Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

The LaGa_1−x−yCo_xMg_yO_3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa_1−x−yCo_xMg_yO_3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10⁻⁶ K⁻¹ at 300–1100 K. Doping La(Ga,Co)O_3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa_1−x−yCo_xMg_yO_3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.     

Cobalt substituted R---Fe(Co)---C alloys

The formation of tetragonal R₂(FeCo)₁₄C phase has been examined in as-cast and melt-spun R₁₄Fe_78−xCo_xC₈ alloys with cobalt substitutions (R = Y, Dy, Nd). The magnetic properties over a temperature range and the microstructure have been studied as a function of cobalt content. The Curie temperature is increased with Co content but the anisotropy K is decreased. High cobalt content leads to the formation of 1:5 phase. High corecivities have been developed in as-cast and melt-spun Dy₁₄Fe_78−xCo_xC₈ alloys with Co content at zero and 32 at %, respectively. As-cast Nd₁₆Fe_78−xCo_xC₈ alloys did not show any permanent magnetic properties although they had the 2:14:1 phase. However, melt-spun and powdered Nd---Fe---Co---C samples showed a coercivity with the highest value corresponding to a melt-spun Nd₁₄Fe₇₈C₈ sample. Microstructure studies showed that the high H_C in ribbons is due to the fine grain size which is in the range of 500–1000 Å.     

Oxygen permeability of La2Cu(Co)O4+δ solid solutions

Formation of the La₂Cu_1−xCo_xO_4+δ solid solutions with orthorhombic K₂NiF₄-type structure was found to be in the range of 0≤x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La₂Cu_1−xCo_xO_4+δ (x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10⁻⁶ K⁻¹. Studying the dependence of oxygen permeation fluxes through La₂Cu(Co)O_4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La₂Cu_1−xCo_xO_4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

High temperature susceptibility of semimagnetic-semiconductor Zn1-xMnxS

This paper gives an analysis of the high temperature susceptibility of diluted semimagnetic-semiconductor Zn_{1 − x}Mn_xS. The high-temperature susceptibility of Zn_{1 − x}Mn_xS was found to behave in accordance with the Curie-Weiss law. From _χ(T) measurements the exchange integral of Mn²⁺ -Mn²⁺ interaction 2J₁/k_B = (-34.6±0.5) K (effective exchange integral) was obtained. A spin S = 2.6±0.1, close to its atomic value S = , was also found. The role of the superexchange in this alloy is shortly discussed at the end of the paper.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa₂Cu_3−xCo_xO_6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10⁻⁵ and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa₂Cu_3−xCo_xO_6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa₂Cu₃O_6+δ     

Perpendicular magnetic anisotropy of Fe-Nd alloy thin films

A study of the magnetic properties of Fe_{100 − x}Nd_x (18 x 50) films made by rf sputtering has been carried out. The perpendicularly magentized films were fabricated for compositions in the range from x = 35 to 50 at substrate temperatures between 210 to 290°C. The intrinsic perpendicular magnetic anisotropy constant K_u is maximum at about x = 40 where K_u is about 1 × 10⁷ erg/cm³ at room temperature. The temperature dependence of K_u implies that the origin of the perpendicular magnetic anisotropy may be related to some sort of atomic ordering of crystalline clusters in an amorphous matrix.     

Structural and electrical properties of crystalline (1−x)Ta2O5−xTiO2 thin films fabricated by metalorganic decomposition

Polycrystalline (1−x)Ta₂O₅−xTiO₂ thin films were formed on Si by metalorganic decomposition (MOD) and annealed at various temperatures. As-deposited films were in the amorphous state and were completely transformed to crystalline after annealing above 600 °C. During crystallization, a thin interfacial SiO₂ layer was formed at the (1−x)Ta₂O₅−xTiO₂/Si interface. Thin films with 0.92Ta₂O₅–0.08TiO₂ composition exhibited superior insulating properties. The measured dielectric constant and dissipation factor at 1 MHz were 9 and 0.015, respectively, for films annealed at 900 °C. The interface trap density was 2.5×10¹¹ cm⁻² eV⁻¹, and flatband voltage was −0.38 V. A charge storage density of 22.8 fC/μm² was obtained at an applied electric field of 3 MV/cm. The leakage current density was lower than 4×10⁻⁹ A/cm² up to an applied electric field of 6 MV/cm.     

Positive linear magnetoresistance in Fex–C1−x composites

A large positive magnetoresistance (MR) has been found in micro-sized Fe_x–C_1−x composites. At a magnetic field of 5 T, the Fe_0.2–C_0.8 composite has the largest MR, 53.8% and 190% at room temperature and at 5 K, respectively. The magnetic field dependence of the MR can be described approximately as MR∝Bⁿ, and the value of exponent n is determined by the Fe weight concentration and temperature, ranging from 1/4 to 6/4. It appears that Fe_x–C_1−x has a linear field dependence of the positive MR at different temperatures. The possible mechanism for the positive MR is discussed.     

Effects of substitution of Ti for Fe in BiFeO3 films prepared by sol–gel process

Ti substituted BiFe_1−xTi_xO_3+δ films have been prepared on indium–tin oxide (ITO)/glass substrates by the sol–gel process. The films with x=0.00–0.20 were prepared at an annealing temperature of 600 °C. X-ray diffraction patterns indicate that all films adopt R3m structure and the films with x=0 and 0.10 show pure perovskite phase. Cross-section scanning shows the thickness of the films is about 300 nm. Through 0.05 Ti substitution, the 2P_r increases to 8.30 μC/cm² from 2.12 μC/cm² of the un-substituted BiFeO₃ film and show enhanced ferroelectricity at room temperature. The 2P_r values are 2.63 and 0.44 μC/cm² for the films with x=0.01 and 0.2, respectively. Moreover, the films with x=0.05 and 0.10 show enhanced dielectric property since the permittivity increases near 150 at the same measuring frequency. Through the substitution of Ti, the leakage conduction is reduced for the films with x=0.05–0.20.     

Far-infrared and millimeter wave spectroscopy of superionic copper conductors

Far-infrared and millimeter wave spectra of copper ion conducting crystal RbCu₄Cl_3+xI_2−x, which has the same structure as the room temperature silver ion conductor RbAg₄I₅, were investigated. Broad absorption peaks observed around 40, 80, and 110–200 cm⁻¹ at room temperature show doublet structures at low temperature; this may be attributed to the difference of local structure by chlorine and iodine ion. The 110–200 cm⁻¹ bands seem to be symmetric breathing modes of CuX₄ (X = Cl or I) tetrahedron and the frequency shift coincides with the square root of the mass ratio of conduction ions. The 80 cm⁻¹ band seems to be Rb-X vibration in RbX₆ octahedron. The 40 cm⁻¹ band seems to be the attempt mode which is an outward motion of the mobile ion in halogen cage. The increase of the absorption intensity at the low energy side with temperature corresponds to an increase of the DC conductivity. Plasmon fitting in energy loss function spectra was attempted.     

Magnetic and electromagnetic properties of RuZn and RuCo substituted BaFe12O19

Polycrystalline samples of M-type hexaferrites BaFe_12−2xRu_xZn_xO₁₉ and BaFe_12−2xRu_xCo_xO₁₉ with 0x0.45 have been prepared by a classical sintering method. The evolutions with x of the cell parameters, the saturation magnetization and the magnetic transition temperature have been measured; in this range of small doping ratios, saturation magnetization and Curie temperature of substituted hexaferrites remain close to those of the undoped BaFe₁₂O₁₉. X-ray diffraction measurements on oriented powders show that a change of magnetocrystalline anisotropy from axial to planar occurs in both cases for a small substituting ratio x_c=0.375. Microwave electromagnetic characteristics have been studied on the ceramic samples from 0.1 to 10 GHz. The behaviour of the magnetic losses (μ″) corroborates the anisotropy change when doping; a convolution of the dissipation mechanisms (domain wall motions and gyromagnetism) is obtained for x_c. The level of the magnetic losses is discussed in relation with others substituted Ba-hexaferrites.     

Determination of different oxygen transport mechanisms and measurement of the oxygen ion conductivity of Y1Ba2Cu3O7−x

By undertaking AC electrochemical impedance experiments on yttria stabilised zirconia electrolytes with polished Y₁Ba₂Cu₃O_7−x electrodes, the activation energy for oxygen ion transport within the bulk of Y₁Ba₂Cu₃O_7−x, in air, over the temperature range 823 K–1043 K, was determined to be 1.50 ± 0.05 eV. At 1000 K the oxygen ionic conductivity was calculated to be around one order of magnitude lower than that in yttria stabilised zirconia. Typical calculated values were σ=5×10⁻⁵ (ω cm)⁻¹ and 6×10⁻³ (ω cm)⁻¹ at the respective temperatures 823 K and 1043 K. By employing a similar cell but with Y₁Ba₂Cu₃O_7−x paste electrodes, oxygen transfer between the Y₁Ba₂Cu₃O_7−x and the electrolyte was found to occur via a surface diffusional processes. Over the temperature range 873 K–1098 K, in air, the activation energy for in-diffusion at the surface was found to be 1.4±0.1 eV and that for out-diffusion at the surface to be 1.76±0.05 eV.     

The order of magnetic phase transition in La(Fe1−xCox)11.4Si1.6 compounds

Magnetic transitions in La(Fe_1−xCo_x)_11.4Si_1.6 compounds with x=0–0.08, have been studied by DC magnetic measurements and Mössbauer spectroscopy. The temperature dependence of the Landau coefficients has been derived by fitting the magnetization, M(μ₀H), using the Landau expansion of the magnetic free energy. For x0.02 there is a strongly first-order magnetic phase transition between ferromagnetic and paramagnetic (F–P) states in zero external field and a metamagnetic transition from paramagnetic to ferromagnetic (P–F) above T_c. Increasing the cobalt content drives the F–P transition towards second order and eliminates the metamagnetic transition.     

Defect structure of quenched γ-BICOVOX by combined X-ray and neutron powder diffraction

A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi₂V_{1 − x}Co_xO_{5.5 − 3x/2} (x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi₂O₂]_n²ⁿ⁺ and [V_{1 − x}Co_xO_{3.5 − 3x/2}]_n²ⁿ⁻ with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral.     

Magnetic properties of Gd2Fe14−xCoxB compounds

The magnetization of magnetically-aligned Gd₂Fe_14−xCo_xB samples with x = 0, 2, 4, 6, 8.4, 11.2 and 14 has been measured at 4.2 K in fields up to 14 T and the anisotropy fields have been derived. The effect of substitution of Co for Fe on the magnetic properties of Gd₂Fe₁₄B has been investigated.     

Preparation of tantalum oxide thin films by photo-assisted atomic layer deposition

Tantalum oxide thin films were prepared by photo-assisted atomic layer deposition (Photo-ALD) in the substrate temperature range of 170–400 °C using Ta(OC₂H₅)₅ and H₂O as precursors. The constant growth rates of 0.42 and 0.47 Å per cycle were achieved for the films grown by normal ALD and Photo-ALD, respectively. The increased growth rate in Photo-ALD is probably due to the reactive surface by photon energy and faster surface reaction. In Photo-ALD, however, the constant growth rate started at lower temperature of 30 °C and one cycle time shortened up to 5.7 s than that of normal ALD. The films grown by normal ALD and Photo-ALD were amorphous and very smooth (0.21–0.35 nm) as examined by X-ray diffractometer and atomic force microscopy, respectively. Also, the refractive index was found to be 2.12–2.16 at the substrate temperature of 190–300 °C, similar to that of the film grown by normal ALD. However, the remarkably low leakage current density of 0.6×10⁻⁶ A/cm² to 1×10⁻⁶ A/cm² at applied field of 1 MV/cm is several order of magnitude smaller than that of normal ALD, probably due to the presence of reactive atom species.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.