New line of thermodynamic phase transitions on the x-T diagram of a dilute Heisenberg system with short-range interactions Li0.5Fe2.5−x GaxO4

It is established on the basis of an investigation of the temperature dependences of the saturation magnetization σ_S(T) and the magnetic contribution to the specific heat C _m(T) that in the reentrant region of the x-T diagram (0.9⩽x⩽1.5) of dilute iron spinels Li_0.5Fe_{2.5− x} Ga_xO₄ there exists between the line of Curie points T _C(x) and the line of freezing temperatures T _f(x) another line of first-order phase transitions T ₁(x) to a noncollinear ferrimagnetic phase: T _C(x)⩽T ₁(x)⩽T _f(x). Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 329–333 (10 March 1998)     

Transport properties of antiferromagnetic fcc-iron alloys

A study is reported of the electrical resistance and thermopower of Fe_xNi_80−x Cr₂₀ fcc alloys within the 44⩽x⩽70 at. % range. It is shown that, at low temperatures, they typically exhibit minima in the temperature dependences of electrical resistance. The appearance of these anomalies is attributed to the formation of a gap in the conduction electron spectrum due to the onset of long-or short-range antiferromagnetic order in the alloys. The effect of magnetic field on the magnetic states appearing in frustrated antiferromagnetic alloys has been studied, and an H-T magnetic phase diagram constructed. Fiz. Tverd. Tela (St. Petersburg) 40, 101–105 (January 1998)     

Magnetic and electrical properties of Mg1−x CuxO solid solutions and magnetic shielding in Cu-Mg1−x CuxO structures

Mg_1−x Cu_xO solid solutions having an NaCl structure with 0⩽x⩽0.20 are synthesized and Cu-Mg_1−x Cu_xO structures are prepared for superconductivity studies. The magnetic susceptibility χ, electron paramagnetic resonance (EPR), and electrical conductivity of the solid solutions are studied at temperatures of 5–550 K. It is shown that χ ⁻¹(T) obeys the Curie-Weiss law with a paramagnetic Curie temperature Θ close to zero and an effective magnetic moment μ _eff=1.9 μ _B, close to the 1.73 μ _B of a Cu²⁺ ion with spin S=1/2. The width ΔH of the EPR line depends weakly on temperature and increases as x is raised. The volume narrowing of the EPR linewidth ΔH is used to estimate the exchange interaction parameter, 3×10⁻⁴ eV. The g-factor is close to 2 and is temperature independent. The electrical conductivity of Mg_1−x Cu_xO at T=300 K is ≈10⁻¹¹–10⁻¹² Ω⁻¹ cm⁻¹ for x=0 and increases to 10⁻⁵–10⁻⁶ Ω⁻¹ cm⁻¹ for x=0.15–0.20. The conductivity is p-type. Magnetic shielding is observed in Cu-Mg_1−x Cu_xO structures with x=0.15 and 0.20. The possible connection of this phenomenon with interference superconductivity in the contact layer of the structure is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 293–296 (February 1999)     

Ion transport in the anion-deficient nonstoichiometric phases La0.95(Ba1−x Srx)0.05F2.95 (0⩽x⩽1)

The fluoride-ion conductivity of the nonstoichiometric tysonite phases La_0.95(Ba_1−x Sr_x)_0.05Fe_2.95 (0⩽x⩽1) is investigated by impedance spectroscopy. Electrophysical measurements are performed in the frequency range 5–5×10⁵ Hz and temperature range 300–700 K. A discontinuity is observed in the temperature dependence of the conductivity at T _c=410–430 K. The behavior of the temperature dependence of the electrical conductivity is explained within a transport model taking into account the migration of fluoride ions between different inequivalent structural sites. The maximum value of the conductivity at room temperature (293 K) is 2×10⁻⁴ Ω⁻¹ cm⁻¹ for the solid solution La_0.95Sr_0.05F_2.95. The fluorine-ion conductivity in La_0.95(Ba_1−x Sr_x)_0.05F_2.95 single crystals is almost an order of magnitude larger than the value for the commercial solid electrolyte La_0.992Eu_0.008F_2.992 (a fluorine-selective membrane) having a tysonite structure. Fiz. Tverd. Tela (St. Petersburg) 40, 658–661 (April 1998)     

Sound propagation anomalies and phase diagram of chromium alloys

Calculations of the complex elastic moduli Ĉ(T) in dilute Cr alloys are compared to measurements of the velocity and damping of sound near T _N and at high temperatures T>T _N (T _N — Néel temperature). The thermodynamic calculation is based on the covalent bond model of 3d ions in a state with different numbers n of covalent electrons. The parameters A _ij ⁽ⁿ⁾ of indirect exchange between the ions of the i and j sublattices are expressed in terms of the covalent bond energy Γ _ij ⁽ⁿ⁾ . The stability of the charge and spin density waves (CDWs and SDWs) is found by a variational method and is determined by the dispersion of Γ _ij ⁽ⁿ⁾ and by the Coulomb parameters U _n. For a small structural vector Q the phase diagram contains a superantiferromagnetic phase (SAFM) at temperatures T _N<T≲2T _N. The peak of the defect |ΔE(T)| of the modulus and of the sound damping Δ_h(T _N) near the first-order SDW-SAFM transition is determined by the structure of the transitional domain. Measurements of the anomalous growth of E(T) at temperatures T>T _N make it possible to determine the magnetostriction constants λ(T) of Ce alloys in the SAFM phase on the basis of the SAFM theory. Fiz. Tverd. Tela (St. Petersburg) 41, 1467–1472 (August 1999)     

Indirect determination of the ratio R=σL/σT at small x from HERA data

The value of the deep inelastic scattering cross-section ratio R=σ _L /σ _T is found in the range 10⁻⁴⩽x⩽10⁻² from F ₂ and dF ₂/d ln Q ² HERA data using very simple relations based on perturbative QCD. Zh. éksp. Teor. Fiz. 112, 32–36 (July 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Correlation between the electric-field effect and weak-link type in YBa2Cu3−x Oy and YBa2Cu3−x Oy /Agx high-T c superconducting ceramics

An experimental study is reported of the effect of an electric field E⩽120 MV/m and of temperature T on the critical current I _c and I-V characteristics of yttrium-based high-T _c superconducting ceramics. The materials studied were copper-deficient ceramics, YBa₂Cu_3−x O_y (D samples), and YBa₂Cu_3−x O_y/Ag_x ceramics [S samples with silver present in amounts equal to the copper deficiency (0⩽x⩽0.4)]. It has been established that in D samples at 77 K, the electric field increases I _c and reduces substantially R for I>I _c, whereas in S samples no field effect is observed. Measurements of the I _c(T) dependence near the critical temperature showed that they can be described for all samples by a relation of the type I _c =const(1−T/T _c )^α, where α≈1 for the D samples, and α≈2 for the S samples. The results obtained suggest that the electric-field effect correlates with the existence in the ceramic of SIS-type weak links at grain boundaries. Fiz. Tverd. Tela (St. Petersburg) 40, 1195–1198 (July 1998)     

Temperature dependences of the high-field magnetization of dilute frustrated ferrimagnetic spinels

Results are presented of an investigation of the magnetic properties of dilute frustrated ferrimagnetic spinels Li_0.5Fe_2.5−x Ga_xO₄ (x=0.8–1.2), which characterize the main parameters of the ferrimagnetic state and provide evidence of local violation of collinear spin ordering and frustrations. In particular, measurements were made of the concentration dependences of the magnetic moment n ₀(x) and the Curie point T _c (x), the magnetization isotherms σ _T (H) at T=4.2 K and H⩽10 kOe, and also the low-and high-field magnetization polytherms σ _H (T). It was established that for x⩾0.8 in fields exceeding the technical saturation field H _s ∼2 kOe, the temperature dependences of the high-field magnetization σ _H (T) between 4.2 and 230 K cannot be described by the Bloch T ^3/2 law whereas this law is satisfied for undiluted Li spinel (x=0). Over the entire temperature range (4.2–230 K) the experimental curves σ _H (T) may be approximated by σ _H (T)=σ ₀(1−AT ^3/2 − BT ^5/2) for x=0.8–1.0 and σ _H (T)=σ ₀[1−CT ^3/2exp(μ(H−H ₀)/k _B T)] for x=1.1, 1.2, where μH ₀∼15 K is the internal field produced by competition between exchange interactions and frustrations. Fiz. Tverd. Tela (St. Petersburg) 40, 1075–1079 (June 1998)     

Magnetically induced spatial dispersion in the cubic magnetic semiconductors Cd1−x MnxTe

In the transverse geometry we have detected birefringence that is linear in the magnetic field B and the light wave vector k in the cubic magnetic semiconductors Cd_1−x Mn_xTe (0⩽x⩽0.52). The effect was found to be large, ∼1 (deg cm⁻¹ T⁻¹), and highly anisotropic, in contrast to the Faraday and Voigt effects. The phenomenon is represented by terms of type γ _ijklB_kk_l in the permittivity tensor ε _ij and can be described by two parameters, A and g. Spectral studies have shown that the normalized parameters A/x and g/x are independent of x, i.e., the effect can be related to the Mn²⁺ ions. Below the edge E _g of the forbidden band, the dispersion of A is described by a (E _g−E)^−1.4-dependence, while the dispersion of g is nil. Theoretical analysis has shown that the spectral curves for A and g can be explained by the special features of the dispersion laws for electrons and holes (features related to the fact that there is no inversion center) and by the dependence of the parameters of the exchange interaction on the electron wave vector. Zh. éksp. Teor. Fiz. 114, 1018–1033 (September 1998)     

Phase transitions and magnetic-transport phenomena in the system La2/3Ba1/3(Mn1−x Cox)O3

A study is performed of the crystalline structure, magnetization, and magnetotransport properties of the system La_2/3Ba_1/3(Mn_1−x Co_x)O₃ with perovskite structure. It is shown that cubic solid solutions exist over the entire range of cobalt concentrations 0⩽x⩽1. Compositions with x⩽0.2 are ferromagnets with maximum resistance near T _C . Compositions with 0.2<x<0.4 manifest properties of inhomogeneous ferromagnets. Measurements of magnetic properties indicate the absence of long-range magnetic order in compositions with 0.5⩽x⩽0.9, which are probably spin glasses. The spontaneous magnetization of cobaltate (2μ _B per formula unit) corresponds to ferromagnetic ordering of the moments of the Co³⁺ and Co⁴⁺ ions found in the intermediate spin state. It is conjectured that the magnetoresistance consists of an extrinsic and an intrinsic contribution. The first arises as a result of intergrain transport of spin-polarized charge carriers, and the second, as a result of magnetic ordering near T _C . The magnetoresistance is essentially independent of the spontaneous magnetization and decreases abruptly as the cobalt concentration is increased with a corresponding transition from long-range to short-range magnetic order. Zh. éksp. Teor. Fiz. 116, 604–610 (August 1999)     

Paramagnetic susceptibility of Bi-Mn alloys in the liquid state

The magnetic susceptibility of Bi_{100 − x} Mn _x (x = 5, 7.5, 10, 12.5, 15, 17.5, 20, 25) alloys is experimentally studied by the Faraday method in the temperature range T = 300−1200°C and the magnetic-field range B = 0.6−1.3 T. To calculate the electronic characteristics of the Bi-Mn alloys, the experimental results are approximated by the generalized Curie-Weiss law. The calculated parameters of the electronic structure of the alloys demonstrate that manganese is present in the melt in an ionic state with an effective magnetic moment μ_eff ≈ 5μ_B, all Bi-Mn alloys have negative paramagnetic temperatures (which indicate the antiferromagnetic character of the exchange between transition 3d element atoms), and the density of states near the Fermi level n(E _F) is low. Therefore, the Fermi level is outside the d band of manganese and its position is controlled by the sp band of bismuth.     

The effect of bias field on the value and temperature dependence of the piezoelectric coefficient d 31 in (1 ? x )Pb(Mg1/3Nb2/3)O3? x PbTiO3 ( x = 0.06, 0.13) single crystals

It is established that, as a result of application of even a small (∼0.5 kV cm⁻¹) bias field E, the maximum of the piezoelectric coefficient d ₃₁ in (1 − x)Pb(Mg_1/3Nb_2/3)O₃ − xPbTiO₃(x = 0.06, 0.13) crystals shifts from the Vogel-Fulcher temperature to the critical point in the E-T phase diagram of a given composition. The field dependence of the d ₃₁(T) peak magnitude has a maximum near the E values corresponding to the critical point. Original Russian Text ? A.S. Emelyanov, S.I. Raevskaya, F.I. Savenko, I.P. Raevski, M.A. Malitskaya, E.I. Sitalo, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 132–133.     

NMR study of the magnetic properties and electronic structure of amorphous Ni−B−P alloys

The¹¹B and³¹P NMR Knight shift (K) and Korringa spin-lattice relaxation rate (1/T ₁ T) have been measured for amorphous Ni_81.5B_18.5−x P _x alloys with 0≤x≤18.5. In accordance with previous bulk measurements, the³¹P NMR parameters vary markedly with composition whereas the¹¹B NMR parameters remain almost constant except for small P-contents. It is suggested that the d-band contribution toK(³¹P) is positive and that toK(¹¹B) is negative. The data further support our previous conclusion that amorphous Ni_81.5B_18.5 is a Pauli paramagnet rather than a weak itinerant ferromagnet.     

Dielectric properties of perovskite ceramics of (1–x)BiFeO3 ? x(KBi)1/2TiO3 (0.40 < x < 0.85) solid solutions from impedance spectroscopy data

The dielectric properties of the ceramics of (1 − x)BiFeO₃ − x(KBi)_1/2TiO₃ (0.40 < x < 0.85) solid solutions with an orthorhombic structure have been studied using impedance spectroscopy in the frequency range 25–10⁶ Hz at different temperatures. It has been shown that these solid solutions undergo a diffuse ferroelectric phase transition. The Curie temperature is found to be in the range 620–640 K. The activation energies of dielectric polarization relaxation (δE _M ) and dc charge carriers (ΔE _dc) are determined. It has been established that, in the vicinity of 460 K, ΔE _dc increases jumpwise as the temperature increases.     

Universal behavior of CePd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Rh<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> ferromagnet at the quantum critical point

The heavy-fermion metal CePd_1−x Rh _x can be tuned from ferromagnetism at x = 0 to the nonmagnetic state at some critical concentration x _c . The non-Fermi liquid behavior (NFL) at x ≃ x _c is recognized by the power-law dependence of the specific heat C(T) given by the electronic contribution susceptibility X(T) and volume expansion coefficient α(T) at low temperatures: C/T ∝ X(T) ∝ α(T)/T∝ 1/ √T. We also demonstrate that the behavior of the normalized effective mass M _N ^* observed in CePd_1−x Rh _x at x ≃ 0.8 agrees with that of M _N ^* observed in paramagnetic CeRu₂Si₂ and conclude that these alloys exhibit the universal NFL thermodynamic behavior at their quantum critical points. We show that the NFL behavior of CePd_1−x Rh _x can be accounted for within the frameworks of the quasiparticle picture and fermion condensation quantum phase transition, while this alloy exhibits a universal thermodynamic NFL behavior that is independent of the characteristic features of the given alloy such as its lattice structure, magnetic ground state, dimension, etc. The text was submitted by the authors in English.     

Internal friction in directed-crystallization (Cu-Sn)-Nb alloys

The distinctive features of the low-frequency internal friction Q ⁻¹(T) of (Cu-Sn)-Nb composites at high temperatures (up to 400°C) are investigated for strains in the range 10⁻⁵–10⁻⁴. Considerable hysteresis of Q ⁻¹(T) in the heating-cooling cycle is recorded, including the presence of a minimum at ∼175°C when the sample is heated to 400°C and two peaks P ₂ (at 280°C) and P ₁ (at ∼100°C) when the sample is cooled from 400°C. The activation energy of the anomalous internal friction background (up to 175°C), the oxygen diffusion parameters, and the oxygen concentration in the niobium fibers (all of which govern the peak P ₂) are calculated, and the value and temperature dependence of the yield point of the bronze matrix (which govern the peak P ₁) are estimated. Zh. Tekh. Fiz. 68, 114–117 (November 1998)     

Effect of mixed glass formers on the crystallization kinetics in AgI–Ag<Subscript>2</Subscript>O–V<Subscript>2</Subscript>O<Subscript>5</Subscript>–MoO<Subscript>3</Subscript> glassy superionic system

The crystallization and glass transition kinetics using differential scanning calorimetry (DSC) in 50AgI–33.33Ag₂O–16.67[(V₂O₅)_1−x –(MoO₃) _x ] superionic glassy system is discussed. Thermal stability of glass, studied using various criteria, does not vary significantly with glass former variation. However, the activation energies for structural relaxation (E _s) at glass transition temperature and crystallization (E _c) obtained using Moynihan and Kissinger, Matusita-Sakka formulations found to exhibit interesting trends with MoO₃ substitution in the glass matrix. It is noticed that the electrical conductivity (σ)–temperature (T) cycles obtained at a typical heating rate of 1 °C/min do exhibit significant thermal events. The conductivity after first heating cycle at room temperature is found to be increasing with MoO₃ content and maximum for x = 0.3 (~10⁻³ Ω⁻¹ cm⁻¹ at 30 °C) which is comparable to that of the host 50AgI–33.33Ag₂O–16.67V₂O₅ glassy system. The parameters obtained from σ–T plots and DSC scans do complement each other in a particular range of composition.     

Nature of conduction via impurities in uncompensated silicon

The static conductivity σ(E) and photoconductivity (PC) at radiation frequencies ħω=10 and 15 meV in Si doped with shallow impurities (density N=10¹⁶−6×10¹⁶ cm⁻³, ionization energy ε₁≃45 meV) with compensation K=10⁻⁴−10⁻⁵ in electric fields E=10–250 V/cm are measured at liquid-helium temperatures T. Special measures are taken to prevent the high-frequency part of the background radiation (ħω>16 meV) from striking the sample. It is found that the conductivity σ(E) is due to carrier motion along the D ⁻ band, which is filled with carriers under the influence of the field E. In fields E<E _q (E _q ≃100–200 V/cm) the carrier motion consists of hops along localized D ⁻ states in a 10–15 meV energy band below the bottom of the free band (energy ε=ε₁); for E>E _q carriers drift along localized D ⁻ states with energy ε∞ε₁−10 meV. An explanation is proposed for the threshold behavior of the field dependence of the photo-and static conductivities. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 232–236 (25 August 1997)     

Galvanomagnetic properties of the Al1−x Six nonequilibrium substitutional solid solutions

Galvanomagnetic characteristics of the A_1−x Si_x solid solutions (x<12 at. %) have been studied within a broad range of temperatures (1.8–290 K) and magnetic fields (up to 15 T). An anomaly in the concentration dependence of the Hall coefficient R _H (x,T=290 K) has been revealed near the boundary of absolute instability (x<8.5 at. %) of compounds in the Al_1−x Si_x series. The variation of the Hall coefficient and of the magnetoresistance in the A_1−x Si_x series at low (T<77 K) temperatures is analyzed within models taking into account the anisotropy in conduction-band electron scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 3–10 (January 1999)     

Composition dependence of density of states in a-Se<Subscript>100−<Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films

The present paper reports the DC conductivity measurements at high electric fields in vacuum evaporated amorphous thin films of a-Se_100−x Sn _x (x = 0, 2, 4, 6) glassy alloys. Current-voltage (I–V) characteristics have been measured at various fixed temperatures. In these samples, at low electric fields, ohmic behaviour is observed. However, at high electric fields (E ∼ 10⁴ V/cm), non-ohmic behaviour is observed. An analysis of the experimental data confirms the presence of space charge limited conduction (SCLC) in the glassy materials studied in the present case. From the fitting of the data to the theory of SCLC, the density of defect states (DOS) near Fermi level is calculated. Increase in DOS with increase in Sn concentration has been found which could be correlated with the electronegativity difference between the two elements used here in making the glassy alloys. The peculiar role of the element Sn as an impurity in the pure Se glassy alloy is also discussed.