Influence of Ho substitution on structural, magnetic and microwave absorption properties of PbM-type hexaferrites nanoparticles

Ho substituted lead hexaferrites, PbCo_0.5Sn_0.5Ho _x Fe_11?x O₁₉ (x = 0.00–0.75) were successfully prepared by a citrate combustion process. Saturation magnetization and coercivity were measured from MH-loops taken on vibrating sample magnetometer. X-ray analysis reveals that in all samples M-type structure exist with few secondary phases. The results of X-ray diffraction analysis show that the particle size is inversely related to the dopant content. Microwave absorption properties of the nanocrystalline lead ferrite (PbCo_0.5Sn_0.5Ho _x Fe_11?x O₁₉) for single-layer investigated in a frequency range of 2–18 GHz. Results indicated that the substituted Ho can greatly improve the microwave absorption properties of PbCo_0.5Sn_0.5Fe₁₁O₁₉ hexaferrites. For sample with x = 0.75, a minimum reflection loss of ?33 dB was obtained at 16.5 GHz for a layer of 1.8 mm in thickness. The microwave absorption performance of PbCo_0.5Sn_0.5Ho_0.5Fe_10.5O₁₉ ferrite gets significant improvement: The bandwidth below ?15 dB expands from 12 to 16.5 GHz, and the peak value of reflection loss decreases from ?18 to ?37 dB. The minimum reflection loss reaches ?34.2 dB when the substitution amount of the Ho³⁺ ions is 0.25. Also results show that the peak positions of the minimum reflection loss shift toward higher frequency band.     

Magnetic properties of Ni–Co doped barium strontium hexaferrite

A series of Ni–Co substituted barium strontium hexaferrite materials, Ba_0.5Sr_0.5Ni _x Co _x Fe_12–2x O₁₉ (x = 0.0, 0.2, 0.4, 0.6, 0.8 mol%) was synthesized by the sol–gel method. X-ray diffraction analysis has shown that the Ni–Co substitutions maintain in a single hexagonal magnetoplumbite phase. The room temperature magnetic properties and the cation site preferences of Ni–Co substituted ferrite were investigated by VSM. Substitutions led to decrease in coercivity while saturation magnetization remains the almost same. It indicates that the saturation magnetization (52.81–59.8 Am²/kg) and coercivity (69.83–804.97 Oe) of barium strontium hexaferrite samples can be varied over a very wide range by an appropriate amount of Ni–Co doping contents.     

Optical and electrical properties of Ba1−x Sr x TiO3 nanopowders at different Sr2+ ion content

Barium strontium titanate (BST) Ba_1?x Sr _x TiO₃ nanopowders have been successfully synthesized using oxalate precursor route. The effect of Sr²⁺ ion content from 0.3 to 0.7 on the crystal structure, crystallite size, microstructure, electrical and optical properties was systematically studied. The results revealed that well crystalline single BST phase was formed by annealing the oxalate precursor at 1,000 °C for 2 h. The crystallite size of the BST powders was decreased with increasing the Sr²⁺ ion molar ratios. The crystallite size was decreased from 56.0 to 33.1 nm when the Sr²⁺ ion content increased from 0.3 to 0.7. Additionally, the lattice parameter (a), unit cell volume and X-ray density of BST ware decreased whereas the porosity, % were increased with Sr²⁺ ion concentration. The BST phase appeared as cubic-like structure. The spectrophotometer measurement results demonstrated that the room temperature band gap energy varied with the Sr²⁺ ion composition x. The band gap energy was shifted to low energy and it was decreased from 3.6 to 3.2 eV with increasing the Sr²⁺ ion content from 0.3 to 0.7. Moreover, the DC resistivity was enhanced with increasing the Sr²⁺ ion ratio. The dielectric response obtained for the stressed samples corresponds to a true resonance rather than a dispersion process with a characteristic frequency around 1 GHz at room temperature. However, the peaks commonly observed at GHz frequency were changed with varying the Sr²⁺ ion composition. The high imaginary components of dielectric permittivity for x = 0.3 was found at higher frequency region around 1.6 GHz compared with the samples with x values of 0.5 and 0.7 in which the frequency regions were around 1.25 and 1.15 GHz, respectively.     

Effects of composition on the microwave absorbing properties of FexNi100−x (x = 0–25) submicro fibers

The morphology and composition of a material have important influences on its electromagnetic parameters and microwave absorbing property. In this paper, the Fe_xNi_100?x (x?=?0–40) powders were prepared by an oxalate precipitation-thermal decomposition process. When x?=?0–25, the powders show submicro fibrous morphologies, however, when x?>?25, some granular particles appear due to the weak coordination ability of ethylenediamine with Fe²⁺ in the oxalate precipitation process. The electromagnetic parameters and microwave absorbing properties of the composites of the Fe_xNi_100?x (x?=?0–25) submicro fibers with 80?wt% paraffin were studied in 2–18?GHz. Compared with the Ni/paraffin composite, enhanced electromagnetic losses and microwave absorbing properties are observed for the Fe₁₀Ni₉₀/paraffin and Fe₂₅Ni₇₅/paraffin composites. The Fe₁₀Ni₉₀/paraffin shows the best microwave absorbing property that at the composite thickness of 2.0?mm, the effective bandwidth and minimum RL reach 3.78?GHz (12.21–15.99?GHz) and ?45.37?dB at 13.90?GHz, respectively. The excellent microwave absorbing property of the Fe₁₀Ni₉₀/paraffin composite is from its high loss factor and the quarter-wavelength cancellation mechanism.     

Effect of Ho-doping on structural,electrical and magnetic properties of La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> ceramics prepared by Plasma-Activated Sintering

Different components of La_0.7?x Ho _x Sr_0.3MnO₃ (LHSMO, x = 0, 0.1, 0.2, 0.3) ceramics were fabricated by Plasma-Activated Sintering (PAS), so as to study the correlation between the contents of Ho³⁺ and the structural, electrical, magnetic properties. XRD and SEM confirmed that LHSMO ceramics prepared by PAS exhibited high-purity phase and dense microstructure. The measurement of electrical resistivity showed that the resistivity of LHSMO ceramics increased, and the metal–insulator transition temperature decreased with the increasing Ho-doping content. The resistivity data were then fitted using various empirical equations, and the conduction mechanism of LHSMO ceramics was found to be in accord with the electron–magnon scattering process in the low-temperature region and the small polaron hopping model in the high-temperature region. Lastly, we calculated the values of magnetoresistance of the LHSMO ceramics, which increased with increasing Ho-doping content, from 3.5% for x = 0 to 14.6% for x = 0.3. Therefore, the doping of Ho³⁺ into La_0.7Sr_0.3MnO₃ can effectively enhance the low-field magnetoresistance effect.     

Enhanced multiferroic and dielectric properties of Sr2+-doped BiFe0.94(Mn0.04Cr0.02)O3 thin films

Sr²⁺-doped B_1?xSr_xFe_0.94(Mn_0.04Cr_0.02)O₃ (B_1?xSr_xFMC, x = 0.00, 0.05 and 0.09) thin films were prepared on FTO/glass (SnO₂: F) substrates by using a sol–gel spin-coating method. X-ray diffraction (XRD), Rietveld refined XRD data and Raman scattering spectra illustrate a structural evolution from trigonal (R3c: H) to tetragonal (P4) occurs in B_1?xSr_xFMC thin films with the increase of Sr²⁺ concent. Superiorly multiferroic and dielectric properties were obtained in the B_1?xSr_xFMC thin films, e.g., large remanent polarization value, a high dielectric constant (P _r = 139.21 μC/cm² and ε_r = 396.7 for x = 0.09) and large saturation magnetization (M _s = 2.08 emu/cm³ for x = 0.05). The leakage current density of B_1?xSr_xFMC thin films is increased with increasing Sr²⁺ concentration, wherein the leakage current density of all the B_1?xSr_xFMC films is of the order of magnitude of 10^?5 A/cm², which are still lower than that in the pure BFO film (10^?3 A/cm²).     

Critical behavior in Fe-doped manganites La0.67Ba0.22Sr0.11Mn1−x Fe x O3 (0 ≤ x ≤ 0.2)

The critical properties of the manganese perovskite La_0.67Ba_0.22Sr_0.11Mn_1?x Fe _x O₃ (0 ≤ x ≤ 0.2) around the paramagnetic (PM)–ferromagnetic (FM) phase transition were investigated using modified Arrott plots and Kouvel–Fisher method based on the data of static magnetic measurements recorded around the curie temperature T _C. Based on the evaluated critical exponents for the compounds with different x values (0.378 ≤ β ≤ 0.411, 1.247 ≤ γ ≤ 1.393, and 4.018 ≤ δ ≤ 4.73) we concluded that the PM–FM phase transition in the compound with x = 0 belongs to the three-dimensional Heisenberg universality. However, the substitution of Fe³⁺ for Mn³⁺ resulted in shifting the value of γ toward the 3D-Ising model value and a slight increase in β. This behavior was discussed and correlated with the random distribution of Fe³⁺ at the Mn³⁺ sites and the consequent suppression the double exchange interaction.     

Synthesis of Fe–Co alloy and cobalt–magnetite composites doped with Nd3+ by using iron disproportionation

Iron–cobalt alloy and cobalt–magnetite composites doped with Nd³⁺ (Co _x Fe_1?x /Co _y Fe_1?y Nd _z Fe_2?z O₄) in which the Fe alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe(OH)₂ and the reduction of Co(II) by Fe⁰ in a concentrated KOH solution. Powder X-ray diffraction, scanning electron microscope and vibrating sample magnetometer were employed to characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe(II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by Nd³⁺ ions on structure, magnetic properties of the composites were investigated. From the perspective of thermodynamics, we explain the postulated mechanism of the disproportionation reaction.     

Dielectric constant, magnetic permeability and microwave absorption studies of hot-pressed Ba-CoTi hexaferrite composites in X-band

The microwave absorption, complex permittivity and complex permeability studies of hot-pressed hexaferrite composites prepared with Ba(CoTi)_xFe_12-2xO₁₉ (x = 0.0, 0.2, 0.4, 0.8, and 1.0) were made in the frequency range from 8.0 to 12.4 GHz. The hexaferrite composites with x > 0.0 exhibit significant dispersion in the complex permittivity (ε_r′-jε_r″). However the dispersion in complex permeability (μ_r′-jμ_r″) is not significant and is attributed to the shielding effect of polymer matrix over the ferrite crystallites. The reflection loss has been studied as function of frequency, composition and thickness of absorber. A comparison of reflection loss of hot-pressed ferrite composites with that of normal sintered ferrite composites was made and analyzed. A minimum reflection loss of—24.0 dB is obtained at 9.9 GHz for 2.8 mm thick sample of BaCo_0.4Ti_0.4Fe_11.2O₁₉ hot-pressed hexaferrite composite.     

Structural and magnetic properties of Ba0.7Sr0.3Fe12 − 2x Co x Ti x O19 M-type hexaferrites

We have studied the effect of “double” substitution in Ba_0.7Sr_0.3Fe_{12 ? 2x} Co _x Ti _x O₁₉ on the structural and magnetic properties of M-type barium hexaferrite. The basic composition of Ba_{1 ? x} Sr _x Fe₁₂O₁₉ obtained by heat-treating carbonate-hydroxide precipitates has been optimized (x = 0.3). 2Fe³⁺ → Co²⁺ + Ti⁴⁺ substitutions considerably reduce the coercive force (H _c) and increase the magnetization (M _s) relative to Ba_0.7Sr_0.3Fe₁₂ O₁₉.     

Synthesis and microwave absorbing properties of Mn–Zn nanoferrite produced by microwave assisted ball milling

In this paper, well dispersed spinel Mn_xZn_1?xFe₂O₄ (x = 0.3,0.5 and 0.7) were obtained by microwave assisted ball milling at 2.45 GHz through only one step. The synthesized products were characterized by X-ray diffraction, high resolution transmission electron microscope, vibration sample magnetometer, and vector network analysis. Synthesized Mn–Zn nanoferrite showed the saturation magnetization reached 84.91emu/g when the x was 0.7 and the largest magnetic loss tangent at the frequency of 2.45 GHz. Microwave absorbing properties of these composites were studied at the frequency range of 2–18 GHz. Two microwave reflection loss peaks appeared for all the spinel ferrite. When x was 0.5, the minimum reflection loss appeared at the highest frequency. When x was 0.7, these two minimum reflection loss peaks, ?17.36 and ?48.13 dB, were calculated with the ?10 dB bandwidth at the frequency ranges of 2.24–5.04 and 13.28–14.88 GHz, respectively. Resonance reflection loss peaks shifted to lower frequencies when the matching thickness increased.     

Synthesis and characterization of SrFe12O19 nanoparticles produced by a low-temperature solid-state reaction method

Ultra-fine nanoparticles of strontium hexaferrite, SrFe₁₂O₁₉, have been synthesized by low-temperature solid-state reaction method without ball-milling process. The effects of the preparing conditions of samples such as calcination temperature, Fe³⁺/Sr² and Na¹⁺/Sr²⁺ mole ratio on the phase composition, particle size and shape and magnetic properties of the calcined samples have been investigated by differential thermal analysis, X-ray diffraction, field emission scanning electronic microscopy, and vibrating sample magnetometery. The X-ray diffraction patterns showed the formation of SrFe₁₂O₁₉ single phase at temperatures as low as 750° C. Fine particles with particle size around 30–100 nm obtained at 750° C with Fe³⁺/Sr² mole ratio of 10. The magnetic measurements and structural analysis showed that particles were single domain and exhibited better magnetic properties than those obtained by the ceramic method.     

Synthesis and characterizations of Nd doped SrFe12O19 nanoparticles

This is the first report ever on Nd³⁺ doped M-type hexaferrite nanoparticles: SrNd_xFe_12−xO₁₉ (0 ≤ x ≤ 1) prepared by citrate precursor using the sol–gel technique followed by gel to crystallization. The influence of the Nd³⁺ substitution, Fe³⁺/Sr²⁺ molar ratio and the calcination temperature on the crystallization of ferrite phase have been examined using powder X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), inductance capacitance resistance meter bridge (LCR) and vibrating sample magnetometer (VSM). The structural analysis reveals that the Nd³⁺ ions rearrange themselves in the host lattice without disturbing the parent lattice and Fe³⁺/Sr²⁺ molar ratio less than 12 is more favorable to achieve single phase hexaferrite at calcination temperature 900 °C for 4 h. Mid-IR analysis confirms that Nd³⁺ occupies the octahedral site. Detailed studies of electrical properties of prepared materials have been investigated in the frequency range 100–1000 Hz at room temperature by LCR meter and two probe technique. The result shows that the electrical properties strongly depend upon the frequency of applied field and dopant concentration. The magnetic measurements showing a considerable improvement in coercivity with the substitution of Nd³⁺ on iron sites, while the unsubstituted hexaferrites have highest value of specific saturation magnetization.     

Effect of Sm3+ on dielectric and magnetic properties of Y3Fe5O12 nanoparticles

The Sm³⁺ doped Y_3?xSm_xFe₅O₁₂ (x = 0–3) nanopowders were prepared using modified sol–gel route. The crystalline structure and morphology was confirmed by X-ray diffraction and atomic force microscopy. The nanopowders were sintered at 950 °C/90 min using microwave sintering method. The lattice parameters and density of the samples were increased with an increase of Sm³⁺ concentration. The room temperature dielectric (ε′ and ε″) and magnetic (μ′ and μ″) properties were measured in the frequency range up to 20 GHz. The room temperature magnetization studies were carried out using Vibrating sample magnetometer using filed of 1.5 T. Results of VSM show that the saturation and remnant magnetization of Y_3?xSm_xFe₅O₁₂ (0–3) decreases on increasing the Sm concentration (x). The low values of magnetic (μ′ and μ″) properties makes them a good candidates for microwave devices, which can be operated in the high frequency range.     

Structural and Magnetic Properties of Nano-structured Eu 3+ Substituted M-Type Hexaferrites Synthesized by Sol-Gel Auto-combustion Technique

Nano-structured M-type hexaferrites having the nominal composition Sr_0.8Ca_0.2Eu _x Fe_12?x O₁₉ (x=0.0, 0.05, 0.1, 0.15, 0.2, 0.25) have been synthesized by a sol-gel auto-combustion technique. The aim of the present study is to investigate the effect of rare-earth Eu³⁺ ions substitution at Fe³⁺ site on the structural and magnetic properties of M-type hexaferrites that might have not been previously explored especially using the sol-gel auto-combustion technique. The samples have been characterized by Differential Scanning Calorimetry (DSC), Fourier Transform Infra-Red (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray fluorescence (ED-XRF) and vibrating sample magnetometer (VSM). The XRD analysis confirms the formation of single M-type hexaferrite phase. The ratio ‘c/a’ lies in the expected range of 3.946–3.951 for M-type hexaferrites phase. The crystallite size was found to be in the range of 15–45 nm, which is sufficient to obtain the suitable signal to noise ratio in the high density recording media. Scanning Electron Microscopy (SEM) analysis exhibits the morphology of grains to be hexagonal platelet. The values of remanence (M _r ) and maximum magnetization (M) lie in the range 31–68 emu/g and 47–90 emu/g, respectively. The coercivity (H _c ) values lie in the range of 2412–4046 Oe and enhancement in the coercivity may be due to increase in the shape anisotropy. The magnetic properties such as coercivity (H _c ), magnetization (M), and retentivity (M _r ) make the synthesized materials useful for applications in the recording media.     

Synthesis and Properties Investigation of Non-equivalent Substituted W-Type Hexaferrite

A novel composed W-type hexaferrite Ba_1?x La _x Co₂Fe₁₆O₂₇ was rapidly synthesized via a sol–gel self-combustion reaction. The effects of lanthanum ions on the oxidation state of iron ions and cobalt ions in hexaferrite were explored by X-ray photoelectron spectroscopy. The changes of the Fe 2p X-ray absorption spectra indicated that the nonequivalent substitution can lead to the transition Fe³⁺→ Fe²⁺ in Ba_1?x La _x Co₂Fe₁₆O₂₇. However, the oxidation state of cobalt ions was maintained as Co²⁺. Moreover, the effects of La content on the phase composition, structural parameters, morphology, and static magnetic properties were also investigated in detail by using the X-ray diffractometer, scanning electron microscope, and vibrating sample magnetometer. The results indicated that the structural parameters decreased regularly with increasing the La content, and the magnetic properties were enhanced after substitution, which is beneficial for their application in various electrical devices employed for industrial and military applications.     

Ba<Subscript>1−x</Subscript>La<Subscript>x</Subscript>Fe<Subscript>12</Subscript>O<Subscript>19</Subscript> (x = 0.0, 0.2, 0.4, 0.6) hollow microspheres: material parameters,magnetic and microwave absorbing properties

Ba_1?xLa_xFe₁₂O₁₉ (x = 0.0, 0.2, 0.4, 0.6) hollow ceramic microspheres (HCMs) have been prepared by combining self-reactive quenching method with heat treatment. Their material parameters, magnetic and microwave absorbing properties were investigated. It was observed that after doping of lanthanum, the material parameters showed a little change except hexagonal crystal disappearing. And the magnetic properties of HCMs were decided by lanthanum content and material parameters. With the lathanum increases from 0.0 to 0.6, the saturation magnetization (Ms) values initially increased, and then decreased sharply to a minimum value, and increased again, moreover, the coercive force (Hc) values were reduced first, and then increased, and decrease to a minimum value. Absorbing properties tests indicate that after La³⁺-doped, at 2 mm thickness, the effective absorbing band (<?10 dB) was reduced to 4.7, 5 and 4.4 GHz, respectively, the minimum reflectance would decrease in low substituted level (x ≤ 0.4) and increase in high level (x = 0.6), and the frequency shifts to low frequency with the increasing of doping content. In 1.5–3 mm range, with the increasing of thickness, the absorption peak of Ba_1?xLa_xFe₁₂O₁₉ (x = 0.2, 0.4, 0.6) HCMs shifts to low frequency and the absorption intensity increases, the effective absorbing band can up to 10, 8.1 and 8 GHz, respectively.     

Dielectric tunable properties of Ba0.5Sr0.5TiO3–MgMoO4 composite ceramics for microwave applications

(1?x) Ba_0.5Sr_0.5TiO₃–xMgMoO₄ (x = 0, 5, 10, 20 and 30 wt%) composite ceramics were prepared via solid state reaction processing. Their structure and dielectric properties were systematically characterized. The introduction of MgMoO₄ resulted in a change in lattice constant of the perovskite phase and partial reaction between MgMoO₄ and Ba_0.5Sr_0.5TiO₃ occurred in the sintering process. Both X-Ray Diffraction (XRD) and Back-scattered Electron Images (BEI) analysis show the co-existence of three phase structures of BST, MgMoO₄ and BaMoO₄. With increasing of MgMoO₄ content, the tunability of the composite ceramics was decreased due to the increase of the amount of non-ferroelectric phases. The Curie temperature Tc of the samples gradually shifted to low temperatures with increasing of MgMoO₄ content. Dielectric constant can be adjusted in the range from 2035 to 150, meanwhile maintain a relatively high tunability and Q values. The sample with 20 wt% MgMoO₄ possesses a tunability of 10 %, a low dielectric constant of 111 and an appropriate Q value of 183 (2.240 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Microwave dielectric properties of (1 − y)Nd(1−2x/3)Bax(Mg0.5Sn0.5)O3–yCa0.8Sr0.2TiO3 ceramic

The (1 ? y)Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were prepared by the conventional solid-state method. The X-ray diffraction patterns of the Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ ceramics revealed that Nd_(1?2x/3)Ba_x(Mg_0.5Sn_0.5)O₃ is the main crystalline phase, which is accompanied by a little Nd₂Sn₂O₇ as the second phase. An apparent density of 6.89 g/cm³, a dielectric constant (ε _r ) of 19.1, a quality factor (Q × f) of 212,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?68 ppm/°C were obtained when the Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃ ceramics were sintered at 1,550 °C for 4 h. The temperature coefficient of resonant frequency (τ _f ) increased from ?68 to +55 ppm/°C as y increased from 0 to 0.7 when the (1 ? y)Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sr_0.2TiO₃ ceramics were sintered at 1,600 °C for 4 h. 0.4Nd_2.94/3Ba_0.03(Mg_0.5Sn_0.5)O₃–0.6 Ca_0.8Sr_0.2TiO₃ ceramic that was sintered at 1,600 °C for 4 h had a τ _f of ?7 ppm/°C.     

Microwave dielectric properties and low temperature firing of (1 − x)Li2Zn3Ti4O12 –xLi2TiO3 (0.2 ≤ x ≤ 0.8) ceramics with B2O3–CuO addition

In the present work, the (1 ? x)Li₂Zn₃Ti₄O₁₂–xLi₂TiO₃ (0.2 ≤ x ≤0.8) ceramics were prepared via the solid state reaction method. The 0.8Li₂Zn₃Ti₄O₁₂–0.2Li₂TiO₃ ceramic sample sintered at 1,160 °C for 2 h demonstrated high microwave dielectric property with a relative permittivity of 18.0, a high quality factor (Q × f) ~ 100,000 GHz (at 7.2 GHz), and a temperature coefficient of resonant frequency about ?47.8 ppm/°C. With 2.0 wt% 0.4B₂O₃–0.6CuO addition, a relative permittivity of 17.5, a Q × f value of 71,000 GHz and a temperature coefficient of resonant frequency of ?44.4 ppm/^oC can be obtained in 0.8Li₂Zn₃Ti₄O₁₂–0.2Li₂TiO₃ ceramic sintered at 925 °C for 5 h and the chemical compatibility with silver electrode indicates that the ceramics may be a suitable candidate for the low temperature co-fired ceramic technology application.