Synthesis and electromagnetic properties of microwave absorbing material: Cox(Cu0.5Zn0.5)1−xFe2O4 (0 < x < 1) nanoparticles

Spinel ferrite Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ over a compositional range 0 < x < 1 was prepared using a simple hydrothermal method. Particle sizes could be varied from 14 to 25 nm by changing the x value. X-ray diffraction results confirmed that all the as-prepared nanoparticles revealed typical spinel structure and transmission electron microscopy images showed that the particle size of the samples increased with increasing x value. The magnetic properties of the as-prepared Co_x(Cu_0.5Zn_0.5)_1−xFe₂O₄ nanoparticles have been systematically examined. The maximum saturation magnetization existed at the highest Co content (x = 1). The electromagnetic properties of all the samples have been measured by an Agilent network analyzer and the results showed that Co_0.1(Cu_0.5Zn_0.5)_0.9Fe₂O₄ possessed the best microwave absorbing properties.     

Structure, magnetic and magnetostrictive properties of Sm0.7Pr0.3Fex alloys

The structure, magnetic and magnetostrictive properties of Sm_0.7Pr_0.3Fe_x (1.45 ≤ x ≤ 1.95) alloys is investigated. X-ray diffraction analysis confirms the presence of single cubic Laves phase in Sm_0.7Pr_0.3Fe_x alloys with 1.45 ≤ x ≤ 1.85. The saturation magnetization of the alloys tends to increase with increasing Fe content. Sm_0.7Pr_0.3Fe_1.55 has the highest magnetostriction among all the Sm_0.7Pr_0.3Fe_x alloys at low fields and shows a large magnetostriction λ_// − λ_⊥ = −1008 ppm at a magnetic field of 12 kOe.     

Magnetic, magnetocapacitance and dielectric properties of Cr doped bismuth ferrite nanoceramics

BiFe_1−xCr_xO₃ (x = 0, 0.04, 0.06 and 0.08) nanoceramics were prepared by sol-gel method. Nanoceramics were calcined at 450 °C. Calcined powders were leached in diluted nitric acid to get single phase. TEM analysis shows the particle size to be ∼80 nm. Thermogravimetric analysis of as prepared powder indicates that the single phase is formed at around 450 °C. Magnetization was found to increase as the concentration of Cr was increased. Dielectric constant and dielectric loss were found to decrease with increase in frequency for all the compositions. Magnetocapacitance was found to increase with magnetic field. For BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, the change of dielectric constant induced by magnetic field may be well approximated by Δ?/? = γM², here, γ (magnetoelectric interaction) is small and positive. A linear fit gave the value of γ of ∼18.4 × 10⁻², 12.3 × 10⁻² and 3.3 × 10⁻² for BiFe_1−xCr_xO₃ (x = 0.04, 0.06 and 0.08) nanoceramics, respectively.     

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.     

High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.     

Structural, dielectric, magnetic, magnetodielectric and impedance spectroscopic studies of multiferroic BiFeO3-BaTiO3 ceramics

Polycrystalline (1−x)BiFeO₃-xBaTiO₃ (x = 0.00, 0.10, 0.20 and 0.30) ceramics have been prepared via mixed oxide route. The effect of BaTiO₃ substitution on the dielectric, ferroelectric and magnetic properties of the BiFeO₃ multiferroic perovskite was studied. From XRD analysis it revealed that BaTiO₃ substitution does not affect the crystal structure of the (1−x)BiFeO₃-xBaTiO₃ system up to x = 0.30. Improved dielectric properties were observed in the prepared system. An anomaly in the dielectric constant (?) was observed in the vicinity of the antiferromagnetic transition temperature. Experimental results suggest that in the (1−x)BiFeO₃-xBaTiO₃ system, the increase of BaTiO₃ concentration leads to the effective suppression of the spiral spin structure of BiFeO₃, resulting in the appearance of net magnetization. The dependence of dielectric constant and loss tangent on the magnetic field is a evidence of magnetoelectric coupling in (1−x)BiFeO₃-xBaTiO₃ system. The impedance analysis suggests the presence of a temperature dependent electrical relaxation process in the material, which is almost similar for all the concentrations in the present studies. The electrical conductivity has been observed to increase with rise in temperature showing a typical negative temperature coefficient of the resistance (NTCR) behaviors analogous to a semiconductor and suggests a non-Debye type of electrical relaxation.     

Improvement of magnetic characteristics and electrical properties of FeCoHfO/AlOx multilayered films

High permeability magnetic films can enhance the inductance of thin-film inductors in DC-DC converters. In order to obtain high permeability, effective uniaxial anisotropic field should be as low as possible. A multilayered technique (laminating the magnetic layers with oxide spacers) was exploited to improve the magnetic properties of thick films. The FeCoHfO/AlO_x multilayered films were fabricated by dc reactive magnetron sputtering. Inserting an insulator (AlO_x) layer can decrease the magneto-elastic anisotropy by reducing the residual stress of the FeCoHfO magnetic films. The anisotropic field and resistivity of the FeCoHfO/AlO_x multilayered films were evidently improved by multilayered coating. With this optimum configuration of 9 layers structure [FeCoHfO (133 nm)/AlO_x (10 nm)]₉, low anisotropic field (H_K = 65 Oe) and high resistivity (ρ ∼ 1350) μΩ cm were achieved.     

Dielectric, electrical and magnetoelectric characterization of (x)Ni0.8Zn0.2Fe2O4 + (1 − x)Pb0.93La0.07(Zr0.60Ti0.40)O3 composites

The structural, electrical, dielectric, magnetic and magnetoelectric properties of (x)Ni_0.8Zn_0.2Fe₂O₄ + (1 − x)Pb_0.93La_0.07(Zr_0.60Ti_0.40)O₃ (x = 0, 0.15, 0.30, 0.45 and 1) have been studied by means of various experimental techniques. Polycrystalline samples of this series have been prepared by the double sintering ceramic method. X-ray diffraction data analysis revealed purity of the composites. Microstructural analysis using scanning electron microscopy mode depicts the presence of two phases in contact with each other. Dielectric properties were studied at and well above room temperature. Temperature dependent variation of the dielectric constant show diffused phase transition which can be well described by fitting the Lorentz-type relation, . Observation of well-saturated ferroelectric hysteresis loop and magnetic hysteresis loop for composites indicates that ferroelectric and magnetic ordering exist simultaneously at room temperature. The static value of magneto electric voltage coefficient (α_E) has been studied as a function of magnetic field at room temperature for all the composites. The maximum value of α_E is 7.53 mV/(cm Oe) for 85% PLZT-15% NZFO composites.     

Variation in structural,optical and magnetic properties of Zn1−xCrxO (x = 0.0, 0.10, 0.15, and 0.20) nanoparticles: Role of dopant concentration on non-saturation of magnetization

Cr-doped ZnO, i.e. Zn_1−xCr_xO (x = 0.00, 0.05, 0.10, 0.15 and 0.20) nanoparticles were synthesized by sol–gel route. The structural and morphological properties of these nanoparticles were investigated by high resolution transmission electron microscope (HRTEM). The average particle size of Zn_1−xCr_xO nanoparticles decreases from 75 to 40 nm with the increase in x from 0.00 to 0.20. The rings observed in selected area diffraction pattern revealed that up to x = 0.10 these nanoparticles have single phase ZnO. However, a secondary spinel phase of ZnCr₂O₄ was observed for higher Cr doping (x ≥ 0.15). The optical band gap calculated using UV–visible absorption was decreased from 3.27 to 2.27 eV with the increase in Cr-doping from 0.00 to 0.20 in ZnO nanoparticles. The undoped ZnO (Zn_1−xCr_xO; x = 0.00) nanoparticles did not show any hysteresis loop at room temperature, however, clear loops were obtained for x = 0.05–0.20. Additionally, magnetization (M) vs. applied magnetic field (H) loops for lower Cr-concentration (x = 0.05) saturate at 5 kOe, and while those with higher Cr concentration (x > 0.05) do not show saturation even at 10 kOe. This may be attributed to increase in the defects at higher Cr-doping into ZnO. The value of saturation magnetization was found to decrease from 4.24 emu g⁻¹ to 1.96 emu g⁻¹ with the increase in Cr doping from x = 0.05 to 0.20 in ZnO and may be due to the secondary ZnCr₂O₄ phase.     

Development of “tuned microwave absorbers” using U-type hexaferrite

The paper presents that microwave absorption is tunable with sample thickness over a frequency range if electromagnetic properties of the sample do not change with frequency in that range. The work summarizes the results of polycrystalline samples of two U-type hexaferrite series: Ba₄ (Co_1−5x P_2x)₂ Fe₃₆ O₆₀ (0.0 ? x ? 0.20 in step 0.05) and (Ba_1−3x La_2x)₄ Co₂ Fe₃₆ O₆₀ (0.10 ? x ? 0.25 in step 0.05) that have been prepared through solid state reaction route. The complex permittivity, permeability and microwave absorbing properties have been discussed in detail for X-band (8.2–12.4 GHz) frequencies. The results show that the substitution of Co²⁺ ion with P⁵⁺ ion and Ba²⁺ ion with La³⁺ ion in the U-type-hexaferrite provides the desired electromagnetic properties for a “tuned microwave absorber” in samples with x = 0.05 and 0.20 of first and 0.20 sample of second series. In these samples, the microwave absorption peak (of >98%) shifts linearly from lower to upper side of X-band with sample thickness. These materials can provide significant application for reduction of radar cross section (RCS) and in electromagnetic interference (EMI) shielding.     

Application of secondary neutral mass spectrometry in the investigation of doped perovskites

Electromagnetic properties of doped perovskites depend sensitively on the doping level. Both the superconducting transition temperature of Bi₂Sr₂Ca(Pr)Cu₂O_8+δ compounds and the magnetic and electronic transport properties of La(Sr)Co(Fe)O₃ perovskites change dramatically with the doping level. Apart from doping, oxygen deficiency is influenced by the details of preparation processes such as calcination and sintering. Simultaneous determination of constituents is of crucial importance from sample characterization point of view. Quantitative analysis of perovskite oxides can be performed by Secondary Neutral Mass Spectrometry (SNMS) which is a suitable technique to measure the chemical composition of almost any sample because the flux of atoms sputtered from the sample is representative of the stoichiometry of the top-most layers. The composition and oxygen content of Bi₂Sr₂Ca_0.86Pr_0.14Cu₂O_8.4 and La_1−xSr_xCo_0.975Fe_0.025O_3−δ, where 0 ≤ x ≤ 0.25, were determined by SNMS. The results show that the method is equally applicable for insulating and conducting compounds. The observed electromagnetic properties reflect well the compositions obtained experimentally.     

Polymorphous Fe/FexOy composites: One-step oxidation preparation,composition control,and static magnetic and electromagnetic characteristics

Polymorphous Fe/Fe_xO_y core–shell and urchin-like composites were synthesized via a facile oxidation process at relatively low temperatures (100–300 °C) in the absence of surfactants or an external magnetic field. The oxidation temperature plays a key role in determining the morphology, crystal size, and composition of the resulting products. The static magnetic and electromagnetic (EM) properties of Fe/Fe_xO_y composites are influenced by their morphology, crystal size, and composition. In this study, excellent soft magnetic properties and enhanced permeability were obtained from core–shell Fe/Fe_xO_y composites with low Fe_xO_y shell contents and low surface anisotropy. In contrast, high coercivity and dielectric performance were exhibited by urchin-like Fe/Fe_xO_y composites with high shape and surface anisotropy. This work provides insights into the absorption mechanism of urchin-like complex absorption materials.     

Nonlinear optical diagnostic of semimagnetic semiconductors Pb1−xYbxX (X = S, Se, Te)

Nonlinear optical measurements were performed to elucidate the influence of magnetic ions on the behavior of charge carriers in magnetic semiconductors—Pb_1−xYb_xX (X = S, Se, Te at x = 1-3%). It was shown that nonlinear optical methods could be used as sensitive tools for investigations of electron-phonon anharmonicity near low-temperature semiconductor-insulator phase transitions. There exists a difference between surface and bulk-like contributions to the nonlinear optical effects. It was shown that only low-temperature Two Photon Absorption (TPA) oscillator may be related to the number of the electron-phonon anharmonic modes responsible for the observed phase transformation. The explanation of the anomalous temperature dependences is given in accordance with dipole momentum's behaviors determined by low-temperature spin-spin interactions and by electron-phonon anharmonic interactions. We have discovered that low-temperature dependence of specific heat of Pb_1−xR_xTe (R = Yb, Pr with x = 3% and 1.6%, respectively) exhibits a non-magnetic order caused by large electron-phonon contributions and structural disorder effects.     

Magnetic properties of epitaxial thin films and bulk of Eu(Ni,Mn)O3 perovskites

Magnetic properties of EuNi_xMn_1 − xO₃ were studied in bulk form and, for a specific composition (x = 1/3), as epitaxial thin film. Paramagnetic properties are interpreted as a VanVleck contribution of the ⁷F Eu³⁺ configuration, showing linear decrease of the effective moment and VanVleck susceptibility as x(Ni) increases. The ordered state was studied for three particular compositions which describe different regimes: canted-antiferromagnetism for x(Ni) = 1/4 and ferromagnetism for x(Ni) = 1/2, the composition x(Ni) = 1/3 being at the frontier of both regimes. Nominal composition x(Ni) = 1/3 was chosen for epitaxial films, since it corresponds to a critical concentration for which a maximum number of pairs Mn³⁺-Mn⁴⁺ and optimal double-exchange interactions are found. The coercive field is enhanced, and the field dependence of T_max is reduced as if the coherent ferromagnetic phase is favoured at the boundary of the magnetic domains.     

Shape controlled synthesis of iron-cobalt alloy magnetic nanoparticles using soft template method

Fe_xCo_1 − x alloy nanoparticles of spherical (x = 0.25, 0.68, 0.85), hollow spherical (x = 0.60) and sheet like (x = 0.60) shapes were prepared at room temperature by reduction of iron chloride tetrahydrate and cobalt chloride hexahydrate with sodium borohydride, using N-Cetyl-N,N,N-trimethyl ammonium bromide (CTAB)/water/hexanol system as soft template. The size and shapes of nanostructures were found to depend on the concentrations of CTAB and hexanol in water. Composition and shape dependence of magnetic properties of spherical, hollow spherical and sheet like Fe_xCo_1 − x alloy nanostructures were discussed. The highest saturation magnetization of 235 emu/g with a coercivity of 160 Oe was obtained for spherical Fe_0.68Co_0.32 nanoparticles.     

Synthesis and weak ferromagnetism of Dy-doped BiFeO3 powders

Dy-doped BiFeO₃ powders were synthesized by sol-gel auto-combustion method. X-ray diffraction of the powders were recorded and analyzed for the confirmation of crystal structure and the calculation of the lattice parameters. X-ray photoelectron spectroscopy was used to identify the oxidation state of Fe. Detailed studies of vibrating sample magnetometer indicated a significant effect of Dy doping on the magnetic properties of Bi_1 − xDy_xFeO₃ powders at room temperature. When x = 0.1, BDFO shows the best magnetic property. The origin of weak ferromagnetism was discussed in this paper.     

Electrical conduction, dielectric behavior and magnetoelectric effect in (x)BaTiO3 + (1 − x)Ni0.94Co0.01Mn0.05Fe2O4 ME composites

Electrical and magnetoelectric properties of magnetoelectric (ME) composites containing barium titanate as electrical component and a mixed Ni-Co-Mn ferrite as the magnetic component are reported. The ME composites with a general formula (x)BaTiO₃ + (1 − x)Ni_0.94Co_0.01Mn_0.05Fe₂O₄ where x varies as 0, 0.55, 0.70, 0.85 and 1 were prepared by standard double sintering ceramic method. The presence of both the phases was confirmed by X-ray diffraction technique. The dc resistivity was measured as a function of temperature. The variation of dielectric constant (?) and loss tangent (tan δ) with frequency (100 Hz-1 MHz) and with temperature was studied. The conduction is explained on the basis of small polaron model based on ac conductivity measurements. The static value of ME conversion factor i.e. dc (ME)_H was studied as function of intensity of magnetic field. The changes were observed in dielectric properties as well as ME effect as the molar ratio of the components was varied. A maximum value of ME conversion factor of 610 μV/cm Oe was observed in the case of a composite containing 15 mol% ferrite phase.     

Microstructure and magnetic properties of BaTiO3-(Ni,Zn)Fe2O4 multiferroics

The microstructures of composite xBaTiO₃-(1−x)(Ni_0.5Zn_0.5)Fe₂O₄ (BT-NZF) multiferroics with various mixing ratios (x = 0.50, 0.60 and 0.70) are investigated by means of electron backscatter diffraction (EBSD) and magnetic force microscopy (MFM). The EBSD measurements reveal a change in the texture of the ferrite and the BaTiO₃ grains upon increasing the ferrite content in the sample. The sample with x = 0.70 exhibits the best ferrite texture, where only some directions are present. Furthermore, the resulting grain sizes vary from several µm (x = 0.50) to about 100 nm in the sample with x = 0.70. The MFM images reveal the presence of magnetic domains being extended over several adjacent grains, which according to the EBSD data may comprise different crystallographic orientations. In this way, we can explain the differences in the magnetic contrast obtained.     

Effect of chromium substitution on structure and magnetic properties of Bi2Fe4O9

Orthorhombic Bi₂Fe_4 − xCr_xO₉ (x = 0.0, 0.25, and 0.75) nanoplatelets were synthesized by a simple hydrothermal method. The structure, morphology, and magnetic properties of the obtained powders have been characterized. Calculation of the lattice parameters of Bi₂Fe_4 − xCr_xO₉, as well as bond lengths and angles, was carried out by X-ray diffraction Rietveld refinement. The volumes of the metal-oxygen tetrahedra and octahedra were calculated to be sequentially increasing as the Cr doping level increases. The samples undergo an antiferromagnetic transition at 250 ± 5 K. The magnetic moments of the samples increase with higher Cr doping level. The 3d electron spin state for Fe³⁺ in the as-prepared samples is different, which is possibly due to the distortion of Fe-O tetrahedra and octahedra in the crystal structure after chromium substitution.     

The influence of Al content on glass forming ability and magnetic properties of high Ms nanocrystalline FeSiBPCuAl alloy

Al content on the influence of glass forming ability and magnetic properties of nanocrystalline (Fe_83.3Si₄B₈P₄Cu_0.7)_100 − xAl_x (x = 0 ∼ 1.25 at.%) alloy was investigated in our work. Experimental results show that addition of 0.5 at.% Al is effective in improving the glass forming ability of Al-free alloy and thus amorphous precursor prepared by industry-grade raw materials can be obtained. Meanwhile, the addition of Al is beneficial in decreasing coercive force and maintaining high saturation magnetization above 180 emu/g, which makes Fe-Si-B-P-Cu-Al alloy a promising soft magnetic material in potential applications.