Structural and magnetic properties of Li-Cu mixed spinel ferrites

Li_0.5−x/2Cu_xFe_2.5−x/2O₄ (where x=0.0-1.0) ferrites have been prepared by solid-state reaction. X-ray diffraction was used to study the structure of the above investigated ferrites at various sintering temperatures. Samples were sintered at 1000, 1100 and 1200 °C for 3 h in the atmosphere. For the sintering temperature of 1000 °C, Li_0.5−x/2Cu_xFe_2.5−x/2O₄ undergoes cubic to tetragonal transformation for higher Cu content. However, for the sintering temperature of 1100 and 1200 °C, X-ray diffraction patterns are mainly characterized by fcc structure, though presence of tetragonal distortion was found by other temperature dependence of initial permeability curves. The lattice parameter, X-ray density and bulk density were calculated for different compositions. Curie temperature was measured from the temperature dependence of initial permeability curves. Curie temperatures of Li-Cu mixed ferrites were found to decrease with the increase in Cu²⁺ content due to the reduction of A-B interaction. As mentioned earlier, temperature dependence of initial permeability curves was characterized by tetragonal deformation for the samples containing higher at% of Cu. The complex initial permeability has been studied for different samples. The B-H loops were measured at constant frequency, f=1200 Hz, at room temperature (298 K). Coercivity and hysteresis loss were estimated for different Cu contents.     

Electromagnetic properties of manganese-zinc ferrite with lithium substitution

Polycrystalline manganese-zinc ferrite with lithium substitution of composition Li_0.5xMn_0.4Zn_0.6−xFe_2+0.5xO₄ (0.0≤x≤0.4) was prepared by the usual ceramic method. X-ray diffraction analysis confirmed that the samples have a spinel structure and are of single phase for some values of Li content. Lithium doping considerably modifies saturation magnetization since its value increases from 57.5 emu/g for x=0.0 to 82.9 emu/g for x=0.4. Lithium inclusion increases the real permeability (over 1 MHz) while the natural resonance frequency shifts to lower values as the fraction of Li increases. These ferrites show good electromagnetic properties as absorbers in the microwave range of 1 MHz - 1 GHz.     

Zn- and Cu-substituted Co2Y hexagonal ferrites: Sintering behavior and permeability

Y-type polycrystalline hexagonal ferrites Ba₂Co_2−x−yZn_xCu_yFe₁₂O₂₂ with 0≤x≤2 and 0≤y≤0.8 were prepared by the mixed-oxide route. Single phase Y-type ferrite powders were obtained after calcinations at 1000 °C. Samples sintered at 1200 °C show a permeability that increases with the substitution of Zn for Co and display maximum permeability of μ′=35 at 1 MHz for x=1.6 and y=0.4. A resonance frequency f_r=500 MHz is observed for Zn-rich ferrites with y=0 and 0.4. The saturation magnetization increases with substitution of Zn for Co. Addition of Bi₂O₃ shifts the temperature of maximum shrinkage down to T≤950 °C. Moreover, an increase of the Cu-concentration further lowers the sintering temperature to T≤900 °C, enabling co-firing of the ferrites with Ag metallization for multilayer technologies. However, low-temperature firing reduces the permeability to μ′=10 and the resonance frequency is shifted to 1 GHz. Thus substituted hexagonal Y-type ferrites can be used as soft magnetic materials for multilayer inductors for high frequency applications.     

γ irradiation effect on the polarization and resistance of Li-Co-Yb-ferrite

The effect of γ irradiation on some physical properties of rare earth ferrite of the general formula Li_0.5+zCo_z Yb_xFe_2.5−2z−xO₄, (z=0.1, x=0.00, 0.025, 0.050, … , 0.200) is discussed. The temperature dependence of the polarization and resistance is studied in the range (300 K≤T≤700 K) at different frequencies (10 kHz≤f≤1 MHz). The relaxation time and the activation energy have been calculated before and after irradiation with γ rays doses of 1 and 3 Mrad. A comparison was made between the ac resistance before and after irradiation for the samples with (0.0≤x≤0.2). The results after irradiation with 1 Mrad γ rays showed that the resistance at the critical concentration decreases from 800 to 25 kΩ at room temperature. Furthermore, with increasing temperature the resistance ranged from R≈130 kΩ at T≈310 K to R≈0.13 kΩ at T≈640 K. Thus, it is possible to improve the conductivity of this type of rare earth ferrite materials to be used in technological applications at room as well as at high temperature.     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Doping effect of Mn on the magnetic behavior in Ni-Zn ferrite nanoparticles prepared by sol-gel auto-combustion

The ferrite samples of a chemical formula Ni_0.5−xMn_xZn_0.5Fe₂O₄ (where x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) were synthesized by sol-gel auto-combustion method. The synthesized samples were annealed at 600 °C for 4 h. An analysis of X-ray diffraction patterns reveals the formation of single phase cubic spinel structure. The lattice parameter increases linearly with increase in Mn content x. An initial increase followed by a subsequent decrease in saturation magnetization with increase in Mn content is observed showing inverse trend of coercivity (H_c). Curie temperature decreases with increase in Mn content x. The initial permeability is observed to increase with increase in Mn content up to x=0.3 followed by a decrease, the maximum value being 362. Possible explanation for the observed structural, magnetic, and changes of permeability behavior with various Mn content are discussed.     

Synthesis, characterization and electromagnetic properties of Fe1−xCox alloy flower-like microparticles

Fe_1−xCo_x alloy microparticles with size 3-5 μm and novel flower-like shapes were prepared by a simple low temperature reduction method. The electromagnetic properties for the paraffin matrix composites containing Fe_1−xCo_x alloy microparticles were measured using a vector network analyzer in the 2-18 GHz frequency range. As a consequence of large surface- and shape-anisotropy energy for the flower-like shaped 3D microstructures, the strong natural resonance around 8-12 GHz and remarkable dielectric relaxation were observed in the complex permittivity and permeability spectrum, which are dominant in the enhanced electromagnetic wave absorption (EMA) performance. It was found that both the electromagnetic parameters of complex permittivity and permeability and the intensity and location of absorption band were remarkably dependent on the Co/Fe molar ratio. The enhanced EMA performance was obtained in these Fe_1−xCo_x-paraffin (x=0.4, 0.5, and 0.6) composites system. For the Fe_0.5Co_0.5 alloy, the reflection loss (RL) exceeding −20 dB was obtained in the broad frequency range of 5.4-18 GHz with a thin sample thickness of between 1.0 and 2.9 mm. In particular, an optimal RL of −59 dB was obtained at 3.61 GHz with a thin thickness of 3.6 mm for the Fe_0.4Co_0.6 sample. The Fe_1−xCo_x alloy microparticles may be attractive candidates for applications of microwave absorption materials with a wide frequency range and strong absorption in the high frequency region.     

Electrical conductivity and complex electric modulus of titanium doped nickel-zinc ferrites

The samples Ni_1+x−yZn_yTi_x Fe_2−2xO₄; y=0.1, 0.0≤x≤0.5 were prepared in a single-phase spinel structure as indicated from X-ray analysis. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 600 K in the frequency range from 42 Hz to 5 MHz have been analyzed. The relation of conductivity with temperature revealed a semiconductor to semimetallic behavior as Ti⁴⁺ concentration increases. The conduction mechanism depends mainly on the valence exchange between the different metal ions in the same site or in different sites. The dielectric constant as a function of temperature and frequency showed that there is a strong dependence on the compositional parameter x. The electrical modulus has been employed to study the relaxation dynamics of charge carriers. The result indicates the presence of correlation between motions of mobile ion charges. The activation energies extracted from M′(ω) and M″(ω) peaks are found to follow the Arrhenius law. The electrical conductance of the samples found to be dependent on the temperature and frequency.     

Microstructure and nanohardness of the diluted magnetic semiconducting Cd1−xMnxS nano-crystalline films

Cd_1−xMn_xS nano-crystalline films (0 ≤ x ≤ 0.5) were formed on glass substrates by thermal evaporation technique at room temperature (300 K). AFM studies showed that all the films were in nano-crystalline form with the grain size varying in the range between 36 and 58 nm and exhibited hexagonal structure of the host material. The lattice parameters varied linearly with composition, following Vegard's law in the entire composition range. The nanohardness and Young's modulus decreased sharply with ‘Mn’ content upto x = 0.3 and increased with high Mn content.     

Enhancement of initial permeability due to Mn substitution in polycrystalline Ni0.50−xMnxZn0.50Fe2O4

The structural and magnetic properties of Mn substituted Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.00, 0.10 and 0.20) sintered at various temperatures have been investigated thoroughly. The lattice parameter, average grain size and initial permeability increase with Mn substitution. Both bulk density and initial permeability increase with increasing sintering temperature from 1250 to 1300 °C and above 1300 °C they decrease. The Ni_0.30Mn_0.20Zn_0.50Fe₂O₄ sintered at 1300 °C shows the highest relative quality factor and highest initial permeability among the studied samples. The initial permeability strongly depends on average grain size and intragranular porosity. From the magnetization as a function of applied magnetic field, M(H), it is clear that at room temperature all samples are in ferrimagnetic state. The number of Bohr magneton, n(μ_B), and Neel temperature, T_N, decrease with increasing Mn substitution. It is found that Mn substitution in Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.20) decreases the Neel temperature by 25% but increases the initial permeability by 76%. Possible explanation for the observed characteristics of microstructure, initial permeability, DC magnetization and Neel temperature of the studied samples are discussed.     

Ferromagnetism in amorphous Ge1−xMnx grown by low temperature vapor deposition

Magnetic properties of amorphous Ge_1−xMn_x thin films were investigated. The thin films were grown at 373 K on (100) Si wafers by using a thermal evaporator. Growth rate was ∼35 nm/min and average film thickness was around 500 nm. The electrical resistivities of Ge_1−xMn_x thin films are 5.0×10⁻⁴∼100 Ω cm at room temperature and decrease with increasing Mn concentration. Low temperature magnetization characteristics and magnetic hysteresis loops measured at various temperatures show that the amorphous Ge_1−xMn_x thin films are ferromagnetic but the ferromagnetic magnetizations are changing gradually into paramagnetic as increasing temperature. Curie temperature and saturation magnetization vary with Mn concentration. Curie temperature of the deposited films is 80-160 K, and saturation magnetization is 35-100 emu/cc at 5 K. Hall effect measurement at room temperature shows the amorphous Ge_1−xMn_x thin films have p-type carrier and hole densities are in the range from 7×10¹⁷ to 2×10²² cm⁻³.     

Improvement in electrical and magnetic properties of mixed Mg-Al-Mn ferrite system synthesized by citrate precursor technique

In the present work, mixed magnesium-manganese ferrites of composition Mg_0.9Mn_0.1Al_0.3Co_zFe_1.7−zO₄ where z=0.3, 0.5 and 0.7 have been synthesized by the citrate precursor technique. X-ray diffraction patterns of the samples confirmed the formation of single-phase spinel structure. The ferrites have been investigated for their electric and magnetic properties such as dc resistivity, Curie temperature, saturation magnetization, initial permeability and relative loss factor (RLF). Fairly constant value of initial permeability over a wide frequency range (0.1-20 MHz) and low values of the relative loss factor of the order of 10⁻⁴-10⁻⁵, in the frequency range 0.1-30 MHz, are the cardinal achievements of the present investigation. In addition to this, initial permeability was found to increase with an increase in temperature while RLF was observed to be low at these temperatures. The dc resistivity and Curie temperature were found to increase with an increase in cobalt content. The mechanisms contributing to these results are discussed in detail in this paper.     

Na1−xLixNbO3 ceramics studied by X-ray diffraction, dielectric, pyroelectric, piezoelectric and Raman spectroscopy

Na_1−xLi_xNbO₃ ceramics with composition 0.05≤x≤0.30 were prepared by solid-state reaction method and sintered in the temperature range 1100-1150 °C. These ceramics were characterised by X-ray diffraction as well as dielectric permittivity measurements and Raman spectroscopy. Dielectric properties of ceramics belonging to the whole composition domain were investigated in a broad range of temperatures from 300 to 750 K and frequencies from 0.1 to 200 kHz. The Rietveld refinement powder X-ray diffraction analysis showed that these ceramics have a single phase of perovskite structure with orthorhombic symmetry for x≤0.15 and two phases coexistence of rhombohedral and orthorhombic above x=0.20. The evolution of the permittivity as a function of temperature and frequency showed that these ceramics Na_1−xLi_xNbO₃ with composition 0.05≤x≤0.15 present the classical ferroelectric character and the phase transition temperature T_C increases as x content increases. The polarisation state was checked by pyroelectric and piezoelectric measurements. For x=0.05, the piezoelectric coefficient d₃₁ is of 2pC/N. The evolution of the Raman spectra was studied as a function of temperatures and compositions. The results of the Raman spectroscopy study confirm our dielectric measurements, and they indicate clearly the transition from the polar ferroelectric phase to the non-polar paraelectric one.     

High temperature thermoelectric properties of Nb-doped ZnO ceramics

Solid-state reaction processing technique was used to prepare Zn_xNb_1−xO (0≤x≤0.02) polycrystalline bulk samples. In the present study, we find that their lattice parameters a and c tend to decrease with increasing amount of Nb additive. The electrical conductivity of all the Zn_1−xNb_xO samples increased with increasing temperature, indicating a semiconducting behavior in the measured temperature range. The addition of Nb₂O₅ to ZnO led to an increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient. The best performance at 1000 K has been observed for nominal 0.5 at% Nb-doped ZnO, with an electrical resistivity of about 73.13 (S cm⁻¹) and Seebeck coefficient of ∼257.36 μV K⁻¹, corresponding to a power factor (S²σ) of 4.84×10⁻⁴ Wm⁻¹ K⁻². The thermal conductivity, κ, of the oxide decreased as compared to pure ZnO. The figure of merit ZT values of ZnO-doped Nb₂O₅ samples are higher than the ZnO pure sample, demonstrating that the Nb₂O₅ addition is fairly effective for enhancing thermoelectric properties.     

Structural,optical and dielectric properties of Ni substituted NdFeO3

Present study reports the structural, optical and dielectric properties of Ni substituted NdFe_1−xNi_xO₃ (0 ≤ x ≤ 0.5) compounds prepared through the ceramic method. X-ray diffraction (XRD) confirmed an orthorhombic crystal structure of all the samples. Both unit cell volume and grain size were found to decrease with an increase in Ni concentration. Morphological study by Scanning electron microscope (SEM) shows less porosity with Ni substitution in present system. From UV–vis spectroscopy, the optical band gap was found to increase with Ni doping. This observed behavior was explained on the basis of reduction in crystallite size, unit cell volume and its impact on the crystal field potential of the system after Ni substitution. The dielectric properties (?′ and tanδ) as a function of frequency or temperature, and the ac electrical conductivity (σ_ac) as a function of frequency have been studied. Hopping of charge carriers between Fe²⁺ → Fe³⁺ ions and Ni²⁺ → Ni³⁺ ions are held responsible for both electrical and dielectric dispersion in the system. Wide optical band gap and a very high dielectric constant of these materials promote them to be a suitable candidate for memory based devices in electronic industry.     

Structure and RT ferromagnetism of Fe-doped AlN films

Al_1−xFe_xN_1−δ thin films with 0 ≤ x ≤ 13.6% were deposited by dc magnetron co-sputtering at room temperature (RT). It is found that Fe atom will substitutes the Al atom in the lattice when x ≤ 1.2%, while it will embed into the interstice of the lattice at larger Fe content. RT ferromagnetism was observed in all doped samples. A maximum saturated magnetization 2.81 emu/cm³ of the film is found to be induced by AlFeN ternary alloy when x = 1.2%.     

Properties of doped ceria solid electrolytes in reducing atmospheres

In this work we present investigations of stability of rare earth-doped ceria electrolytes in reducing atmosphere. The effect of dopant type and dopant concentration on reducibility was studied on the basis of thermogravimetric and impedance spectroscopy measurements on materials grouped into two series: Ce_1 − xGd_xO_2 − x/2 (0 ≤ x ≤ 0.4) and Ce_0.85R_0.15O_1.925 (R = Y, Nd, Sm, Gd, and Dy). Relationship between an initial vacancy concentration introduced by the amount of dopant and the characteristic temperature of reduction was found. Much less pronounced dependence was observed for different dopants with the same concentration, which indicates that it is the dopant and vacancy concentration and not the dopant type, which is responsible for reducibility of ceria electrolytes. Impedance spectroscopy measurements allowed for calculation of changes of oxygen ions transport number during the reduction process.     

Magnetic and electrical properties of amorphous Ge1−xCrx thin films grown by low temperature vapor deposition

Amorphous Ge_1−xCr_x thin films are deposited on (1 0 0)Si by using a thermal evaporator. Amorphous phase is obtained when Cr concentration is lower than 30.7 at%. The electrical resistivities are 1.89×10⁻³–0.96×10² Ω cm at 300 K, and decrease with Cr concentration. The Ge_1−xCr_x thin films are p-type. The hole concentrations are 5×10¹⁶–7×10²¹ cm⁻³ at 300 K, and increase with Cr concentration. Magnetizations are 7.60–1.57 emu/cm³ at 5 K in the applied field of 2 T. The magnetizations decrease with Cr concentration and temperature. Magnetization characteristics show that the Ge_1−xCr_x thin films are paramagnetic.     

Parallel syntheses and thermoelectric properties of Ce-doped SrTiO3 thin films

Thermoelectric properties of single crystalline Ce_xSr_1−xTiO₃ films (0 ≤ x ≤ 0.5) have been studied by using combinatorial pulsed-laser deposition. Temperature gradient method was used for identifying an optimum growth temperature for SrTiO₃ homoepitaxial growth, at which both oxygen stoichiometry and persisting layer-by-layer growth mode could be accomplished. Electrical conductivity (σ) and Seebeck coefficient (S) were measured at room temperature for the composition-spread films grown at the optimized temperature and found to be considerably higher than those reported for bulk poly-crystalline compounds. Hall measurement revealed that carrier density linearly increased with increasing x, suggesting that a trivalent Ce ions substituted divalent Sr ions to supply electrons. A maximum power factor (S²σ) was obtained for the x = 0.2 film, being 7 and 14 μW/K² cm at 300 and 900 K, respectively.     

The influence of Mn content on luminescence properties in Mn-doped ZnO films deposited by ultrasonic spray assisted chemical vapor deposition

Mn-doped ZnO (Zn_1−xMn_xO, 0 ≤ x ≤ 0.1) films are prepared by an ultrasonic spray assisted chemical vapor deposition method. X-ray diffraction and Raman scattering show that all the Zn_1−xMn_xO films are good wurtzite structures without any impurity phases. Cathodoluminescence spectra show that ultraviolet emission and green luminescence can be observed. The intensity of ultraviolet emission decreases with the increment of x, while the intensity of green luminescence increases with the increment of x when x ≤ 0.02. However, when x (x > 0.02) is further increased, the intensity of green luminescence decreases gradually, and the green luminescence disappears when x is above 0.075. We consider that the change of the luminescence is related to the competition between the radiative recombination and the non-radiative recombination.