Finite size effect on Gd doped CoGdxFe2−xO4 (0.0≤x≤0.5) particles

Nanoparticles of CoGd_xFe_2−xO₄ (where x=0.0, 0.1, 0.3, 0.5) series have been prepared by chemical co-precipitation. The effect of Gd³⁺ ion concentration on crystalline phase, crystallinity, crystallite size, molecular vibrations and magnetic resonance has been investigated in detail. The crystallinity decreases with an increase in Gd³⁺ ion concentration and changes the structural parameters. The spin lattice relaxation has been correlated with the doping ion concentration. Similarly, the superparamagnetic behavior of these particles has been observed with EPR spectroscopy.     

The suppression of structural phase transformation in LaVO3 and La1−xSrxVO3 thin films fabricated by pulsed laser deposition

Highly oriented (100) thin films of LaVO₃ and La_1−xSr_xVO₃ have been fabricated by pulsed laser deposition in a reducing atmosphere. The films show a transition from insulating to metallic behaviour in the composition region of x, 0.175<x<0.200. In the single crystals of the antiferromagnetic insulating phase, a first-order structural phase transition is observed few degrees below the magnetic transition, which manifests itself as a kink in the temperature dependence of resistivity. In the highly oriented thin films of LaVO₃ and La_1−xSr_xVO₃ fabricated on lattice matched substrates in this study, the structural phase transformation in the insulating phase has been suppressed. The electrical conduction is found to take place via hopping through localized states at low temperatures. The metallic compositions show a non-linear (T^1.5) behaviour in the temperature dependence of resistivity. V (2p) core level spectra of these films show a gradual change in the relative intensities of V³⁺ and V⁴⁺ ions as the value of x increases.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

Effect of Sr concentration on microstructure and magnetic properties of (Ba1−xSrx)(Ti0.3Fe0.7)O3 ceramics

Fe-doped (Ba_1−xSr_x)TiO₃ ceramics were prepared by solid-state reaction, and ferromagnetism was realized at room temperature. The microstructure and magnetism were modified by the Sr concentration control (0≤x≤75 at%) at a fixed Fe concentration, and the relevant magnetic exchange mechanism was discussed. All the samples are shown to have a single perovskite structure. When increasing the Sr concentration, the phase structure is transformed from a hexagonal perovskite into a cubic perovskite, with a monotonic decrease in lattice parameters induced by ionic size effect. The room-temperature ferromagnetism is expected to originate from the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti sites mediated by the O²⁻ ions. The increase in Sr addition modifies two main influencing factors in magnetic properties: the ratio of pentahedral to octahedral Fe³⁺ and the concentration of oxygen vacancies, leading to a gradually enhanced saturation magnetization. The highest value, obtained for Fe-doped (Ba_0.25Sr_0.75)TiO₃, is an order of magnitude higher than that of the Fe-doped BaTiO₃ system with similar Fe concentration and preparation conditions, which may indicate (Ba_1−xSr_x)TiO₃ as a more suitable matrix material for multiferroic research.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Synthesis and ferroelectric properties of bismuth layer-structured (Bi7−xSrx)(Fe3−xTi3+x)O21 solid solutions

Bismuth layer-structured (Bi_7−xSr_x)(Fe_3−xTi_3+x)O₂₁ (BSFT) ceramics were synthesized and the ferroelectric properties and crystal structure were investigated. X-ray powder diffraction profiles and refinement of the lattice parameters indicated single phase BSFT was obtained in the composition range 0-1.5. The lattice parameter b of BSFT remained almost constant, while a slight decrease in the lattice parameter a was observed by the Sr and Ti substitution for Bi and Fe, respectively, which indicated an increase in the orthorhombicity. The dependence of the BSFT lattice parameter on temperature implied a phase transition from the orthorhombic to the tetragonal phase, which was in good agreement with the Curie temperature. The remnant polarization P_r, of BSFT was significantly improved by the Sr and Ti substitution for Bi and Fe, and ranged from 9 to 16 μC/cm², although no remarkable variation in the coercive field E_c was observed. As a result, a well-saturated P-E hysteresis loop of BSFT ceramic was obtained at x=0.5 with a P_r of 30 μC/cm at an applied voltage of 280 kV/cm.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Formation of room-temperature ferromagnetic Zn1−xCoxO nanocrystals

Optical and magnetic properties of Co²⁺-doped ZnO nanocrystals were studied. Optical measurements confirm the incorporation of Co²⁺ in ZnO lattice with tetrahedral geometry. Optical absorption spectra also reveal the partial bleaching of the excitonic feature attributable to an increase in electron concentration. Magnetization measurements indicate the ferromagnetic ordering in Co²⁺-doped ZnO nanocrystals with saturation magnetization . No structural changes were observed in lightly doped ZnO nanocrystals. The present investigations are important in obtaining the ferromagnetic Zn_1−xCo_xO nanocrystals.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Structural, magnetic and magnetotransport behavior of La0.7SrxCa0.3−xMnO3 compounds

A systematic investigation of the structural, magnetic and electrical properties of a series of nanocrystalline La_0.7Sr_xCa_0.3−xMnO₃ materials, prepared by high energy ball milling method and then annealed at 900 °C has been undertaken. The analysis of the XRD data using the Win-metric software shows an increase in the unit cell volume with increasing Sr ion concentration. The La_0.7Sr_xCa_0.3−xMnO₃ compounds undergo a structural orthorhombic-to-monoclinic transition at x=0.15. Electric and magnetic measurements show that both the Curie temperature and the insulator-to-metal transition temperature increase from 259 K and 253 K correspondingly for La_0.7Ca_0.3MnO₃ (x=0) to 353 K and 282 K, respectively, for La_0.7Sr_0.3MnO₃ (x=0.3). It is argued that the larger radius of Sr²⁺ ion than that of Ca²⁺ is the reason to strengthen the double-exchange interaction and to give rise to the observed increase of transition temperatures. Using the phenomenological equation for conductivity under a percolation approach, which depends on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions, we fitted the resistivity versus temperature data measured in the range of 50-320 K and found that the activation barrier decreased with the raising Sr²⁺ ion concentration.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Superconductivity and magnetism of La2Cu1−xZnxO4+δ chemically oxidized by NaClO

The structural, superconducting and magnetic properties of La₂Cu_1−xZn_xO_4+δ (0≤x≤0.1) chemically oxidized by NaClO at room temperature were studied. All the samples before and after oxidation are single phase with orthorhombic structure, as indicated by their powder X-ray diffraction analysis. The iodometric titration results indicate that Zn-substituted La₂Cu_1−xZn_xO₄ is more favorable for the insertion of the excess oxygen, as compared to the Zn-free La₂CuO₄. The T_c suppression rate resulting from Zn substitution in La₂Cu_1−xZn_xO_4+δ is −12.4 K/%. The effective magnetic moment induced by the non-magnetic Zn ion is the order of one Bohr magneton, which decreases with increasing the Zn concentrations in the range examined. The latter two results are qualitatively well consistent with those obtained in La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping. This reveals that the non-magnetic Zn ions play the same role in both of the La₂Cu_1−xZn_xO_4+δ with the excess oxygen content of about 0.1 and the La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping.     

Synthesis and physical properties of negative thermal expansion materials Zr1−xMxW2O8−y (M=Sc, In, Y) substituted for Zr(IV) sites by M(III) ions

Zr_1−xM_xW₂O_8−y (M=Sc, In and Y) solid solutions substituted up to x=0.04 for Zr(IV) sites by M(III) ions were synthesized by a solid-state reaction. X-ray diffraction experiments from 90 to 560 K revealed that all solid solutions had a cubic crystal structure and showed negative thermal expansion coefficients. The lattice parameters of Zr_1−xM_xW₂O_8−y were smaller than that of ZrW₂O₈ probably due to oxygen defects, though the ionic radii of substituted M³⁺ ions were larger than that of Zr⁴⁺. Order-disorder phase transition temperatures of the substituted samples drastically decreased in the order of Y, In and Sc compared to the percolation theory, and decreased with increasing M content.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Simulation of Al/Fe disorder in Ca2FexAl2−xO5

A method is described which is capable of simulating disorder across a range of composition. It is used to study the disorder of iron and aluminium on the octahedral and tetrahedral cation sites in the Brownmillerite phase Ca₂Fe_xAl_2−xO₅. The arrangements with the lowest lattice energies are those that maximize the number of Fe³⁺ on octahedral sites throughout the composition range. An exchange of one Fe³⁺ with one Al³⁺, which results in a decrease in the number of iron ions on octahedral sites, increases the lattice energy by an amount that is dependent on x but is independent of the number of exchanges. The exchange of trivalent cations also results in an expansion in the b lattice direction, accompanied by a decrease in the a and c directions across the full range of composition.     

Structural and photoluminescence properties of thermally evaporated Cd1−xMnxS nano-crystalline films

Thin films of Cd_1−xMn_xS (0≤x≤0.5) were formed on glass substrates by resistive vacuum thermal evaporation. All the films were deposited at 300 K and the films were annealed at 373, 473 and 573 K for 1 h in a vacuum of 10⁻⁶ mbar. Atomic force microscopy (AFM) studies showed that all the films investigated were in nano-crystalline form with a grain size in the range 36-82 nm. All the films exhibited a wurtzite structure of the host material. The lattice parameters varied linearly with composition following Vegard’s law in the entire composition range. Photoluminescence studies showed that two distinct emission bands were observed for each Cd_1−xMn_xS compound. One corresponds to internal transition and the other one is due to the transition of Mn²⁺ ions in interstitial sites or in small ‘Mn’ chalcogenic clusters.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Crystal structure and photoluminescence of (Y1−xCex)2Si3O3N4

Oxonitridosilicate phosphors with compositions of (Y_1−xCe_x)₂Si₃O₃N₄ (x=0−0.2) have been synthesized by solid state reaction method. The structures and photoluminescence properties have been investigated. Ce³⁺ ions have substituted for Y³⁺ ions in the lattice. The emission and excitation spectra of these phosphors show the characteristic photoluminescence spectra of Ce³⁺ ions. Based on the analyses of the diffuse reflection spectra and the PL spectra, a systematic energy diagram of Ce³⁺ ion in the forbidden band of sample with x=0.02 is given. The best doping Ce content in these phosphors is ∼2 mol%. The quenching temperature is ∼405 K for the 2 mol% Ce content sample. The luminescence decay properties were investigated. The primary studies indicate that these phosphors are potential candidates for application in three-phosphor-converted white LEDs.     

P and Al NMR study of Al1−xGaxPO4

Mixed phosphates Al_1−xGa_xPO₄ (0.0≤x≤1.0), with orthorhombic structure, have been prepared by co-precipitation method followed by annealing at high temperature (T≈1350 °C) and investigated by using powder X-ray diffraction and NMR spectroscopy. A systematic decrease observed for the unit cell parameters with increasing Ga³⁺ contents, suggests the formation of a solid solution between AlPO₄ and GaPO₄. ³¹P MAS NMR spectra of both AlPO₄ and GaPO₄ show a single peak at δ=−24.6 and −8.5 ppm, respectively. For mixed phosphates, multi-component ³¹P NMR spectra characterized by varying values of chemical shifts, have been observed due to the existence of different types of ³¹P structural configurations formed by varying number of Al³⁺ and Ga³⁺ as its next nearest neighbours. The intensity analysis of these component peaks suggested a perfectly random distribution of Al³⁺ and Ga³⁺ cations in these mixed phosphates. Chemical shift anisotropy parameters for the different structural configurations of ³¹P have been evaluated from the intensity analysis of the spinning side bands corresponding to different isotropic peaks. ²⁷Al NMR spectra of mixed phosphate samples exhibited a slightly asymmetric broad peak at δ≈42.0 ppm, which is characteristic of the tetrahedral configuration of Al³⁺ ions with significant covalent bonding between the metal and oxygen ions.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.