Large-distance rf- and dc-sputtering of epitaxial La1 − xSrxMnO3 thin films

Studies on large-distance sputtering as an effective alternative to molecular beam epitaxy, pulsed laser deposition or off-axis sputtering for the deposition of epitaxial La_1 − xSr_xMnO₃ (LSMO) thin films, are reported. The focus of this study is on the quality of the samples and their structural and magnetic properties. The dependence of the characteristics of the LSMO films on the sputtering mode (rf, dc) and the sputtering parameters, in particular on the oxygen partial pressure is established and discussed. It is shown that large-distance sputtering can provide high quality LSMO thin films without the need for post-annealing.     

Controlling temperature coefficient of resistivity in La1−xSrxMnO3 ceramics

We propose La_1−xSr_xMnO₃ as a new lead-free and ruthenium-free conductive oxide used for thick film resistors. The temperature coefficient of resistivity (TCR) of the La_1−xSr_xMnO₃ was controlled systematically by changing the composition x. The TCR behavior depended on the change of the crystal symmetries and the average valence of Mn ions. The highest value of 9356 ppm/°C was obtained at the x = 0.35. Zero TCR was realized around 0.200 < x < 0.225 and 0.45 < x < 0.50, where the critical x values were related to the characteristic change from Mott-insulator to metallic behavior. The systematical controlling TCR and the zero TCR are the first to be demonstrated for conductive oxide.     

Structure and magnetism in the oxygen-deficient perovskites Ce1−xSrxCoO3−δ (x ≥ 0.90)

We have examined the structure and phase behaviour of strontium-doped Ce_1−xSr_xCoO_3−δ and found that the perovskite form is stabilised over a relatively narrow solid solution range (x > 0.85). A combination of electron, powder X-ray and neutron diffraction has revealed tetragonal superstructures of the basic perovskite unit; (I4/mmm) 2a_p × 2a_p × 4a_p (x = 0.90) and (P4/mmm) a_p × a_p × 2a_p (x = 0.95). Magnetisation measurements show ferromagnetic behaviour under applied magnetic fields. Low temperature neutron diffraction of Ce_0.10Sr_0.90CoO_2.80 in zero field reveals a magnetic cell of dimension 2a_p × 2a_p × 4a_p with an ordered cobalt moment of 1.7 B.M. at 25 K.     

Electrochemical reduction of nitrous oxide on La1−xSrxFeO3 perovskites

The electrochemical reduction of nitrous oxide and oxygen has been studied on cone-shaped electrodes of La_1−xSr_xFeO_3−δ perovskites in an all solid state cell, using cyclic voltammetry. It was shown that the activity of the La_1−xSr_xFeO_3−δ perovskites for the electrochemical reduction of nitrous oxide mainly depends on the amount of Fe(III) and oxide ion vacancies. The activity of the La_1−xSr_xFeO_3−δ perovskites towards the electrochemical reduction of nitrous oxide is much lower than the activity of the La_1−xSr_xFeO_3−δ perovskites towards the electrochemical reduction of oxygen, making the possibility of electrochemically reducing nitrous oxide selectively in an exhaust gas containing excess oxygen on this type of materials very doubtful.     

Nanostructured Ce1−xZrxO2 solid solutions produced by mechanochemical processing

The nanostructured Ce_1−xZr_xO₂ solid solutions (x ≤ 0.2) have been successfully synthesized from CeCl₃–ZrCl₄–NaOH mixtures by mechanochemical processing as a gradual transformation, involving in-situ CeO₂ and amorphous ZrO₂ formation as intermediates. Solid solutions type-Ce_1−xZr_xO₂ along with NaCl as diluent were obtained at different milling times, with a final composition of Ce_0.8Zr_0.2O₂ after 5 h and 15 h under high and low energetic milling conditions, respectively. The NaCl formed during the mechanochemical reaction, which is eliminated by washing after calcination of the as-milled sample, allows to obtain a Ce_0.8Zr_0.2O₂ solid solution with high surface area and nanometric grains. The nanostructured Ce_0.8Zr_0.2O₂ solid solution shows good thermal stability after prolonged heating at 600 °C. However, the addition of an extra amount of diluent during mechanochemical reaction evidences a detrimental effect, avoiding the formation of solid solution or deteriorating the textural/microstructuctural characteristics of the obtained Ce_1−xZr_xO₂ solid solution. Removal of NaCl previous to calcination improves notably the textural/microstructural characteristics of the Ce_0.8Zr_0.2O₂ solid solution.     

Synthesis and characterization of pyrochlore-type solid solutions Y2−xLaxRu2O7

New pyrochlore-type solid solutions Y_2−xLa_xRu₂O₇ have been synthesized for x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 as single-phase materials. The lattice parameters for these solid solutions are linear with x and range from 1.0140 nm (x=0.0) to 1.0257 nm (x=0.6). All the samples show semiconducting behavior with an activation energy of about 0.1 eV. They show the spin-glass-like magnetic transition at low temperatures. Its transition temperature T_G decreases from 77 K for Y₂Ru₂O₇ with increasing the La³⁺ content.     

The tetragonal-orthorhombic transition and the associated changes in the properties of Pr0.5−xNdxSr0.5MnO3

The transition from the tetragonal (I4/mcm) to the orthorhombic (Imma) structure associated with a change in A- to CE-type antiferromagnetism in the Pr_0.5−xNd_xSr_0.5MnO₃ has been investigated by X-ray diffraction as well as magnetization and resistivity measurements. The transition is found to occur between x=0.1 and 0.2.     

Spin states and glassy magnetism in LaCo1−xNixO3 (0 ≤ x ≤ 0.5)

We investigated the effect of Ni substitution on electronic structure and magnetic properties of perovskite LaCoO₃ in the substitution range 0 ≤ x ≤ 0.5. A homovalent +3 state and spin state transition of Co⁺³ has been observed upon Ni substitution in x-ray absorption measurements at the Co and the Ni K-edges. Thermally driven spin state transition has been found to disappear with Ni substitution. A change in nature of magnetic interactions from antiferromagnetic to ferromagnetic and spin glass behavior with substitution is observed in dc and ac magnetization measurements. Ni substitution has been found to lower the average effective magnetic moment which has been ascribed to the decrease in Co/Ni ratio. Changes in fine structure and magnetic properties due to Ni substitution have been explained through the stabilization of intermediate spin state of Co⁺³ by the lattice expansion induced changes in crystal field. The Jahn-Teller distortion is assumed to be suppressed in the expanded lattice and possibility of antiferro-orbital ordering has been proposed for the ferromagnetic super-exchange interactions Co⁺³(IS)–O–Co⁺³(IS). The present work provides possible explanation for the ambiguity in the origin of FM in Co and Ni based perovskite cobaltites as well as supports the idea of lattice expansion induced ferromagnetism.     

Tartarate precursors to CoxZn1−xGa2O4 spinel oxides

The Co_xZn_1−xGa₂O₄ spinels (x = 0.25, 0.5, 0.75, 1) were synthesized by a wet chemistry method–the precursor route via tartarate decomposition. The complex precursors have been characterized by IR, UV–Vis, magnetic measurements and luminescence spectroscopy. XRD, SEM, IR, UV–Vis and luminescence spectroscopy were used for the structural and morphological investigation of Co_xZn_1−xGa₂O₄ spinels; magnetic susceptibilities of the spinel were also measured. The X-ray diffraction patterns confirmed the formation of gallate spinel phase, Co_xZn_1−xGa₂O₄.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.     

Induced ferromagnetic metallic state and Griffiths phase in La1/2Ca1/2Mn1−xMoxO3 compounds

We have investigated the magnetic and transport properties of the La_0.5Ca_0.5Mn_1−xMo_xO₃ (0.0 ≤ x ≤ 0.1) compounds. Partial substitution of Mn by Mo destroys the charge-ordering (CO) phase and induces the ferromagnetic (FM) metallic transition at low temperatures in the samples. The thermo-magnetic irreversibility observed in both resistivity ρ (T) and magnetization M (T) curves suggest a competing scenario due to the coexistence of different phases in the Mo doped samples. In addition, a deviation from the Curie–Weiss behavior in the inverse susceptibility far above the Curie temperature (T_C) indicates the existence of Griffiths phase (GP) in the Mo-doped compounds. The above results can be interpreted based on the induced Mn²⁺ species by Mo substitution in the La_0.5Ca_0.5Mn_1−xMo_xO₃ system.     

Specific heat of hole-doped vanadates: Y1−xCaxVO3, Pr1-xCaxVO3 and Nd1−xSrxVO3

We have investigated the specific heat of hole-doped vanadium oxides Y_1−xCa_xVO₃ (0 ≤ x ≤ 0.11), Pr_1−xCa_xVO₃ (0 ≤ x ≤ 0.3) and Nd_1−xSr_xVO₃ (0 ≤ x ≤ 0.2) probably for the first time by applying the Modified Rigid ion model (MRIM). The results obtained on temperature dependent (1 K ≤ T ≤ 300 K) specific heat are in reasonably good agreement with the experimental data. The impact of lattice distortions on the elastic and thermal properties of the present Mott insulators is portrayed by an atomistic approach. The scope of further improvement in the present model has also been discussed.     

Mössbauer and magnetic studies of Mg1+2xSbxFe2−3xO4 spinel ferrites

Spinel-related Mg_1+2xSb_xFe_2−3xO₄ samples (x = 0.0, 0.05, 0.10, 0.15, 0.20, and 0.30) prepared using the conventional double sintering technique were investigated using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra favor a cationic distribution of the form (Mg_δFe_1−δ)^A[Mg_1+2x−δSb_xFe_1+δ−3x]^BO₄ among the tetrahedral-A and octahedral-B sites of the spinel structure. The cation distribution parameter (δ) was found to vary with the Sb⁵⁺ concentration (x). The Mössbauer hyperfine magnetic fields at both sites and the Curie temperatures of the ferrites decrease as x increases. This was attributed to gradual weakening in the magnetic exchange interaction as more Fe³⁺ ions are substituted by diamagnetic Sb⁵⁺ and Mg²⁺ ones. The sample with x = 0.30 exhibits short range magnetic order due to cationic clustering and/or superparamagnetism. The magnetization of all samples was found to be temperature-dependent implying that δ depends on temperature in addition to x. At low temperatures the substituted ferrites (x ≠ 0.0) unexpectedly exhibit higher magnetization values relative to that of the pure ferrite MgFe₂O₄. This behavior, while at variance with the Néel's model for ferrimagnetism, is explicable in terms of the spin canting mechanism proposed in the Yafet–Kittel model.     

Critical behaviour and magnetocaloric effect of nano crystalline La0.67Ca0.33Mn1−xFexO3 (x = 0.05, 0.2) synthesized by nebulized spray pyrolysis

The structure, critical exponents and magnetocaloric effect (MCE) of Nebulized Spray Pyrolysis (NSP) synthesized nano crystalline La_0.67Ca_0.33Mn_1−xFe_xO₃ (x = 0.05, 0.2) were investigated. The Reitveld refinement of XRD patterns show that the samples adopt an orthorhombic structure with Pnma space group. TEM inspection reveals that the average particle size is about 15 nm and 42 nm for NSP synthesized LCMFe_0.05 and LCMFe_0.2 samples respectively. The temperature and field dependent magnetization studies reveal the superparamagnetic state of La_0.67Ca_0.33Mn_0.95Fe_0.05O₃ and spin-glass-like state of La_0.67Ca_0.33Mn_0.8Fe_0.2O₃. The critical behaviour at the transition region studied using modified Arrott plot provides a second order nature of phase transition for both samples. The magnetocaloric studies show the maximum value of magnetic entropy change (ΔS_max) is in the range 2.3 J kg⁻¹ K⁻¹ at 158 K for LCMFe_0.05 and 0.3 J kg⁻¹ K at 92 K for LCMFe_0.2 respectively at 5 T field. The field dependence of the magnetic entropy changes are also analysed, which show a power law dependence (ΔS_M ∞ Hⁿ, n = 0.72 (2)) at transition temperature, T_C = 162 K for LCMFe_0.05 and n = 1.11(3) at 92 K for LCMFe_0.2.     

La1−xSrxCuO2.5−δ as new cathode materials for intermediate temperature solid oxide fuel cells

La_1−xSr_xCuO_2.5−δ (LSCu), which exhibit excellent electrical conductivity and oxygen vacancies were investigated as potential cathode materials for solid oxide fuel cell (SOFC) applications. The structure stability, electrical conductivity, cathodic overpotential, and the reactivity with yttria-stabilized zirconia (YSZ) were examined in this study. It was found that the LSCu perovskite was obtained only when the addition of strontium fell in the range between 15 and 30%. With more than 20% of strontium addition, this material showed excellent electrical property and immunity to the reaction with YSZ at 800 °C. The conductivities of LSCu were as high as 900 S/cm at 600 °C, and 800 S/cm at 800 °C. The cathodic overpotential of this material was approximately 3.8 and 10.6 mV at a current density of 100 mA/cm² at 850 and 750 °C, respectively. These properties are superior to Sr-doped lanthanum manganite (LSM), which is the state-of-the-art cathode material of SOFCs.     

Synthesis,structure and magnetic properties of distorted YxLa1−xFeO3: Effects of mechanochemical activation and composition

The influence of mechanochemical treatment on the synthesis and properties of Y_xLa_1−xFeO₃ (0 ≤ x ≤ 1) orthoferrites is studied. Solid mixtures of the corresponding metal oxides were treated in a high-energy ball-mill. X-ray diffraction revealed that during the milling the disappearance of the reactants and a fast conversion to orthoferrite phase take place. Magnetic measurements showed a weak ferromagnetic behavior of the obtained materials, observing higher magnetization for larger x. The activated powders heated at 600 and 800 °C showed a progressive crystalline ordering together with a significant drop of magnetization. Thermal treatments at 1000 °C produced the formation of the phase Y₃Fe₅O₁₂ for the samples richer in yttrium, increasing the magnetization. Rietveld refinements of the diffraction patterns and dynamical scanning calorimetry were used respectively to determine the lattice parameters and Néel temperatures for the formed orthoferrites. The effect of the composition on the structure and magnetic behavior is discussed.     

Single domain limit for NixCo1−xFe2O4 (0 ≤ x ≤ 1) nanoparticles synthesized by coprecipitation route

Ni_xCo_1−xFe₂O₄ (0 ≤ x ≤ 1) nanoparticles (sizes: 8–52 nm) were synthesized by chemical coprecipitation route. Single domain limit (d_sdl) for the nanoparticles, determined from the coercivity (H_C) versus particle-size curves, was explored as a function of nickel concentration (x). The coercivity of the particles attains a peak value at d_sdl, and it was found that coercivity decreases linearly with increasing nickel concentration in the samples. The saturation magnetization (M_S) and blocking temperature (T_b) of the system show increasing trends with increasing cobalt concentration in the nanoparticles.     

Microwave driven hydrothermal synthesis of Ba1−xSrxTiO3 nanoparticles

Ba_1−xSr_xTiO₃ (BST) nanopowders with composition x = 0.1-0.4 have been prepared using microwave driven hydrothermal synthesis (MDHS). A low temperature process has been chosen in order to avoid high temperature heat treatment leading to particle growth and agglomeration. MDHS method allows obtaining the nanocrystalline powder samples during relatively short period of time (10 min) and therefore MDHS was optimized. In case of the phase evolution studies the XRD measurements were performed. The average size of crystallites was estimated using Scherrer equation. TEM and SEM images were taken for the detailed analysis of the grain size, surface and morphology.     

Synthesis, structural and magnetic properties of La1−xCdxFeO3 (0.0 ≤ x ≤ 0.3) orthoferrites

Nanocrystalline La_1−xCd_xFeO₃ (0.0 ≤ x ≤ 0.3) solid solutions have been synthesized by a single-step solution combustion method at a relatively low temperature of 400 °C. The combustion-synthesized solid solutions were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and magnetic measurements. The crystal structure examined by XRD indicates that the samples were single-phase, and crystallize in an orthorhombic (space group, Pbnm no. 62) structure. The parent and doped compounds showed canted antiferromagnetic behavior associated with an increase in magnetic moment with Cd doping. The changes in magnetic properties of the materials are correlated to the changes in structural features resulting from the Rietveld structural refinement of the materials.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.