Structural,optical and magnetic study of (1−x)ZnO–xMgO composites prepared through solid state reaction method

We report the study of structural, optical and magnetic properties of (1−x)ZnO–xMgO (x=0.35, 0.40, 0.45 and 0.50) composites prepared by solid state reaction method. X-ray diffraction pattern confirms the presence of both the phases associated with ZnO (hexagonal) and MgO (cubic), which is revealed through the existence of (1 1 1) and (2 0 0) peaks in addition to ZnO peaks. The lattice parameter c as calculated using X-ray analysis undergoes shrinkage with increasing content of MgO. Microstructural analysis suggests that there is no variation in spherical elongated shape of grains with increasing concentration of MgO, where the average grain size is found to be ∼600 nm. The band gap as calculated from optical absorption spectra obtained by diffuse reflectance method recorded at room temperature is tuned from 3.16 to 3.55 eV. Photoluminescence spectra consist of near band edge UV emission (389 nm) and defect level emission (503 nm). The increase of MgO concentration leads to blue shift of UV emission peaks. The magnetic measurements conducted using SQUID at 5 K temperature reveals ferromagnetism along with paramagnetic and superparamagnetic components. Saturation magnetisation (M_s) is observed to be enhanced with MgO doping.     

Low dielectric loss Ba0.6Sr0.4TiO3/MgTiO3 composite thin films prepared by a sol–gel process

The dielectric properties of Ba_0.6Sr_0.4TiO₃ (BST)/MgTiO₃ (MT) composite thin films deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by the sol–gel method were investigated. The X-ray pattern analysis indicates that the thin films exhibit good crystalline quality with perovskite phase and that insertion of MT layer does not obviously affect the phase structure of BST thin films. The characterization of dielectric properties demonstrates that configuration of BST/MT/BST thin films is an effective approach to obtain low dielectric loss and dielectric tunability of BST thin films. At room temperature, the tunability of pure BST60 films and BST/MT (15 nm)/BST composite thin films is 47% and 36%, respectively, at the frequency of 1 MHz with an applied electric field of 400 kV/cm. For BST/MT/BST composite thin films, considerable reduction in the dielectric loss values is observed, which renders them attractive for tunable microwave device applications.     

Effects of Zn substitution on the magnetic and transport properties of La0.6Sr0.4Mn1−yZnyO3−δ (0≤y≤0.3)

Phase-singular solid solutions of La_0.6Sr_0.4Mn_1−yZn_yO_3−δ(0≤y≤0.3) [LSMZO] perovskite of rhombohedral symmetry (space group: ) with y up to 30 at.% could be synthesized notwithstanding the differences in ionic radii of Mn_{V I}³⁺ (i.r.=0.645 Å) and Zn_{V I}²⁺ (i.r.=0.74 Å). The LSMZO≤02 compositions are ferromagnetic metallic (FMM) at room temperature whereas LSMZO-02-08 are ferromagnetic insulators (FMI) and LSMZO>08 are paramagnetic insulators (PMI). Total obliteration of the FM transition is unique to Zn-doping at leading to PMI even at low temperatures, measured up to 8 K (presently). The FM to PM transition (T_c) and the peak (T_p) in resistivity-temperature curves decreases with the Zn-content. The charge-transport in p-type LSMZO is predictable by variable range hopping (VRH), which changes to nearest-neighbor hopping of small polarons (NNHP) at T>T_p. Non-stoichiometry (0.005≤δ≤0.21) evaluated chemically from redox titrations indicated the prevalence of excess oxygen vacancy rather than charge compensatively predictable values which, in turn, indicates the diminishing Mn⁴⁺ content in LSMZO. The ’s act as electron donors in p-LSMZO and this increases the resistivity (ρ_RT) associated with the shift in T_c to low temperatures. Increased ρ_RT on annealing in low is a clear evidence on the role of in LSMZO.     

Magnetic properties,microstructure, and phase evolution of PrxFebal.TiyB20−x (x=4–9; y=2.5–5) nanocomposites

Magnetic properties, microstructure, and phase evolution of Pr lean and boron-enriched Pr_xFe_bal.Ti_yB_20−x (x=4–9; y=2.5–5) melt-spinning ribbons with nanostructures have been investigated. Based on thermal magnetic analysis (TMA), for y=2.5, two phases, namely Pr₂Fe₁₄B and α-Fe, were found for ribbons with x=9, while additional two metastable phases, Pr₂Fe₂₃B₃ and Fe₃B, existed for x=4, 7 and 8. With the decrease of Pr content, the remanence increases but coercivity decreases. The optimal properties of B_r=9.5 kG, _iH_c=10.7 kOe, and (BH)_max=17.8 MG Oe are achieved in Pr₉Fe_bal.Ti_2.5B₁₁ nanocomposites. On the other hand, higher Ti substitution for Fe in Pr₇Fe_bal.Ti_yB₁₃ ribbons could refine the grain size and suppress the metastable Pr₂Fe₂₃B₃ and Fe₃B phases effectively. The excellent permanent magnetic properties are mainly dominated by the nanoscaled microstructures and the coexistence of sufficient magnetically soft phases, Fe₃B, Pr₂Fe₂₃B₃ and α-Fe, with magnetically hard Pr₂Fe₁₄B phase.     

Low-temperature synthesis of nanocrystalline Ca1−xHoxMnO3−δ (0?x?0.3) powders

Nanocrystalline Ca_1−xHo_xMnO_3−δ (0?x?0.3) manganites were synthesized as phase-pure by a simple and instantaneous solution autogel combustion method, which is a low temperature initiated synthetic route to obtain fine grain size. All the samples, heated at 800 °C for 18 h, can be produced as phase-pure; the polycrystalline powders are homogeneous and possess ultrafine particle size. The holmium-doped calcium manganites retain the orthorhombic phase of the undoped sample. The scanning electron microscope (SEM) images revealed that the combustion-derived compounds exhibit particle size that decreases with holmium content from 300 to 80 nm. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹. The high temperature dependence of resistivity was measured using a standard four-probe method in the range 25-600 °C. All the samples exhibit semiconductor behaviour and holmium induces a marked decrease in the electrical resistivity when compared with the parent CaMnO₃. The results can be well attributed to the Mn⁴⁺/Mn³⁺ ratio and to the particle grain size.     

Structure and magnetic properties of magnetostrictive compounds Tb0.36Dy0.64(Fe0.85Co0.15)2−xBx (0⩽x⩽0.15)

Crystal structure and magnetic properties of magnetostrictive compounds Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x (0?x?0.15) have been investigated at room temperature. The matrix of these compounds keeps a cubic MgCu₂-type structure. Lattice parameter a of the Laves phase decreases to reach a minimum at x=0.10, then increases with increasing boron content. Through analyzing the Mössbauer spectra, the easy magnetization direction (EMD) for all samples is confirmed to lie along 〈111〉 direction at room temperature, suggesting the presence of the giant magnetostriction. The mean hyperfine field H_hf and the deduced iron moment μ_Fe increase with increasing boron content, resulting in the enhancement of both Curie temperature T_C and spin reorientation temperature T_r. Although the addition of B enlarges the magnetocrystalline anisotropy constant K₁, the composition dependence of the ratio λ/K₁ for Tb_0.36Dy_0.64(Fe_0.85Co_0.15)_2−xB_x, however, reaches a maximum value at x=0. 05 under high magnetic fields.     

Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Electrical and switching properties of the Se90Te10−xAgx (0?x?6) films

Amorphous Se₉₀Te_10−xAg_x (0?x?6) films are obtained by thermal evaporation technique under vacuum from the synthesized bulk materials on pyrographite and glass substrates. X-ray analysis shows the amorphous nature of the obtained films. The dc electrical conductivity was studied for different thicknesses (165-711 nm) as a function of temperature in the range (298-323 K) below the corresponding T_g for the studied films. The obtained results show that the conduction activation energy has a single value through the investigated range of temperature which can be explained in accordance with Mott and Davis model. The I-V characteristic curves for the film compositions are found to be typical for a memory switch. The mean value of the threshold voltage increases linearly with increasing film thickness (165-711 nm), while it decreases exponentially with increasing temperature in the investigated range for the studied compositions. The results are explained in accordance with the electrothermal model for the switching process. The effect of Ag on the studied parameters is also investigated.     

Influence of Nb substitution for Fe on the magnetic and magneto-impedance properties of amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0–7) ribbons

We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe_76.5−xSi_13.5B₉Cu₁Nb_x (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (M_s) and decreases the coercivity (H_c) until x=3, for which the largest M_s∼152 emu/g and the smallest H_c∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (M_s∼156 emu/g and H_c∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.     

Structural and magnetic properties of Zn1−xCoxO (0<x⩽0.30) nanoparticles

We present a systematic investigation on the structural and magnetic properties of Zn_1−xCo_xO nanoparticles synthesized by an auto-combustion method. The single-phase Zn_1−xCo_xO crystallize in the wurtzite-type structure with a homogeneity range as large as x≈0.30, which enables the observation of some anomalies. The lattice parameter a and the unit cell volume V increase with the Co content, and anomalies are discernable around x=0.15 on the a−x and V−x curves. The magnetization data show no evidence of ferromagnetic (FM) ordering in our samples down to T=5 K, and the magnetization at 5 K and 5 T exhibits a maximum around x=0.125. Based on the detailed analysis of the magnetization data and the donor impurity band exchange model, the anomalies on composition dependence of both the lattice parameters and magnetization can be associated with an occurrence of cation percolation around the threshold x_p (≈1.5/Z=0.125 for three-dimensional lattice with coordination number Z=12). Within the framework of the donor impurity band exchange model, the absence of FM in the well-characterized Zn_1−xCo_xO can be attributed to insufficient donor electron concentration.     

Heterogeneous magnetic and electronic states in Ca1−xLaxMnO3−δ (x⩽0.12) single crystals with electron doping

The magnetic, transport, and optical properties of electron-doped Ca_1−xLa_xMnO_3−δ single crystals with x  ?0.12 were studied. The magnetic measurements show that in single crystals with x=0$x=0$ and 0.05 the G-type AFM phase with weak FM component is realized and in crystals with x=0.10$x=0.10$ and 0.12 the G- and C-type AFM phases coexist. The C-type magnetic structure arises at less concentration of La than in polycrystalline samples as a result of oxygen vacancies being additional source of electrons. Under magnetic transitions in the G- and C-type phases, resistivity and magnetoresistance of the doped single crystals have anomalies. Optical absorption in IR range indicates formation of a charge gap in crystals with x=0.10$x=0.10$ and 0.12 at appearance of the C-AFM and monoclinic phase with orbital/charge ordering. By comparing optical and transport properties, heterogeneous electronic state and its relation with heterogeneous magnetic state are shown.     

Performance and stability of niobium-substituted Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes

The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1 − xNb_xO_3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb⁵⁺ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800-900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO₂-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.     

Magnetostriction of TbxHo0.75−xPr0.25(Fe0.9B0.1)2 (x=0–0.3) compounds

The Tb_xHo_0.75−xPr_0.25(Fe_0.9B_0.1)₂ (x=0, 0.1, 0.15, 0.2, 0.25, and 0.3) compounds are found to stabilize in a cubic Laves phase structure. The lattice parameter, magnetostriction (at 10 kOe), and Curie temperature are found to increase with increasing Tb content. The compound with x=0.15 exhibits a possible anisotropy compensation between the Tb and (Ho/Pr) sublattices. The easy magnetization direction rotates towards the 〈1 1 1〉 from the 〈1 0 0〉 direction, with increasing Tb content. The splitting of the (4 4 0) peak accompanied by the spontaneous magnetostriction-induced rhombohedral distortion is observed for compounds with x?0.15 and the spontaneous magnetostriction (λ_1 1 1) is found to increase with Tb content.     

Glass transition behavior of binary GaxSe100−x (0?x?10) glass systems

The glass transition behavior of glassy Ga_xSe_100−x (x=0, 2.5, 5, 7.5 and 10) systems were investigated using differential scanning calorimetry (DSC). The variation of glass transition temperature, T_g, with Ga concentration has been studied. The value of activation energy of glass transition, E_g, has been found to increase with increase in Ga content. This increase in E_g has been explained in terms of the average heat of atomization for these glasses.     

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La_0.6−xSr_0.4MnO_3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO₃ perovskite structure being the dominant phase and Mn₃O₄ being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn²⁺ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature T_C vs. the Mn⁴⁺ ion content ratio at the B sites of ABO₃ perovskite phase is in accord with the experimental result of the samples La_1−xSr_xMnO₃.     

Study on the structure and optical property of Zn1−xCuxO sol–gel thin films on quartz substrate

Zn_1−xCu_xO thin films (x=0, 1.0, 3.0, 5.0%) are prepared on quartz substrate by sol–gel method. The structure and morphology of the samples are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results show that Cu ions were effectively penetrated into the ZnO crystal lattices with substitutional and interstitial impurities to form stable solid solutions without changing the polycrystalline wurtzite structure. Two peaks at 420 nm (2.95 eV, violet), 485 nm (2.56 eV, blue) have been observed from the photoluminescence (PL) spectra of the samples. It is concluded that the violet peak may correspond to the exciton emission; the blue emission corresponds to the electron transition from the bottom of the conduction band to the acceptor level of zinc vacancy. The optical test shows that the optical band gap E_g is decreased with the increase amount of Cu doping in ZnO. The band gap decrease from 3.40 eV to 3.25 eV gradually. It is also found that the transmission rate is increased rapidly with the increase of Cu ions concentration.     

Studies on sintering of solid state synthesized Ba1−xKxBiO3−δ superconducting phase

Superconducting Ba_1−xK_xBiO_3−δ pellets were synthesized by solid state reaction followed by sintering. Thermo-gravimetric and differential thermo-gravimetric analysis (TG-DTA) of the mixture of nitrates was carried out to study the reactions during the phase formation. The effect of different sintering temperatures on the phase formation was studied. The X-ray diffraction data confirms the formation of superconducting Ba_0.6K_0.4BiO_2.23 phase at 700 °C. The surface morphological studies as a function of sintering temperature were studied by SEM. It is observed that the pellets prepared by solid state reaction followed its sintering at 700°C shows the superconducting transition at 26.8 K.     

Magnetic characterization and thermoelectric power of Ni1−yZny Cu0.3Fe1.7 O4; 0.0⩽y⩽0.6

Samples of Ni_1−yZn_yCu_0.3Fe_1.7O₄; 0.0?y?0.6 were prepared by the solid state reaction method. X-ray investigations were carried out in order to assure the formation of the samples in single spinel phase. The analysis of X-ray data shows that the unit cell parameter increases with increasing Zn concentration and ascribed to the variation of the predicted cation distribution. Seebeck coefficient measurements were performed to know the type of charge carriers participating in the conduction mechanism. The magnetic susceptibility for the prepared samples was measured using Faraday^’s method at different temperatures as a function of the magnetic field intensity. The magnetic parameters were calculated from the magnetic susceptibility data, in the temperature range (300–800 K) at three different magnetic field intensities of (1280, 1733 and 2160 Oe). The effective magnetic moment (μ_eff) showed that, the critical Zn content was y=0.2$y=0.2$.     

Observation of spin reorientation in layered manganites La1.2Sr1.8(Mn1−yRuy)2O7 (0.0⩽y⩽0.2) by Lorentz transmission electron microscopy

The effect of Ru substitution for Mn in bilayered oxides La_1.2Sr_1.8(Mn_1−yRu_y)₂O₇ (0?y?0.2$0?y?0.2$) was investigated by magnetization measurements and low-temperature Lorentz transmission electron microscopy. It was found that the magnetic anisotropy is controlled by the Ru content y and temperature T. The easy axis changes from 〈1 1 0〉 for the y=0$y=0$ crystal to the c  -axis for y=0.2$y=0.2$, and it rotates away from the c-  axis for the y=0.05$y=0.05$ and y=0.07$y=0.07$ crystals with decreasing temperature. Furthermore, maze-shaped magnetic domain structures were observed in the (0 0 1) thin crystals with 0.05?y?0.2$0.05?y?0.2$. Changes in domain size and structure indicate that the uniaxial magnetic anisotropy becomes stronger as Ru content y increases.