Том 59, № 5 (2017)

In this paper, we report ferromagnetism in copper doped zinc-blende BeO. Our first-principles calculations based on spin density functional theory predicts a total magnetic moment of 1 μB per copper when copper substitutes beryllium in BeO, where 0.58 μB is localized at Cu atom. The results obtained show that the ferromagnetic state is 34 meV lower than the antiferromagnetic state. Calculations indicate an appreciable band gap reduction in BeO. The analysis of the partial density of states reveals that ferromagnetism and reduction of BeO band gap are principally due to the strong p–d coupling of О and Cu.

Gadolinium molybdate Gd₂(MoO₄)₃ orthorhombic ferroelectric ferroelastic (β'-phase) is simulated by the method of interatomic potentials. The simulation is performed using the GULP 4.0.1 code (General Utility Lattice Program), which is based on the minimization of the energy of the crystal structure. Parameters of the gadolinium–oxygen interatomic interaction potentials are determined by fitting to the experimental structural data and elastic constants by a procedure available in the GULP code. Atomistic modeling using the effective atomic charges and the system of interatomic potentials made it possible to obtain reasonable estimates of structural parameters, atomic coordinates, and the most important physical, mechanical, and thermodynamic properties of these crystals. Temperature dependences of the heat capacity and vibrational entropy of the crystal are obtained. The calculated parameters of gadolinium–oxygen interaction potentials can be used to simulate more complex gadolinium-containing compounds.

A theory of the inverse magnetoelectric effect in layered structures has been presented. The theory is based on solving the equations of elastodynamics and electrostatics separately for the magnetostrictive and piezoelectric phases, taking into account the conditions at the interface between the phases. Expressions for the coefficient of inverse magnetoelectric conversion through the parameters characterizing the magnetostrictive and piezoelectric phases have been obtained. Theoretical dependences of the inverse magnetoelectric conversion coefficient on the frequency of the alternating-current electric field for the three-layer PZT–Ni–PZT structure and the two-layer terfenol-D–PZT structure have been calculated. The results of the calculations are in good agreement with the experimental data.

Based on the single-band t–t' Anderson–Hubbard model, the effect of disorder on the parameters and ranges of existence of incommensurate helical spin waves is studied. The problem is solved within the functional integration theory in static approximation, taking into account longitudinal fluctuations of the magnetic moment. Magnetic phase diagrams and parameters of incommensurate helical spin waves are obtained as functions of temperatures and electron and impurity concentrations. It is shown that disorder can lead to the first-order transition from the antiferromagnetic phase to the (Q, π) phase and the metal–dielectric transition from antiferromagnetic metal to antiferromagnetic dielectric far from the half-filled band. The results obtained are used to explain the incommensurate magnetic order observed in cuprates in the overdoped mode.

Peculiarities of the magnetization dynamics induced in iron garnet films by laser pulses with a frequency detuning near the absorption edge have been studied experimentally. It has been found that the dependence of the observed signal amplitude on the pumping energy becomes nonmonotonic with an increase in the pumping frequency. At the same time, the pumping energy corresponding to the maximum amplitude of the signal and this maximum signal amplitude decrease. Moreover, the signal amplitude starts to decrease with an increase in the pumping energy at frequencies within the absorption band. The observed phenomena are possibly caused by generation of magnetostatic spin waves and the effect of ultrafast optically induced demagnetization.

Solid solution Sr_0.5Ba_0.5Nb₂O₆ films have been synthesized on a (111)Pt/(001)Si substrate by rf deposition in an oxygen atmosphere. The depolarized Raman spectra, the structure, and the dielectric characteristics of the films have been studied over a wide temperature range. It is found that the films were singlephase, had the tetragonal tungsten bronze structure, and had a pronounced axial texture with axis 001 directed perpendicular to the substrate surface. It is shown that the film material undergoes a diffuse phase transition to the state of a relaxor ferroelectric in the temperature range 300–425 K. Possible reasons of the regularities observed are discussed.

The mechanisms of high-speed deformation of ultrafine-grained copper produced during severe plastic deformation by equal-channel angular pressing were analyzed using numerical modeling in comparison with those in the case of coarse-crystalline copper. The activity of annihilation processes during nonconservative motion and double cross slip of dislocations was estimated. Their effect on the macroscopic behavior of samples is shown.

The Green’s function method for hexagonal crystals within the Lifshitz–Rosenzweig (1947) and Kröner (1953) approaches has been used to obtain analytical expressions for the energy of elastic interaction of radiation-induced point defects with dislocation loops of three types: the basal edge dislocation loop (cloop), the basal shear dislocation loop, and the edge a-loop (bedding plane {11 20}, Burgers vector b^D = 1/3〈11 20〉). In the case of the basal edge dislocation loop, a similar expression has been obtained independently by solving the equilibrium equations using the Elliott method. A numerical comparison of the derived expressions for zirconium has demonstrated a complete identity of the results obtained within the approaches considered in this study.

It has been found that the friction of a heterogeneous material, namely, sandstone, leads to the appearance of triboluminescence. The phenomenon of triboluminescence corresponds to luminescence of ≡Si–O free radicals and Fe³⁺ ions. These radicals and ions are formed as a result of the breaking of Si–O–Si bonds in nanocrystals of quartz and feldspar entering into the composition of the sandstone. The time dependence of the triboluminescence intensity represents a set of flashes, each having the duration of a few nanoseconds. It has been assumed that triboluminescence flashes correspond to the appearances of cracks in the material. Сrack opening is found to be approximately 180 nm. The size distribution of the cracks is exponential.

The second-rank spin Hamiltonian parameters of Gd³⁺ and Eu²⁺ orthorhombic centers in crystals of the yttrium aluminum garnet Y₃Al₅O₁₂ have been analyzed within the framework of the superposition model for the zero-field splitting of the ground state. It has been shown that the description of the experimental data in this model is possible only under the assumption of relaxation of the ligand environment of the paramagnetic impurity.

Local deformations of the crystal lattice under ultrasonic loading over a wide range of amplitudes have been investigated by means of recording the X-ray rocking curves for paratellurite and lithium fluoride single crystals.

It has been experimentally found that, under the static compression of a calcium crystal at room temperature, it undergoes a series of structural phase transitions: face-centered cubic lattice → body-centered cubic lattice → simple cubic lattice. It has been decided to investigate precisely the simple cubic lattice (because it is an alternative lattice) with the aim of elucidating the possibility of the existence of other (nonstructural) phase transitions in it by using for this purpose the Hubbard model for electrons with half-filled ns-bands and preliminarily transforming the initial electronic system into an electron–hole system by means of the known Shiba operators (applicable only to alternative lattices). This transformation leads to the fact that, in the new system of fermions, instead of the former repulsion, there is an attraction between electrons and holes. Elementary excitations of this new system are bound boson pairs—excitons. This system of fermions has been quantitatively analyzed by jointly using the equation-of-motion method and the direct algebraic method. The numerical integration of the analytically exact transcendental equations derived from the first principles for alternative (one-, two-, and three-dimensional) lattices has demonstrated that, in systems of two-species (electrons + hole) fermions, temperature-induced metal–insulator phase transitions of the Mott type are actually possible. Moreover, all these crystals are in fact excitonic insulators. This conclusion is in complete agreement with the analytically exact calculations of the ground state of a one-dimensional crystal (with half-filled bands), which were performed by Lieb and Wu with the aim to find out the Mott insulator–metal transition of another type.

The GeO_x films and multilayer nanoperiodic Ge/SiO₂ structures containing germanium nanocrystals were prepared by physical vapor deposition in vacuum. The properties of the films and multilayer structures were controlled by varying the deposition temperature in the range of 35–590°C and the annealing temperature in the range of 400–1000°C. A comparative study of the optical and structural characteristics of the nanosystems was performed using the methods of Raman scattering spectroscopy, IR spectroscopy, photoluminescence, and electron microscopy, which demonstrated a qualitative similarity of the nanosystems. It was found that annealing at temperatures in the range of 600–800°C leads to the formation of germanium nanocrystals with a high density (~10¹² cm^–2), whereas in the materials not subjected to annealing, their density did not exceed ~10¹⁰ cm^–2. The average size of the nanocrystals was found to be 5 ± 2 nm. For both nanosystems, three luminescence bands were observed at 1.2, 1.5–1.7, and 1.7–2.0 eV. It was assumed that the origin of these bands is associated with germanium nanocrystals, oxygen-deficient centers in GeO^x, and defects at the Ge/dielectric interface, respectively.

Composite Fe_mO_n–SiO₂ particles have been obtained by precipitation of iron oxide from aqueous solution with the addition of tetraethoxysilane. The shapes and sizes of separate single particles and their aggregates have been determined by scanning electron microscopy. Changes in the stability of the colloidal suspension due to external magnetic field and the addition of NaCl have been evaluated. Based on magnetization curves obtained on a vibrating-sample magnetometer and characteristics calculated within the theoretical model of magnetostatically interacting particles, it is shown that the composite particles are superparamagnetic.

The structure and composition of SiC nanolayers are comprehensively studied by X-ray reflectometry, IR-spectroscopy, and atomic-force microscopy (AFM) methods for the first time. SiC films were synthesized by the new method of topochemical substitution of substrate atoms at various temperatures and pressure of CO active gas on the surface of high-resistivity low-dislocation single-crystal n-type silicon (111). Based on an analysis and generalization of experimental data obtained using X-ray reflectometry, IR spectroscopy, and AFM methods, a structural model of SiC films on Si was proposed. According to this model, silicon carbide film consists of a number of layers parallel to the substrate, reminiscent of a layer cake. The composition and thickness of each layer entering the film structure is experimentally determined. It was found that all samples contain superstoichiometric carbon; however, its structure is significantly different for the samples synthesized at temperatures of 1250 and 1330°C, respectively. In the former case, the film surface is saturated with silicon vacancies and carbon in the structurally loose form reminiscent of HOPG carbon. In the films grown at 1330°C, carbon is in a dense structure with a close-to-diamond density.

By means of electron microscopy, the spatial correlation of gold nanoparticles are studied, obtained by thermal vacuum sputtering of metal on a bromine-activated surface of films of adamantane-containing polyimides and copolyimides based on pyromellitic anhydride (PMDA), 4,4'-oxydianiline (ODA), and 1-aminoethyl-3-(4'-aminophenyl)adamantane (AEAPhA) or 3,4,3',4'-tetracarboxydiphenyloxide dianhydride (ODPA), ODA, and AEAPhA with varying ratio of ODA and AEAPhA fragments. It is shown that with increasing ratio of mole fractions [AEAPhA]/[ODA], the texture of the film surface, free volume, the correlation length, and the frequency of alternation of the particle distribution density change. The shortrange order decreases, while the long-range order increases in ensembles of particles. The supramolecular structure of the polymer is transformed similarly because of structural transitions in the amorphous state of ODPA–ODA–AEAPhA and the mesomorphic state of PMDA–ODA–AEAPhA.

The oxide YBiGeO₅ and GdBiGeO₅ compounds have been synthesized by solid-phase synthesis. The high-temperature heat capacity has been measured using differential scanning calorimetry in the range 373–1000 K. The results were used to calculate the thermodynamic properties (the change in the enthalpy and entropy).