Isotropic positive magnetoresistance in Co-Al<Subscript>2</Subscript>O<Subscript>n</Subscript> nanocomposites

The magnetotransport properties of Co_x(Al₂O_n)_{100 ? x} nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Co_x(Al₂O_n)_{100 ? x} composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.     

Multiferroic properties and structural features of <Emphasis Type="Italic">M</Emphasis>-type Al-substituted barium hexaferrites

Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (0.1 ≤ x ≤ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe_12–x Al _x O₁₉ (x ≤ 1.2) with a collinear ferromagnetic structure.     

Investigation of magnetic phase transitions in dilute copper ferrites

Mössbauer spectra of copper ferrites CuGa _x Al_2x Fe_{2 ? 3x} O₄ (x = 0.3, 0.4, 0.5) have been measured at temperatures in the range from 90 to 295 K. The degree of disturbance of long-range magnetic order has been determined as a function of the temperature. The Curie temperature has been found for the composition with x = 0.5, and the dependence of the Curie temperature on the concentration of the nonmagnetic impurity has been constructed.     

Effect of substitutional order on the relaxation of aluminum nuclei in Y<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>Lu<Subscript>x</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> mixed garnets

The spin-lattice relaxation time of ²⁷Al nuclei residing in the octahedral and tetrahedral oxygen environment in Y_{3 ?x}Lu_xAl₅O₁₂ mixed aluminum-yttrium garnets was measured by the NMR method for 0≤x≤3. A maximum in the concentration dependence of the relaxation time was observed for both crystallographic positions at x=0.75; this maximum correlates with the minimum in the NMR linewidth, indicating the ordering obtained as the yttrium and lutetium ions occupy the dodecahedral positions in the garnet lattice.     

Long-range magnetic order in Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Na<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>2</Subscript>O<Subscript>2</Subscript>

Single crystals of Li _x Na_{1 ? x} Cu₂O₂ solid solutions have been grown from the melt and crystal lattice parameters have been studied. It is found that the solid solution of this system exists in the region x ≤ 0.25. Specific heat and magnetic susceptibility are investigated on monocrystalline samples with x = 0.07, 0.14, and 0.21. Long-range magnetic order in these compounds is formed at T = 13.8, 4.1, and 14.8 K, respectively. It is found that the Néel temperature T _N in these compounds increases with Li content.     

Magnetoresistance and the hall effect of the ordered alloys Fe<Subscript>100 −<Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (25 < <Emphasis Type="Italic">x</Emphasis> < 35 at. %)

The transverse magnetoresistance and the Hall effect of Fe_{100 ?x} Al _x ordered alloys with x = 26.5?34.1 at. % are studied at temperatures of 77 and 295 K. No saturation is found in the field dependences up to 17 kOe. The magnetoresistance is negative, depends nonmonotonically on the Al concentration, and reaches a maximum |ΔR/R| = 1.6% in the concentration range from 28 to 30 at. %. The coefficient of the anomalous Hall effect depends nonlinearly on the Al concentration and resistivity: it increases to a lesser extent as compared to the resistivity at x = 26–30 at. % and decreases with increasing resistivity at x > 30 at. %. An explanation is proposed for the observed anomalies in the magnetotransport properties in the framework of the model developed for the inhomogeneous magnetic microstructure of the Fe_{100 ?x} Al _x ordered alloys.     

Structural features and the microscopic dynamics of the three-component Zr<Subscript>47</Subscript>Cu<Subscript>46</Subscript>Al<Subscript>7</Subscript> system: Equilibrium melt,supercooled melt,and amorphous alloy

The structural and dynamic properties of the three-component Zr₄₇Cu₄₆Al₇ system are subjected to a molecular dynamics simulation in the temperature range T = 250–3000 K at a pressure p = 1.0 bar. The temperature dependences of the Wendt–Abraham parameter and the translation order parameter are used to determine the glass transition temperature in the Zr₄₇Cu₄₆Al₇ system, which is found to be T_c ≈ 750 K. It is found that the bulk amorphous Zr₄₇Cu₄₆Al₇ alloy contains localized regions with an ordered atomic structures. Cluster analysis of configuration simulation data reveals the existence of quasi-icosahedral clusters in amorphous metallic Zr–Cu–Al alloys. The spectral densities of time radial distribution functions of the longitudinal (C?_L(k, ω)) and transverse (C?_T(k, ω)) fluxes are calculated in a wide wavenumber range in order to study the mechanisms of formation of atomic collective excitations in the Zr₄₇Cu₄₆Al₇ system. It was found that a linear combination of three Gaussian functions is sufficient to reproduce the (C?_L(k, ω)) spectra, whereas at least four Gaussian contributions are necessary to exactly describe the (C?_T(k, ω)) spectra of the supercooled melt and the amorphous metallic alloy. It is shown that the collective atomic excitations in the equilibrium melt at T = 3000 K and in the amorphous metallic alloy at T = 250 K are characterized by two dispersion acoustic-like branches related with longitudinal and transverse polarizations.     

Influence of magnetic ordering on the structural properties of dilute copper ferrites

The x-ray structural properties of samples in the CuGa_xAl_xFe_2?2x O₄ (x = 0?0.7) and CuGa_xAl_2x Fe_2?3x O₄ (x = 0?0.5) systems are studied. It is found that magnetic ordering in dilute copper ferrites affects their structural properties. It is concluded that the frustration of magnetic coupling leads to suppression of the cooperative Jahn-Teller effect in dilute copper ferrites with a frustrated magnetic structure.     

Electrical conductivity and thermopower in Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Mn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> S sulfides

This paper reports on the results of investigations into the structural, electrical, and thermoelectrical properties of sulfides Co _x Mn_{1 ? x} S (0 ≤ x ≤ 0.4) in the temperature range 80–950 K. It is established that the thermopower coefficient α decreases significantly with an increase in the cobalt concentration in the lattice of the α-MnS compound. The Co _x Mn_{1 ? x} S compounds with cobalt concentrations in the range 0 ≤ x ≤ 0.3 are semiconductors with hole conduction (α > 0), whereas the compound with x = 0.4 exhibits metallic conduction (α < 0). It is found that the band gap E _g of the compounds under investigation varies in the range from 1.46 eV for α-MnS (x = 0) to 0.26 eV for Co _x Mn_{1 ? x} S (x = 0.4).     

Susceptibility of the paraprocess in spinel ferrites with a frustrated magnetic structure

The temperature dependence of the susceptibility of the paraprocess χ_para (T) is investigated for samples in the CuGa_xAl_xFe_2?2xO₄ (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7), CuGa_xAl_2xFe_2?3xO4 (x = 0.1, 0.2, 0.3, 0.4, 0.5), and Ga_xFe_{1? x}NiCrO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8) systems. It is found that long-range magnetic order arises in spinel ferrites at the temperature T_tran of the transition from a cluster spin-glass state to a frustrated magnetic structure with a maximum in the temperature dependence of the susceptibility of the paraprocess.     

Galvanomagnetic properties of thin films of bismuth and bismuth–antimony alloys on substrates with different thermal expansions

Temperature dependences of the galvanomagnetic properties of films of bismuth and Bi_{100 – x}Sb_x (x ≤ 12) on substrates with different temperature expansion coefficients were studied in the temperature range of 77–300 K. The block films were prepared through thermal deposition, and single-crystal Bi_{100 – x}Sb_x were grown by zone recrystallization under a coating. It was found that the temperature expansion coefficient of a substrate substantially influenced the galvanomagnetic properties of Bi and Bi_{100 – x}Sb_x films. Using the experimental data, the change in the charge-carrier concentration in the Bi and Bi_{100 – x}Sb_x films on different substrates at 77 K was estimated.     

Growth of (GaAs)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>(ZnSe)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solution films and investigation of their structural and some photoelectric properties

Single-crystal films of the substitutional solid solution (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) on GaAs substrates have been grown using liquid phase epitaxy. The X-ray diffraction patterns, photoluminescence spectra, and current-voltage characteristics of the n-(GaAs)-p-(GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) heterostructures prepared have been investigated. The lattice parameters of the film a _f = 5.6544 Å and the substrate a _s = 5.6465 Å have been determined, and the profile of the molecular distribution of the solid solution components has been obtained. The photoluminescence spectrum of the (GaAs)_{1 ? x} (ZnSe) _x (0 ≤ x ≤ 0.80) films exhibits a narrow peak (against the background of the broad luminescence band) with a maximum in the luminescence intensity at a photon energy of 2.67 eV due to the presence of Zn-Se bonds in the structure (ZnSe is covalently bonded to the tetrahedral lattice of the GaAs matrix). It has been shown that the direct branch of the current-voltage characteristics of the structures under investigation is described by an exponential dependence I = I ₀exp(qV/ckT) at low voltages (V > 0.3 V) and by a power-law dependence I ～ V ^α with exponents α = 4 at V = 0.4–0.8 V, α = 2 at V = 0.8–1.4 V, and α = 1.5 at V > 2 V. The experimental results have been explained in the framework of the double-injection model for the n-p-p ⁺ structure under the condition that the concentration distribution of nonequilibrium charge carriers has a minimum.     

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 %.     

The Nernst-Ettingshausen coefficient in the normal phase of doped HTSCs of the YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>y</Subscript> system

The temperature dependence of the Nernst-Ettingshausen coefficient Q(T) in the normal phase of doped HTSCs of the yttrium system was studied. The main features characterizing the behavior of this coefficient were revealed, and the character and mechanism of the effect that various nonisovalent substituents exert on the Q(T) dependence were analyzed. It is shown that the narrow-band model permits one not only to describe all the specific features observed in the Q(T) curves but also to perform a simultaneous quantitative analysis of the temperature dependences of four kinetic coefficients (the electrical resistivity and the Seebeck, Hall, and Nernst-Ettingshausen coefficients) with the use of a common set of model parameters characterizing the band structure and carrier system in the normal phase of an HTSC. This approach was employed to determine the carrier mobilities and the asymmetry of the dispersion curve in the systems studied (YBa₂Cu₃O_y, y = 6.37–6.91; YBa₂Cu_3?xCo_xO_y, x = 0–0.3; Y_1?xCa_xBa₂Cu₃O_y, x = 0–0.25; Y_1?xCa_xBa_2?xLa_xCu₃O_y, x = 0–0.5) and to analyze the effect of the substitutions involved on the variation of these parameters.     

Relaxation processes in an alternating-current electric field and energy loss mechanisms in hafnium diselenide cointercalated with copper and silver atoms

Samples based on hafnium diselenide intercalated with atoms of two types, Cu_xAg_yHfSe₂ at (x + y) ≤ 0.2, have been synthesized for the first time. The frequency dependences of the components of the complex impedance have been measured using impedance spectroscopy in the frequency range from 1 Hz to 10 MHz, and the specific features of the relaxation processes occurring in samples of different compositions have been analyzed. It has been shown that the characteristic times of these processes depend not only on the total concentration of intercalated atoms, but also on the ratio between them. As the total concentration of copper and silver increases, the onset of frequency dispersion of the complex admittance shifts to the higher frequency range. The relative contributions from the conduction and relaxation polarization losses also change depending on the total and element concentrations of the intercalated atoms.     

Hyperfine nuclear interaction at copper lattice sites of high-temperature superconductors studied by <Superscript>61</Superscript>Cu(<Superscript>61</Superscript>Ni) Mössbauer emission spectroscopy

Mössbauer emission spectroscopy on the ⁶¹Cu(⁶¹Ni) isotope has been used to determine the quadrupole coupling constant C(Ni) and magnetic induction B(Ni) for the ⁶¹Ni²⁺ probe at copper sites in Cu₂O, CuO, La_{2 ?x}Ba_xCuO₄, Nd_2?xCe_xCuO₄, RBa₂Cu₃O₆, and RBa₂Cu₃O₇ (R=Y, Nd, Gd, Yb). The compounds containing divalent copper were found to exhibit linear C(Ni) vs. C(Cu) and B(Ni) vs. B(Cu) relations [C(Cu) and B(Cu) are the quadrupole coupling constant and magnetic induction for the ⁶³Cu probe, respectively, found by NMR], which is interpreted as an argument for the copper being in divalent state. The deviation of the data points corresponding to the Cu(1) sites in RBa₂Cu₃O₆ and RBa₂Cu₃O₇ from the C(Ni) vs. C(Cu) straight line may be due either to the copper valence being other than 2+ (in the RBa₂Cu₃O₆ compounds) or to the principal axes of the total and valence electric field gradient being differently oriented (in the RBa₂Cu₃O₇ compounds).     

Optical and electrical properties of β-Tl<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>x</Subscript>InS<Subscript>2</Subscript> (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 0.015) single crystals

The peculiarities of fundamental optical absorption, thermally stimulated conductivity, and depolarization currents in β-Tl_{1 ? x} Cu_xInS₂ (0 ≤ x ≤ 0.015) single crystals have been investigated in the temperature range 4.2–300 K. It is found that the temperature coefficient of the band gap E _g changes near the temperature of the structural phase transition.     

Luminescence kinetics of dipolar excitons in circular traps

A detailed investigation of multiband superconductivity and Leggett’s mode in the Mg_1?x Al_xB₂ (0 ≤ x ≤ 0.45) system was carried out using tunneling and Andreev spectroscopy. Temperature dependences of superconducting gaps Δ_σ and Δ_π and their variation upon the degree of disorder and the Al concentration were studied. The dependence of the Leggett’s mode energy ε₀ upon the values of the gaps Δ_σ and Δ_π has been derived.     

Transformation of dielectric properties and appearance of relaxation behavior in Pb<Subscript>5</Subscript>(Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript>x</Subscript>)<Subscript>3</Subscript>O<Subscript>11</Subscript> crystals

The special features of the dielectric properties and conduction of ferroelectric crystals of Pb₅(Ge_1?xSi_x)₃O₁₁ (0 ≤ x ≤ 0.67) solid solutions were studied. Permittivity anomalies close to the temperatures T₁ ≈ 260 K and T₂ ≈ 130 K, the appearance of relaxator behavior at x > 0.35, and critical behavior of the concentration dependences of dielectric and pyroelectric characteristics at x₁ = 0.35 and x₂ = 0.60 were observed and studied. These phenomena were found to be related to the dynamics of charge localization on defects with activation energies of U_a1 ≈ 0.6 eV and U_a2 ≈ 0.23 eV. Relaxator behavior appears when the Curie point lies in the temperature region of thermal charge localization. The concentration dependence features at x₁ and x₂ are explained by the coincidence of the Curie point and the centers of the temperature regions of charge localization on the U_a1 and U_a2 defect levels, respectively.     

Influence of oxidized interlayers on magnetic properties of multilayer films based on amorphous ferromagnet–dielectric nanocomposites

Films of composites (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x, (Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x, (Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x and multilayer heterogeneous composite–composite structures {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + N₂]}_n, {[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x]/[(Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x + O₂]}_n, {[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x]/[(Co₄₁Fe₃₉B₂₀)_x(SiO₂)_100–x + O₂]}_n, and {[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x]/[(Co₈₄Nb₁₄Ta₂)_x(SiO₂)_100–x + O₂]}_n have been deposited using the ionbeam sputtering method with a cyclic supply of reaction gases during deposition. The structure and magnetic properties of the films have been studied. It has been shown that the introduction of an oxidized interlayer makes it possible to suppress the perpendicular magnetic anisotropy in the (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_100–x composite with the metallic phase concentration higher than the percolation threshold.