On the nature of the “double” yield point in Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript> alloy upon high-pressure torsion

The anomalous phenomenon of the “double” yield point attributed with the “crystal ? amorphous state” phase transition during high-pressure torsion of Ti₅₀Ni₂₅Cu₂₅ alloy has been analyzed experimentally and theoretically. Satisfactory correlation between experimental data and theoretical hypothesis has been found.     

Specific physical properties of nanocrystalline (La<Subscript>0.65</Subscript>Sr<Subscript>0.35</Subscript>)<Subscript>0.8</Subscript>Mn<Subscript>1.2</Subscript>O<Subscript>3 ± Δ</Subscript> samples obtained by cold isostatic pressing

Single-phase powders of manganites (La_0.65Sr_0.35)_0.8Mn_1.2O_{3 ± Δ} with average crystallite sizes of 30, 50, and 500 nm were produced by co-precipitation. The samples studied were obtained by cold isostatic pressing of powders at a pressure of 1 GPa without subsequent sintering. It is shown that the size of particles has a significant effect on the electromagnetic properties of the manganite samples. As the crystallite size decreases, the electrical resistance and coercive force increase and the tunneling magnetoresistance of the samples and the Curie temperature decrease.     

Preparation and characterization of organic–inorganic hybrid compound [N(C<Subscript>4</Subscript>H<Subscript>9</Subscript>)<Subscript>4</Subscript>]<Subscript>2</Subscript>Cu<Subscript>2</Subscript>Cl<Subscript>6</Subscript>

Organic–inorganic hybrid sample [N(C₄H₉)₄]₂Cu₂Cl₆ was prepared via the reaction between copper chloride and tetrabutylammonium chloride. The compound was characterized by X-ray powder diffraction, IR, Raman, differential scanning calorimetry (DSC), DTA-TGA analysis and electrical impedance spectroscopy. DSC studies indicate a presence of one-phase transition at 343 K. The complex impedance of compound [N(C₄H₉)₄]₂Cu₂Cl₆ have been investigated in temperature and frequency ranges 300–380 K and 200 Hz–5 MHz, respectively. The Z′ and Z″ versus frequency plots are well fitted to an equivalent circuit model. The circuits consist of the parallel combination of bulk resistance R _p and constant phase elements CPE. The frequency dependence of the conductivity is interpreted in term of Jonscher's law: s(w) = s_dc + Awⁿ \sigma (\omega ){ } = {\sigma_{\rm{dc}}} + { }A{\omega^n} . The conductivity follows the Arrhenius relation. The variation of the value of these elements with temperatures confirmed the availability of the phase transition at 343 K detected by DSC and electrical measurements.     

Magnetic and magnetoresistive properties of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–Sr<Subscript>2</Subscript>FeMoO<Subscript>6–δ</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoheterostructures

A method has been developed for fabricating nanoporous matrices based on anodic aluminum oxide for the deposition of ferromagnetic nanoparticles in them. The modes of deposition of strontium ferromolybdate thin films prepared by the ion-plasma method have been worked out, and the magnetic and magnetoresistive properties, structure, and composition of the films have been investigated. It has been revealed that the microstructure and properties of the strontium ferromolybdate films deposited by ionplasma sputtering depend on the deposition rate and the temperature of the substrate. Based on the measurement of the electrical resistivity of nanoheterostructures in a magnetic field, it has been found that the magnetoresistance reaches 14% at T = 15 K and B = 8 T, which is due to the manifestation of tunneling magnetoresistance.     

High-Temperature Heat Capacity of Zn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>–Cu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript> Solid Solutions

Differential scanning calorimetry has been used to study the influence of temperature on the heat capacity of synthesized vanadates Zn₂V₂O₇, (Cu_0.56Zn_1.44)V₂O₇, and (Cu_1.0Zn_1.0)V₂O₇. It is found that dependences C_p = f(T) have extremes. The thermodynamic properties of Zn₂V₂O₇ have been determined.     

Hysteresis of the magnetoresistance of granular HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − δ</Subscript> in weak fields

Hysteresis of the magnetoresistance of ceramic YBa₂Cu₃O_～6.95 HTSC samples is studied at T = 77.3 K in an external magnetic field H _ext changing in 0 → H _max → 0 cycles, where H _max is the maximum magnitude of H _ext. Information is obtained about the dependences of the critical fields of Josephson weak links H _c2J , the lower critical fields of superconducting grains H _c1A , and the critical fields H _BG-VG of the Bragg glass-vortex glass phase transition in the vortex matter on transport current I, magnetic field, and the mutual orientation of I and H _ext. It is found that the magnetoresistance δρ⁺/ρ_{273 K} measured with increasing H _ext is significantly higher than Δρ^?/ρ_{273 K} and that H _c2J ⁺ < H _c2J ^? , H _c1A ⁺ < H _c1A ^? , and H _BG-VG ⁺ < H _BG-VG ^? .     

Paramagnetism of copper–oxygen chains in high-temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript> superconductors

Experimental evidence for the existence of chain paramagnetic Curie-type contributions to the temperature dependence of static magnetic susceptibility χ(T) in the normal (nonsuperconducting) state is obtained for a series of pure YBa₂Cu₃O_{6 + δ} high temperature superconductors with different oxygen contents (0.6 < δ ≤ 1). It is shown that the chain contribution is obvious on χ(T) curves only in the T < 150K range of temperatures, grows along with the number of oxygen vacancies in Cu1–O4 chains, and depends on the ordering of these vacancies.     

Effect of hydrostatic pressure on the superconductivity in fluorinated Hg<Subscript>0.8</Subscript>Ba<Subscript>2</Subscript>Ca<Subscript>2</Subscript>Cu<Subscript>3.2</Subscript>O<Subscript>8+δ</Subscript>

The effect of hydrostatic pressure on the superconducting transition temperature was measured for the Hg-1223 phase of a fluorinated mercury cuprate high-temperature superconductor with T_c(optim)=38 K. The value of the T_c derivative with respect to pressure was found to be rather high (11.0 K/GPa); at P=1.5 GPa, T_c=153.5 K. The results obtained are discussed in connection with works on the synthesis of such samples.     

Magnetic characteristics of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles obtained by glycine–nitrate synthesis

The magnetic properties of magnesium–iron spinel (MgFe₂O₄) powdered nanoparticles obtained by glycine–nitrate synthesis are investigated by X-ray phase analysis and the NMR method. According to the results of X-ray phase analysis, the average size of the crystalline part of nanoparticles of the powder under investigation is 45 ± 4 nm. Magnetization J is determined using the formula J = (B/μ0)–H, where B and H are the induction and strength of the magnetic field in the sample, which are measured by the NMR method. The magnetic characteristics of MgFe₂O₄ are as follows: specific saturation magnetization J_sat = 17.52 A m²/kg, specific residual magnetization J_r = 5.73 A m2/kg, coercive force H_c = 4600 A/m, and magnetic moment P_sat = 371 × 10^–20 A m² in the magnetic saturation state and P_r = 121 × 10^–20 A m² in the residual magnetization state.     

Magnetization and Static Magnetic Susceptibility of Fine-Crystalline High-Temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Superconductors Synthesized by the Sol–Gel Method

A comparative study of the magnetization and static magnetic susceptibility of high-temperature superconductors (HTSC) YBa₂Cu₃O_y synthesized by two variants of the sol–gel method with different average sizes of crystallites 〈 D〉 ranging 0.4–2 μm has been performed in constant magnetic fields (Н ≤ 6 kOe). It has been shown that the different annealing temperatures and times, at which their crystal structure is formed, change both the average sizes of crystallites 〈D〉 and the sizes of the structural homogeneity regions 〈l〉 and, at the same time, the magnetic field penetration depth (λ) and the coherence length (ξ). As a result, such parameters as 〈D〉 ~ λ and 〈l〉 ~ ξ become comparable, leading to a change in the physical characteristics of HTSCs. It has also been shown that the superconducting transition temperature T_c determined from the measurements of magnetic characteristics in constant magnetic fields remains within values optimal for superconductivity (T_c ≈ 92 K) in the case of an optimal number (y) of oxygen atoms, which determine the levels of charge doping for a given compound.     

Percolation nature of the 60-K to 90-K phase transition in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6 + δ</Subscript>

The temperature dependence of the heat capacity of the HTSC YBa₂Cu₃O_{6 + δ} is measured in the temperature range 1.8–300 K and the doping range 0.70 < δ < 0.86. The results obtained suggest that the transition from the 60-K to the 90-K phase in YBa₂Cu₃O_{6 + δ} has a percolation nature and that the underdoped 60-K phase represents a heterophase structure in which numerous superconducting clusters are embedded into an insulating matrix.     

Correlation between raman spectra and structural properties of Zn<Subscript>2 − 2<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>x</Subscript>In<Subscript>x</Subscript>Se<Subscript>2</Subscript> films

The Raman spectra of Zn_{2 ? 2x} Cu_xIn_xSe₂ (ZCIS) semiconductor films designed for use as optically active layers in thin-film solar cells have been investigated. The Raman spectra of ZCIS films are characterized by the presence of the dominant mode A ₁, which is observed in A^IB^IIIC ₂ ^VI compounds with chalcopyrite structure. The spectra of CuInSe₂ films (x = 1) obtained at low temperatures (T ≤ 400°C) contain and additional mode at 258 cm^?1, which is due to the presence of the impurity Cu_xSe phase. All modes observed in the spectra of ZCIS films with a Zn concentration ≤20 at % obtained under optimal conditions (520–540°C) correspond to the symmetry of vibrations in the chalcopyrite structure. The broadening and blue shift of the A ₁ mode occurring with an increase in the Zn concentration are indicative of degradation of the chalcopyrite crystal structure and the chalcopyrite → sphalerite phase transition at Zn concentrations exceeding 20 at %.     

Magnetic capacitance of the Gd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>FeO<Subscript>3</Subscript> thin films

The capacitance, inductance, and dissipation factor of the Gd _x Bi_1–x FeO₃ films were measured in the temperature range of 100 K < T < 800 K in magnetic fields of up to 8 kOe at frequencies of 0.1–100 kHz. The magnetic susceptibility maxima in the low-temperature region and dependences of the relaxation time and inductance on prehistory of the films cooled in zero and nonzero magnetic fields are established. The giant increase in magnetic capacitance in the external bias electric field is found. The results obtained are explained by the domain structure transformation in external electric and magnetic fields.     

Effect of ionizing radiation on dielectric characteristics of Cu<Subscript>2</Subscript>ZnSn(S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript> single crystals

The effect of electron irradiation on conductivity and dielectric permeability of Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ single crystals and solid solutions based on them is studied. It is shown that values of dielectric permeability decrease with an increase in the irradiation dose while those of specific electric conductivity sharply increase.     

Study of the KNO<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system by differential scanning calorimetry

The structural and the thermodynamic properties of potassium nitrate KNO₃ and its composites with nanosized aluminum oxide Al₂O₃ have been studied by differential scanning calorimetry. It has been found that an amorphous phase forms in composites (1–x)KNO_3–xAl₂O₃. The thermal effect corresponding to this phase has been observed at 316°C. It has been found that the phase transition heats of potassium nitrate decreased as the aluminum oxide fraction increased.     

Magnetic field penetration into the weak-link system of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> granular superconductor

The differential magnetic susceptibility χ_d(H) of YBa₂Cu₃O_7?x polycrystalline samples is studied experimentally in fields H<150 Oe. The empirical χ_d(H) dependence is determined. The results are explained on the basis of the critical-state model of a Josephson medium with hypervortices.     

Infrared diffuse reflectance spectra of micropowders of Ni<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites

The results of experimental studies of the IR diffuse reflectance spectra in the range from 4000 to 50 cm^–1 of Ni_1–xZn_xFe₂O₄ 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.     

Normal state conduction in Gd(Ba<Subscript>2–x</Subscript>Pr<Subscript>x</Subscript>)Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Bold">7+δ</Emphasis></Subscript>

The normal state resistivity of single phase polycrystalline Gd(Ba_2–xPr_x)Cu₃O₇₊ samples with 0.0x0.50 have been investigated. There is a distinct metal-insulator transition at and a superconductor-insulator transition at with the increase of x. The two-dimensional variable range hopping is dominant in the normal state resistivity of the samples. The localization length, hopping range, and hopping energy of carriers show that Pr doping strongly localizes the carriers in normal state, and finally causes the suppression of superconductivity.     

Neutron Spectroscopy of the Atomic Dynamics of La<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>7</Subscript> at Fluorite–Pyrochlore Structural Transformations

The formation of the structural and dynamic properties of La₂Zr₂O₇ in the process of crystallization at the isothermal annealing of initially amorphous precursors obtained by the coprecipitation of corresponding salts has been studied by neutron spectroscopy. The existence of vibrational states characteristic of hydrogen, which is in one or another of the possible chemical states and is incorporated into a solid matrix, has been detected in the spectra of amorphous and fluorite phases. The DFT calculation of the phonon density of states has been performed to analyze the energy structure of experimental phonon spectra for various phases of the La₂Zr₂O₇ compound. The amount of hydrogen in the fluorite phase has been estimated.     

Effect of the Composition and the Crystal Structure on the Electrophysical Properties of the Ni<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>W<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> System at Low Temperatures

The problem of establishing the correlation between, on the one hand, the chemical and phase compositions of Ni_1–xW_x alloys (0 ≤ x ≤ 0.5) and, on the other hand, the character of the temperature dependences of the electrical resistivity, is considered. Based on the experimental ρ(T) curves, the concentration dependences of are reconstructed in the wide temperature range (50 K ≤ T ≤ 273 K). The ρ(x) curves have features related to a change in the crystal structures of the alloys (concentration fcc–bcc phase transition), their magnetic structures and percolation processes occurring in the two-phase fcc + bcc medium.