laser annealing
Аннотация
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.



Time-of-flight neutron diffraction of nanocrystalline powders of nonstoichiometric niobium carbide NbC0.77
Аннотация
The evolution of the microstructure of nonstoichiometric niobium carbide NbC0.77 powders subjected to high-energy ball milling has been investigated using time-of-flight neutron diffraction. It has been found that the nanocrystalline powders prepared by milling have a microinhomogeneous structure and contain two clearly identified fractions that differ from each other by the sizes of their particles. One fraction has a particle size of less than 60 nm, whereas the particle size of the second fraction exceeds 250 nm. The average size of coherent scattering regions and the value of microstrains in nanocrystallites, which accounts for the anisotropy of strain-induced distortions, have been estimated.



Surface Physics and Thin Films
Characterization of thin carbon films formed on the iron surface by magnetron sputtering with ion-beam mixing
Аннотация
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.



Fullerenes
Carbon composite based on fullerenes and exfoliated graphite
Аннотация
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.



Graphene
On Estimating the G-peak shift in graphene Raman spectra
Аннотация
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/k0g, 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.



Thermal Properties
Thermal conductivity of gallium antimonide crystals under conditions of uniform compression
Аннотация
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.


