Vol 60, No 12 (2018)

The model of the Mn_Ga acceptor in GaAs in which initial ground state of a coupled hole Γ₈ is changed due to the antiferromagnetic exchange interaction with five 3d electrons of the Mn core is described. The acceptor energy spectrum and the wave functions of its states and also their changes under action of deformations, electric and magnetic fields are considered. Expressions are presented for the description of various properties of isolated Mn_Ga acceptors in GaAs, and the data of some experiments (changes in the \({\text{Mn}}_{{{\text{Ga}}}}^{{\text{0}}}\) luminescence and absorption spectra and polarization under uniaxial pressures and in magnetic field, EPR spectra, temperature dependence of the magnetic susceptibility, circular polarization of photoluminescence during excitation by polarized light in magnetic field, etc.) are analyzed. It is demonstrated that, in some cases, it is necessary to take into account the existence of random electric and strain fields splitting the acceptor states in the crystal. The analysis results show that this model agrees well with most of the experimental results. The exchange interaction constant is in the range of 3–5 meV.

Within the Hubbard model with allowance for Hund’s and spin–orbit interactions, the concepts of thermal spin fluctuations in a strongly correlated system of f electrons in PuCoGa₅ are developed. The effect of fluctuations of the spin magnetic moments of different orbitals on the spin–orbit splitting of the f  electron spectrum is considered. The mean square magnetic moment obtained in this case makes it possible to describe the observed temperature dependence of the magnetic susceptibility (χ(T)) of PuCoGa₅ near and above the electron pairing temperature. Estimates of the radius of spin correlations correspond in order of magnitude to the sizes of Cooper pairs in superconductors with the d symmetry of the order parameter. In the high-temperature region, the magnetic susceptibility obeys the Curie–Weiss law. With decreasing temperature, when approaching the pairing temperature, the ferromagnetic instability is suppressed and the temperature maximum of χ(T) appears.

The strength and resistance of the Ti–6Al–4V titanium alloy to solid particle erosion have been studied in as-delivered state and after the equal-channel angular pressing. The ultrafine-grained material and the initial material have been tested for dynamic tension using an Instron drop tower testing machine and an aerodynamic setup for erosion testing with corundum particles as an abrasive material. Despite a substantial increase in the static strength properties, the ultrafine-grained material demonstrates the properties similar to those of the initial material under dynamic loads, which, in turn, has been analyzed based on a structural–temporal approach using the incubation time criterion.

The atomic structure of the iron–gallium alloy containing 18 at % Ga has been studied by X-ray diffraction. The samples were annealed in the paramagnetic (T > T_C) and ferromagnetic (T < T_C) state. In the first case, the structural state was fixed by quenching from the annealing temperature into water; in the second case, the structural state was obtained by slow cooling. The structural studies of the single-crystal samples were conducted on a four-circle X-ray diffractometer at room temperature. From the X-ray diffraction data, it follows that the alloy, independently on the heat treatment, contains B2 clusters, i.e., locally ordered regions with the CsCl-type structure observed in alloys of iron with silicon (to 10 at % Si) and aluminum (7 at % Al) before. In addition to the B2‑clusters, regions with the D0₃ short-range order are observed in the quenched sample; the sizes of these regions significantly increases after annealing in the ferromagnetic state, i.e., a long-range order forms. The relation of the fine structural changes in the alloy due to various heat treatments with its magnetoelastic and magnetostriction properties is discussed.

Characteristic effects of magnetic ordering and conduction in semiconductor heterostructures with a GaAs : Be/Ga_0.84In_0.16As/GaAs quantum well and manganese δ-layers of different thickness (from 0.4 to 2 monolayers) were studied based on analysis of magnetic field and temperature dependences, galvanomagnetic effects, and magnetization. An anomalous dependence of the conductivity on the manganese atoms concentration in the δ-layer was observed, which was due to a strong scattering of charge carriers in the structures with the low content of magnetic impurities. Magnetic properties of the heterostructures clearly indicated the magnetic ordering of the impurity system (saturation and hysteresis of the magnetization and fulfillment of the Curie–Weiss law at increasing temperature). Parameters of the magnetic subsystem allowed revealing different types of ordering in the systems with different concentrations of the magnetic impurity. Changing the concentration of the Mn admixture in the δ-layer was shown to influence significantly the conductivity and magnetism in the studied structures.

The ionic (proton and deuteron) conductivity of the system CaZr_1 – xSc_xO_3 – α (x = 0.03–0.20) is studied experimentally in H₂ + H₂O + N₂ and D₂ + D₂O + N₂ reducing atmospheres at pH₂O = pD₂O = 3.2 kPa and in the temperature range of 600–900°C. This system exhibits a considerable H/D isotopic effect in conductivity, and its magnitude is estimated. Using theoretical elaborations, we also estimate the magnitude of the thermodynamic isotope effect in the solubility of H₂O/D₂O in the considered system under reducing conditions.

The processes of electric charge transfer (conductivity) and mass transfer (diffusion) in La_1 ‒ ySr_yF_3 – y superionic conductors are determined by mobile fluorine ions. The fluorine random-diffusion coefficients D_σ have been calculated from experimental data on the ionic conductivity σ_dc for La_1 ‒ ySr_yF_3 – y single crystals at the SrF₂ dopant contents of 1, 3, 5, 7.5, 10, and 15 mol %. A maximum in the region of 3–5 mol % SrF₂ is observed in the dependence D_σ(y). The value of the Haven correlation parameter H_r = D_NMR/D_σ (D_NMR is the fluorine diffusion coefficient measured by NMR on ¹⁹F nuclei) is determined; it characterizes the ion transport mechanism in La_1 – ySr_yF_3 – y crystals. The H_r values are 0.75 ± 0.15, 0.45 ± 0.15, and 0.65 ± 0.15 at 400–800 K for 1, 3, and 15–16 mol % SrF₂, respectively. Superionic conductor La_0.97Sr_0.03F_2.97 with maximum σ_dc and D_σ values has a minimum Haven parameter. The obtained H_r values indicate that fluorine diffusion in superionic conductors La_1 – ySr_yF_3 – y goes not according to the vacancy mechanism involving single vacancies but by cooperative motion of F^– conduction ions.

The intrinsic fluorine-ion conductivity σ_lat of BaF₂ (CaF₂ fluorite type) and LaF₃ (tysonite type) crystals is studied by the impedance spectroscopy method. These compounds represent two major structural types taken as the basis to form the best nonstoichiometric fluorine-conducting solid electrolytes. The conductivity σ_lat caused by thermally activated defects is manifested in the field of high temperatures, where conductometric measurements are complicated by pyrohydrolysis. The experiments carried out in inert atmosphere with application of the impedance method have for the first time produced the reliable values of σ_lat of fluoride crystals in conditions of suppression of pyrohydrolysis (BaF₂) or partial pyrohydrolysis (LaF₃). Values of the σ_lat at 773 K for BaF₂ and LaF₃ crystals grown from melt by the Bridgman method using the vacuum technology are 2.2 × 10^–5 and 8.5 × 10^–3 S/cm differing by a factor of ~400. The tysonite structural type has been proved feasible for making high-conductivity solid fluoride electrolytes based on the analysis of energy characteristics of formation and migration of anionic defects.

The relationship of the micromagnetic structure, the contributions of various crystalline phases and hysterons obtained by the first order reversal curves (FORC) method in sintered (NdDy)(FeCo)B magnets is discussed. The distribution map of parameters of magnetic hysteresis partial loops that allows the separation of various spin ensembles to the magnetization has been built.

The Sr_0.8Ce_0.2Mn_1 – yCo_yO_3 – δ (y = 0.2, 0.3, 0.4, 0.5, and 0.6) complex oxides with the perovskite structure obtained from simple oxides by the solid–state reaction technique have been examined using structural analysis and magnetic measurements. Substitution of Co for Mn in the dilute Sr_0.8Ce_0.2MnO₃ antiferromagnet with a Néel temperature of T_N = 210 K leads to a decrease in the degree of tetragonal distortion of the crystal structure and transition to the cubic cell. The degeneracy of the antiferromagnetic interaction (T_N = 138 K at y = 0.2) observed at the first stage of the substitution of Co for Mn changes for its gradual enhancement with an increase in the magnetic transformation temperature up to 239 K at y = 0.6. An increase in the Co content weakens the competition between the ferromagnetic and antiferromagnetic couplings and reduces the temperature of the transition to the spin-glass-like state. The magnetic inhomogeneity and formation of Co²⁺–Mn⁴⁺ ferromagnetic clusters in Sr_0.8Ce_0.2Mn_0.4Co_0.6O_2.69 at 140 K have been observed.

Structural, elastic, and magnetic properties of cobaltites DyBaCo₄O_7 + x produced by various technologies and distinguished by oxygen excess x are experimentally studied. It was found that rhombic distortion of the crystal structure of annealed stoichiometric samples with x = 0 results in frustration removal and elastic property anomalies in the T_N region, caused by cobalt subsystem ordering. At an insignificant oxygen nonstoichiometry, no structural distortions occur in quenched samples, and anomalies in elastic characteristics in the T_N region smooth and disappear. The studies of DyBaCo₄O_7 + x magnetic properties show that the rare-earth (RE) subsystem remains paramagnetic and its contribution exceeds the contribution of the Co-subsystem with strong antiferromagnetic couplings by an order of magnitude. The magnetic susceptibility of the stoichiometric sample does not exhibit an appreciable anomaly at T_N, since cobalt subsystem ordering is not accompanied by the formation of a noticeable effective field on RE ions.

The effect of thermal cycling and sintering temperature on the chemical and thermodynamic stability of the bulk multiferroic xLa_0.7Pb_0.3MnO₃–(1 – x)PbTiO₃ quasi-ceramic and ceramic composites has been experimentally investigated. It is shown that the limiting temperature of the long-term sample firing should not exceed 1070 K. It has been found that sintering at this temperature and/or short-term exposure of the samples at higher temperatures (up to 1220 K) significantly increase the sample compactness, stabilize the thermal expansion, and enhance the quality of the composites. It has been established that the component grain integrity is violated by shrinkage of the samples and a sharp change in their volume during the phase transition of a ferroelectric component.

The polarization kinetics in a transparent Pb(Mg_1/3Nb_2/3)O₃–23Pb(Zr_0.53Ti_0.47)O₃ ferroceramic is investigated in the electric fields of 0 < E < 6 kV/cm. The optical transmittance, dielectric, and acoustic properties of ferroceramics are measured at room temperature. The dielectric and acoustic characteristics are found to instantly switch even in fields below the coercive, due to the emergence of a partially ordered ferroelectric phase. Furthermore, the polarized phase arisen in the electric field is not completely stable.

The change in the bulk modulus of coarse-crystalline and nanocrystalline silver sulfides is determined in the temperature range 300–960 K using the experimental data on the temperature dependences of the heat capacity and the thermal expansion coefficients of these materials. The bulk modulus of the nanocrystalline silver sulfide is found to be lower in magnitude than modulus B of the coarse-crystalline silver sulfide in this temperature range.

Electron paramagnetic resonance of rhombic Eu²⁺ centers in single crystals of Lu₃Al₅O₁₂ doped with europium ions with the natural isotope abundance was studied. Parameters of the hyperfine and quadrupole interactions of ¹⁵¹Eu and ¹⁵³Eu isotopes of these centers were determined.

A temperature effect on fluorescence intensity and polarization of a colloidal system of carbon nanodots in glycerol under linearly polarized pumping conditions is studied. Nanodots are obtained via pyrolysis of paraffin in nanopores of a mesoporous silica. An increase in temperature leads to quenching of nanodots fluorescence, and activation energy of the quenching process is assessed. An experimental relationship between the linear fluorescence polarization and temperature is described by the Levshin–Perrin equation, which takes into account the rotational diffusion of luminescent particles (fluorophores) in a liquid matrix. The size of fluorophores is noticeably smaller than that of carbon nanodots according to the Levshin–Perrin model. A difference between the dimensions of the fluorophore and the nanodot indicates that the small atomic groups responsible for luminescence of the nanodot possess high segmental mobility.

We studied the structure, IR absorption spectra, the spectral characteristics of photoluminescence and morphology of cerium- and terbium-doped orthoborates of gadolinium and yttrium obtained by hydrothermal synthesis at 200°C, as well as solid solutions of orthoborates on the basis of yttrium, gadolinium, and lutetium with composition RECe_0.01Tb_0.1BO₃ (RE = Lu_0.5Gd_0.39, Lu_0.5Y_0.39, and Y_0.5Gd_0.39). The X-ray diffraction spectrum of yttrium orthoborate Y_{1 – x – y}Ce_xTb_yBO₃ is described by a hexagonal lattice with space group P6₃/m, which, after annealing at 970°C, transforms into a monoclinic lattice with space group C2/c. High-temperature annealing of the studied orthoborates leads to a multiple, more than two orders of magnitude, increase in the luminescence intensity of Tb³⁺ ions when the samples are excited in the absorption band of cerium ions. This effect is the result of a significant increase in the concentration of Ce³⁺ ions in the orthoborates at high temperatures. It is shown that the luminescence of terbium ions is due to energy transfer from Ce³⁺ to Tb³⁺, which proceeds with high efficiency (∼85%) by the mechanism of dipole-dipole interaction between cerium and terbium.

The structural and dynamic properties of a tetrafluoroborate ion in n-Bu₄NBF₄ organic salt at different temperatures and phase states were studied by oscillation spectroscopy methods. The results of calculations of the energy and relaxation parameters have shown that in the plastic phase, a \({\text{BF}}_{4}^{ - }\) ion is characterized by a low reorientation energy as compared with that of the crystalline phase.

The electrical conductivity of polycrystalline V_(1 – x)Fe_xO₂ films has been investigated in a wide temperature range, which covers both the metal and insulator phase regions. It is shown that with an increase in the iron concentration the metal–insulator phase transition shifts toward lower temperatures, while the temperature range of the transition in doped samples additionally broadens as compared with pure VO₂. To explain the temperature dependence of the electrical conductivity of the V_(1 – x)Fe_xO₂ insulator phase, a hopping conductivity model has been used, which takes into account the effect of thermal vibrations of atoms on the resonance integral. The values of parameter ε have been calculated as a function of the degree of VO₂ doping.

Magnetic composite (MC) particles consisting of finding hydroxyapatite and iron oxides (HAp/FeOxid), synthesized at both pyrolysis temperatures during MC synthesis, i.e., 800, 900, and 1000°C and at the various ferroxide concentrations in the HАp : FeOxid composite, i.e., 1 : 3, 1 : 2, and 1 : 1 (at a pyrolysis temperature of 1000°C). It is found that the HAp/FeOxid MCs are formed by the hydroxyapatite matrix providing biological compatibility of the MC containing iron oxide particles. Mössbauer studies show that maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄), ε-Fe₂O₃, and akaganeite (β-FeOOH) phases are simultaneously observed in synthesized HAp/FeOxid MCs. The content of ε-Fe₂O₃ component having giant magnetic anisotropy is to ~40% of iron oxides (FeOxid) in HAp/FeOxid MCs, which makes obtained MCs very promising for various applications, including biomedical ones.

Photoluminescence spectra of an isolated GaAs quantum dot within an AlGaAs quantum wire are studied. The examination of behavior of spectra in a magnetic field provided an opportunity to estimate the exciton binding energy in the quantum dot, which turned out to be 10 times higher than the bulk exciton binding energy in GaAs. It is demonstrated that the signal of exciton photoluminescence from the quantum dot emitted along the nanowire axis is linearly polarized. At the same time, the photoluminescence signal propagating in the direction perpendicular to the nanowire axis is almost unpolarized. This may be attributed to the nonaxial dot positioning in the wire under a giant increase in the binding energy of an exciton affected by an image potential.

Nanopowder tungsten oxide and metallic tungsten are obtained via pyrolysis of ammonium metatungstate. Two methods are used for the synthesis of tungsten oxide: the use of a fibrous matrix and pyrolysis of aerosol particles. Tungsten oxide particles are formed during the pyrolysis in air. Metallic tungsten nanoparticles are obtained via subsequent thermal reduction of tungsten oxide in hydrogen. The structure and morphology of the samples are studied with X-ray diffraction and scanning electron microscopy. Tungsten nanopowders with average sizes from 7 to 30 nm are obtained depending on synthesis temperature. The electrochemical characteristics of electrodes coated with tungsten nanoparticles are studied with cyclic voltammetry, impedance spectroscopy, and galvanostatic charge–discharge methods. An electrode with W nanoparticles exhibited a specific low-frequency capacitance of about 90 F/g due to thin tungsten oxide film on the surface of tungsten nanoparticles.

The entrainment of current carriers (electrons) in a two-dimensional semiconductor nanostructure by a flow of neutral particles (atoms or molecules) moving near its surface is considered. It is shown that the physical mechanism is similar to the entrainment of electrons with an ion beam inquantum wires, considered earlier in the works of V.L. Gurevich and M.I. Muradov.

Thin films have been produced via a spray method from commercially available single-walled carbon nanotubes (SWCNTs). A SWCNT film thickness has ranged from ~10 to ~80 nm. The SWCNT diameter has accepted values of 1.6–1.8 nm. The existence of SWCNTs longer than 10 μm is established. The optimal thickness of a SWCNT thin film is found to be ~15 nm at which the transmittance exceeds 85%. The specific resistance of SWCNT thin films goes from ~1.5 × 10^–3 to ~3 × 10^–3 Ohm cm at room temperature. The pioneering study of the temperature dependences of the Seebeck coefficient and surface resistance is performed for this type of SWCNT. A surface resistance is found to increase with rising temperature. Furthermore, the Seebeck coefficient of SWCNT thin films weakly depends on temperature. Its value for all samples is evaluated to be ~40 μV/K. According to the sign of the Seebeck coefficient, thin films exhibit hole-type conductivity. Moreover, the power factor of a 15-nm thin SWCNT-film decreases with a temperature increase to 140◦C from the value of approximately ~120 to ~60 μW m^–1 K^–2. A further rise in temperature has led to a gain in the power factor.

A theoretical description of the principles of nonmechanical transportation of microliter volumes of a liquid crystal (LC) encapsulated in a thin capillary is proposed. By numerical methods within a nonlinear generalization of the classical Ericksen–Leslie theory, various regimes of formation of a hydrodynamic flow in a uniformly oriented LC cavity under the action of a temperature gradient and a double electrostatic layer naturally arising at the LC/solid interface are investigated. The sizes of an LC capillary and the parameters of the necessary thermal effect capable of initiating a flow of the LC phase in the horizontal direction are found.

Establishing the features of interfacial effects on the electrical conductivity of graphene is crucial for successful design of novel graphene-based electronic devices, including chemical sensors and biosensors. We study electrical properties of monolayer graphene, prepared by thermal decomposition of silicon carbide in argon, in the field-effect transistor and the four-probe geometries. Alterations in the electrical properties of graphene in response to placing a quantity of water on its surface followed by removal of the water are investigated. In these geometries, the field effect is shown to play a key role in the way the electrical properties of graphene are affected by the formation of the graphene–water interface.

The thermal conductivity of single-crystal (ZrO₂)_1 – x(Sc₂O₃)_x (x = 0.09, 0.10) and (ZrO₂)_{1 ‒ x ‒ y}(Sc₂O₃)_x(Y₂O₃)_y (x = 0.003–0.10; y = 0.01–0.025) solid solutions has been experimentally determined in the temperature range 50–300 K. The results are analyzed with allowance made for the phase compositions and features of imperfect crystal structures.