Changes of the magnetic interactions in TlCo2Se2 upon metal substitution

Various solid solutions TlCo_2−xMe_xSe₂ (Me=Fe, Ni and Cu) have been investigated by neutron powder diffraction, supplemented by magnetometry. The incommensurate spin-helix running along the c-axis in tetragonal TlCo₂Se₂ prevails for low concentrations of copper and iron but changes pitch. In the copper case, only cobalt carries a magnetic moment. On nickel substitution, however, collinear antiferromagnetic coupling between the ferromagnetic layers occurs. The magnetic moment distribution between the two transition metals in the solid solution TlCo_2−xNi_xSe₂ was tentatively probed with first principle calculations on fictive ordered TlCoNiSe₂, modelled by two types of superstructures. Also the ternary mother compounds, Pauli paramagnetic TlNi₂Se₂ and antiferromagnetic TlCo₂Se₂, were investigated with the same LMTO method.     

Short-range order analysis and some physical properties of InxSe1−x glasses

Bulk In_xSe_1−x (with x=5-25 at%) glasses were prepared using the melt-quench technique. Short range order(SRO) was examined by the X-ray diffraction using Cu(k_α) radiation in the wave vector interval 0.28≤k≤6.5 A⁰⁻¹.The SRO parameters have been obtained from the radial distribution function. The inter-atomic distance obtained from the first and second peak are r₁=0.263 and r₂=0.460 nm, which is equivalent In-Se and Se-Se bond length. The fundamental structural unit for the studied glasses is In₂Se₃ pyramid. Using the differential scanning calorimetry (DSC), the crystallization mechanism of In_xSe_1−x chalcogenide glass has been studied. The glass transition activation energy (E_g) is 289±0.3 kj/mol.There is a correlation amongst the glass forming ability, bond strength and the number of lone pair electrons. The utility of the Gibbs-Di Marzio relation was achieved by estimating T_g theoretically.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.     

Structural and electrical characterization of SxSe100−x thin films

Thin films of samples of the glassy S_xSe_100−x system with 0 ≤ x ≤ 7.28 have been prepared by thermal evaporation technique at room temperature (300 K). X-ray investigations show that the structure of pure selenium (Se) does change seriously by the addition of small amount of sulphur S ≤7.28%. The lattice parameters were determined as a function of sulphur content. Results of differential thermal analysis (DTA) of the glassy compositions of the system S_xSe_100−x were discussed. The characteristic temperatures (T_g, T_c and T_m) were evaluated. Dark electrical resistivities, ρ, of S_xSe_100−x thin films with different thicknesses from 100 to 500 nm, were measured in the temperature range from 300 to 423 K. Two distinct linear parts with different activation energies were observed. The variation of electrical resistivity of examined compositions has been discussed as a function of the film thickness, temperature and the sulphur content. The application of Mott model for the phonon assisted hopping of small polarons gave the same two activation energies obtained from the resistivity temperature calculations.     

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.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.     

Structure and magnetic properties of Sn1−xMnxO2

The microstructure and magnetic properties have been investigated systematically for Sn_1−xMn_xO₂ polycrystalline powder samples with x=0.02-0.08 synthesized by a solid-state reaction method. X-ray diffraction revealed that all samples are pure rutile-type tetragonal phase and the cell parameters a and c decrease monotonously with the increase in Mn content, which indicated that Mn ions substitute into the lattice of SnO₂. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism. Furthermore, magnetic investigations demonstrate that magnetic properties strongly depend on doping content, x. The average magnetic moment per Mn atom decreases with increase in the Mn content, because antiferromagnetic super-exchange interaction takes place within the neighbor Mn³⁺ ions through O²⁻ ions for the samples with higher Mn doping. Our results indicate that the ferromagnetic property is intrinsic to the SnO₂ system and is not a result of any secondary magnetic phase or cluster formation.     

Magnetic properties of the Lu2Fe17−xMnx single crystals

Magnetic properties of the single-crystalline Lu₂Fe_17−xMn_x compounds, in which x=0, 0.5, and 2, with the Th₂Ni₁₇-type crystal structure are reported. The Lu₂Fe_17−xMn_x compounds with x=0 and 0.5 are ferromagnets at low temperatures and antiferromagnets at high temperatures. The compound with x=2 is always a ferromagnet. The easy-plane magnetic anisotropy in the Lu₂Fe_17−xMn_x ferromagnets drastically weakens with increase in Mn content up to x=2. The temperature dependence of the first magnetic anisotropy constant was obtained and compared with the single-ion model prediction.     

High magnetostriction at low fields of (Tb1−xDyx)0.2Pr0.8(Fe0.4Co0.6)1.88C0.05 intermetallic compounds

The structure and magnetostriction of the (Tb_1−xDy_x)_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 intermetallic compounds (0≤x≤1) were studied by X-ray diffraction and magnetic measurements. The formation of an approximate single Laves phase with a MgCu2-type cubic structure was observed in this series of compounds. It was found that the Curie temperature and the saturation magnetization of the compounds would decrease with increase in the Dy content up to x=1. The magnetostriction λ_a (λ_a=λ_‖-λ_⊥) gently rises when x≤0.6, and follows with a precipitous fall when x exceeds 0.6, with the highest value of λ_a being reached in the compounds with x=0.6. The magnetostriction of all the samples was observed to approach their own saturation in the magnetic fields higher than 4 kOe. This indicates that the addition of a small amount of Dy could effectively improve the low-field magnetostriction of the Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.88C_0.05 compounds, which could become a kind of promising magnetostrictive material.     

The structure and luminescence properties of long afterglow phosphor Y3−xMnxAl5−xSixO12

A series of phosphors with the composition Y_3−xMn_xAl_5−xSi_xO₁₂ (x=0, 0.025, 0.050, 0.075, 0.150, 0.225, 0.300) were prepared with solid state reactions. The X-ray powder diffraction analysis of samples shows that the substitution of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance decrease to a certain extent. The emission spectra show that Mn²⁺ in Y₃Al₅O₁₂ emits yellow-orange light in a broad band. With the increment of substitution content, the emission intensity of the phosphors increases firstly then decreases subsequently, and the emission peak moves to longer wavelength. Afterglow spectra and decay curves show that all the Mn²⁺ and Si⁴⁺ co-doped samples emit yellow-orange light with long afterglow after the irradiation of ultraviolet light. The longest afterglow time is 18 min. Thermoluminescence measurement shows that there exist two kinds of traps with different depth of energy level and their depth decreases with the increment of substitution content.     

Structure and electrical properties of Bi5GexSe95−x thin films

Bi₅Ge_xSe_95−x (30, 35, 40 and 45 at.%) thin films of thickness 200 nm were prepared on glass substrates by the thermal evaporation technique. The influence of composition and annealing temperature, on the structural and electrical properties of Bi₅Ge_xSe_95−x films was investigated systematically using X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX). The XRD patterns showed that the as-prepared films were amorphous in nature with few tiny crystalline peaks of relatively low intensity for 30 and 45 at.% and the Bi₅Ge₄₀Se₅₅ annealed film was polycrystalline. The chemical composition of the Bi₅Ge₃₀Se₆₅ film has been checked using energy dispersive X-ray spectroscopy (EDX). The electrical conductivity was measured in the temperature range 300-430 K for the studied compositions. The effect of composition on the activation energy (ΔE) and the density of localized states at the Fermi level N(E_F) were studied, moreover the electrical conductivity was found to increase with increasing the annealing temperature and the activation energy was found to decrease with increasing the annealing temperature. The results were discussed on the basis of amorphous-crystalline transformations.     

The structure and luminescence properties of a novel orange emitting phosphor Y3MnxAl5−2xSixO12

A series of phosphors with the composition Y₃Mn_xAl_5−2xSi_xO₁₂ (x=0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 0.6) was prepared through solid state reactions. X-ray powder diffraction analysis of samples shows that when co-doping content does not exceed 16% of Al³⁺, equimolar co-doping of Mn²⁺ and Si⁴⁺ does not change the garnet structure of phosphors, but makes the interplanar distance to decrease a certain extent. However, if the co-doping content exceeds 16%, new phases will form in the samples. The excitation and emission spectra of samples show that Mn²⁺ in Y₃Mn_xAl_5−2xSi_xO₁₂ emits broadband orange light (peak wavelength varies from 586 to 593 nm). With an increment in co-doping content, the emission intensity of the phosphors increases when the value of x is lower than 0.1 while it decreases when it is higher than 0.1 and the emission peak moves to a longer wavelength.     

Study of the quasi two-dimensional CoxNi1-xTa2O6 compounds by X-ray diffraction and magnetic susceptibility measurements

We report results on the structural and magnetic properties of the Co_xNi_1−xTa₂O₆ series of compounds by X-ray powder diffraction, magnetic susceptibility and magnetization measurements. X-ray refinements carried out by the Rietveld method show that these compounds crystallize in a P4₂/mnm tetragonal structure. Magnetic susceptibility curves show a broadened maximum witnessing that these compounds exhibit two-dimensional antiferromagnetic behaviors. All the Co_xNi_1−xTa₂O₆ compounds order below 10 K and present a large ion anisotropy. The magnetic properties have been determined in both the paramagnetic and antiferromagnetic state. In the hypothesis of two dimensional AF ordering, the near neighbor exchange constants (J₁) and the next near neighbor exchange constants for two different paths (J₂ and J'₂) were determined. The composition dependence of the magnetic properties including ordering temperature, exchange constants and anisotropy factors are discussed. The drastic reduction of the ordering temperature for x=0.20 for Co_xNi_1−xTa₂O₆, suggest the hypothesis of a peculiar magnetic behavior for this composition.     

Luminescent properties of HTP AgGd1−xW2O8:Eux and AgGd1−x(W1−yMoy)2O8:Eux phosphor for white LED

Intense red phosphors, AgGd_1−xEu_x(W_1−yMo_y)₂O₈ (x=0.0-1.0, y=0.0-1.0), have been synthesized through traditional solid-state reaction and characterized by X-ray diffraction (XRD) and photoluminescence (PL). XRD results reveal that AgGd_1−xEu_xW₂O₈ synthesized at 1000 °C has a tetragonal crystal structure, which is named as high temperature phase (HTP) AgGdW₂O₈. All phosphors compositions with Eu³⁺ show red and green emission on excitation either in the charge-transfer or Eu³⁺ levels. Analysis of the emission spectra with different Eu³⁺ concentrations reveal that the optimum dopant concentration for Eu³⁺ is x=0.6 in the HTP AgGd_1−xEu_xW₂O₈ (x=0.0-1.0). Studies on the AgGd_0.4Eu_0.6(W_1−yMo_y)₂O₈ (y=0.0-1.0) and AgGd_1−xEu_x(W_0.7Mo_0.3)₂O₈ (x=0.0-1.0) show that the emission intensity is maximum for compositions with y=0.3 and x=0.5, respectively, and a decrease in emission intensity is observed for higher y or x values. The Mo⁶⁺ and Eu³⁺ co-doped AgGd(WO₄)₂ phosphors show higher emission intensity in comparison with the singly Eu³⁺-doped AgGd(WO₄)₂ in UV region. The intense emission of the tungstate/molybdate phosphors under 394 and 465 nm excitations, respectively, suggests that these materials are promising candidates as red-emitting phosphors for near-UV/blue GaN-based white LED for white light generation.     

The magnetic properties of oxide spinel Li0.5Fe2.5−2xAlxCrxO4 solid solutions

The exchange interactions (J_BB and J_AB are the intra and the inter-sublattice exchange interactions between neighbouring spins, respectively) are obtained by using the general expressions of canting angle and critical temperature obtained by mean field theory of Li_0.5Fe_2.5−2xAl_xCr_xO₄. The expression of magnetic energy of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is obtained for different spin configurations and dilution x. The saturation magnetisation of Li_0.5Fe_2.5-2xAl_xCr_xO₄ is obtained with different values of dilution x. The magnetic phase diagram of Li_0.5Fe_2.5-2xAl_xCr_xO₄ materials is obtained by high temperature series expansions (HTSEs). The critical exponent associated with the magnetic susceptibility of Li_0.5Fe_2.5−2xAl_xCr_xO₄ is deduced.     

Study of polycrystalline bulk CuIn1-xGaxTe2

Polycrystalline CuIn_1−xGa_xTe₂ bulk films were synthesized by reacting, in stoichiometric proportions, high purity Cu, In, Ga and Te in a vacuum sealed quartz ampoule. The phase structure and composition of the bulk films were analysed by X-ray diffraction and energy-dispersive X-ray analysis, respectively. The bulk samples, of p-type conductivity, are found to be near-stoichiometric, polycrystalline, with tetragonal chalcopyrite structure, predominantly oriented along a direction perpendicular to the (1 1 2) plane. Photoluminescence spectra were recorded at 7 K and 700 mW to characterize the defects and the structural quality. The main peak as a function of composition has been studied.     

Sputter deposition of high transparent TiO2−xNx/TiO2/ZnO layers on glass for development of photocatalytic self-cleaning application

In this study, TiO_2−xN_x/TiO₂ double layers thin film was deposited on ZnO (80 nm thickness)/soda-lime glass substrate by a dc reactive magnetron sputtering. The TiO₂ film was deposited under different total gas pressures of 1 Pa, 2 Pa, and 4 Pa with constant oxygen flow rate of 0.8 sccm. Then, the deposition was continued with various nitrogen flow rates of 0.4, 0.8, and 1.2 sccm in constant total gas pressure of 4 Pa. Post annealing was performed on as-deposited films at various annealing temperatures of 400, 500, and 600 °C in air atmosphere to achieve films crystallinity. The structure and morphology of deposited films were evaluated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM). The chemical composition of top layer doped by nitrogen was evaluated by X-ray photoelectron spectroscopy (XPS). Photocatalytic activity of samples was measured by degradation of Methylene Blue (MB) dye. The optical transmittance of the multilayer film was also measured using ultraviolet-visible light (UV-vis) spectrophotometer. The results showed that by nitrogen doping of a fraction (∼1/5) of TiO₂ film thickness, the optical transmittance of TiO_2−xN_x/TiO₂ film was compared with TiO₂ thin film. Deposited films showed also good photocatalytic and hydrophilicity activity at visible light.     

Structure and magnetostriction of Tb0.2Pr0.8(Fe0.4Co0.6)1.93−xCx intermetallic compounds

The structure and magnetostriction of Tb_0.2Pr_0.8(Fe_0.4Co_0.6)_1.93−xC_x intermetallic compounds were studied by X-ray diffraction and magnetic measurements. Almost a single cubic Laves phase forms in the alloys for x ≤0.20, and a small amount of C can inhibit the formation of the 1:3 phase. The lattice parameter increases when 0≤x≤0.15, while the T_c and the spontaneous magnetization decreases with increasing x. The lattice parameter decreases slowly when 0.15≤x≤0.30, while the T_c decreases evidently with increasing x. The magnetostriction λ_a (=λ_∥-λ_⊥) is improved at low magnetic fields at room temperature for the compounds with 0.05≤x≤0.10, indicating that these C-containing compounds are promising magnetostrictive materials.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.     

Characterization and properties of ZnO1−xSx alloy films fabricated by radio-frequency magnetron sputtering

A series of ZnO_1−xS_x alloy films (0 ≤ x ≤ 1) were grown on quartz substrates by radio-frequency (rf) magnetron sputtering of ZnS ceramic target, using oxygen and argon as working gas. X-ray diffraction measurement shows that the ZnO_1−xS_x films have wurtzite structure with (0 0 2) preferential orientation in O-rich side (0 ≤ x ≤ 0.23) and zinc blende structure with (1 1 1) preferential orientation in S-rich side (0.77 ≤ x ≤ 1). However, when the S content is in the range of 0.23 < x < 0.77, the ZnO_1−xS_x film consists of two phases of wurtzite and zinc blende or amorphous ZnO_1−xS_x phase. The band gap energy of the films shows non-linear dependence on the S content, with an optical bowing parameter of about 2.9 eV. The photoluminescence (PL) measurement reveals that the PL spectrum of the wurtzite ZnO_1−xS_x is dominated by visible band and its PL intensity and intensity ratio of UV to visible band decrease greatly compared with undoped ZnO. All as-grown ZnO_1−xS_x films behave insulating, but show n-type conductivity for w-ZnO_1−xS_x and maintain insulating properties for β-ZnO_1−xS_x after annealed. Mechanisms of effects of S on optical and electrical properties of the ZnO_1−xS_x alloy are discussed in the present work.