Optimization of the magnetic properties and microstructure of Co–La substituted strontium hexaferrite by varying the production parameters

The aim of this paper was to improve the magnetic properties of magnetoplumbite-type (M-type) strontium hexaferrite substituted with Co²⁺–La³⁺ produced by conventional ceramic forming techniques. The effect on the magnetic properties of varying the composition of the target compound Sr_1−xLa_xFe_12−yCo_yO₁₉ and the primary and secondary firing temperatures was investigated. Microstructure studies and XRD phase analysis indicated that optimum values of the remanent magnetization B_r and coercive field H_cj were obtained with a primary firing temperature of 1240 °C and a final firing temperature of 1180 °C, where (x=y)_th ¹=0.15, (y/x)_exp ²=0.75 and the molar ratio of ferric oxide to strontium oxide=5.8. The optimized magnetic properties obtained under these conditions were B_r=4070 Gs, H_cj=4710 Oe, (H_k/H_cj)=82.     

A comparative study of the magnetic and microwave properties of Al3+ and In3+ substituted Mg–Mn ferrites

The effect of substitution of diamagnetic Al³⁺ and In³⁺ ions for partial Fe³⁺ ions in a spinel lattice on the magnetic and microwave properties of magnesium–manganese (Mg–Mn) ferrites has been studied. Three kinds of Mg–Mn based ferrites with compositions of Mg_0.9Mn_0.1Fe₂O₄, Mg_0.9Mn_0.1Al_0.1Fe_1.9O₄, and Mg_0.9Mn_0.1In_0.1Fe_1.9O₄ were prepared by the solid-state reaction route. Each mixture of high-purity starting materials (oxide powders) in stoichiometric amounts was calcined at 1100 °C for 4 h, and the debinded green compacts were sintered at 1350 °C for 4 h. XRD examination confirmed that the sintered ferrite samples had a single-phase cubic spinel structure. The incorporation of Al³⁺ or In³⁺ ions in place of Fe³⁺ ions in Mg–Mn ferrites increased the average particle size, decreased the Curie temperature, and resulted in a broader resonance linewidth as compared to un-substituted Mg–Mn ferrites in the X-band. In this study, the In³⁺ substituted Mg–Mn ferrites exhibited the highest saturation magnetization of 35.7 emu/g, the lowest coercivity of 4.1 Oe, and the highest Q×f value of 1050 GHz at a frequency of 6.5 GHz.     

Preparation and characterization of Co2+ substituted Li–Dy ferrite ceramics

Stoichiometric compositions of ferrites with the chemical formula Li_0.5?0.5xCo_xFe_2.4?0.5xDy_0.1O₄ with x=0, 0.25, 0.5, 0.75, 1.0 were prepared by the standard double sintering ceramic method. X-ray diffraction analysis confirmed the cubic spinel structure of the prepared samples. The structural, morphological and magnetic properties were studied by X-ray diffraction, infra-red spectroscopy (IR), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM) and ac susceptibility measurements. Lattice constant, grain size and density increase whereas porosity decreases with the increase in Co²⁺ substitution. IR measurements show the characteristic ferrite bands. Spectral absorption bands were observed in IR spectroscopic analysis at ν₁=564?601 cm^?1, ν₂=486?519 cm^?1 and ν₃=551?578 cm^?1. The cation distribution estimated by the X-ray diffraction is supported by magnetization and susceptibility studies. The saturation magnetization decreases from 44.25 to 17.14 emu/g whereas coercivity remarkably increases from 240.69 to 812.14 emu/g with increasing Co²⁺ substitution. The mechanisms involved are discussed.     

Investigation of structural,magnetic and Mössbauer properties of Co2+ and Cu2+ substituted Ni–Zn nanoferrites

We investigated the effects of Co²⁺ and Cu²⁺ substitution on the super-exchange interactions in Ni–Zn nanoferrites. The cation distribution technique was taken into account to explain the results. To authenticate the cation distribution, we have estimated the cation distribution in the light of X-ray diffraction method, Mössbauer spectroscopic analysis, and magnetization study. Statistical model based on the cation distribution was used to calculate the Curie temperature. The values of magneton number n_B and Curie temperature T_C calculated by using the cation distribution is found to be in agreement with the experimentally obtained values.     

Physical and magnetic properties of Mn–Zr doped La–Sr ferrite prepared by the auto-combustion route

This is the first report ever on (Mn²⁺–Zr⁴⁺) doped M-type lanthanum strontium hexaferrite with general formula, Sr_0.85La_0.15(MnZr)_xFe_12−2xO₁₉ where x=0.0, 0.25, 0.50, 0.75, and 1.0, prepared by citrate auto-combustion method. These ferrites were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDX) and Vibrating sample magnetometer (VSM). X-ray diffraction patterns show the formation of high purity hexaferrite phase without other secondary phases for all the synthesized samples. It was observed from magnetic hysteresis data that the coercive force is reduced from 5692.5 Oe to 1669.2 Oe with increase in doping contents but the net magnetization of the samples varies slightly from 60.6 to 55.2 emu/gm. High saturation magnetization (M_s), low coercivity (H_c) and remanence magnetization (M_r) values of these materials make them particularly suitable for data recording.     

Synthesis,characterization and optical properties of Zr+4/La+3/Nd+3 tri-doped yttria nanopowder by sol–gel combustion method

In this research, zirconium lanthanum and neodymium tri-doped yttria nanopowder was synthesized using the sol–gel combustion method. Citric acid (CA) and glycine (G) were used for the gel and fuel agent, respectively. The effect of CA:G:TM (TM=transition metal) mole ratios on the particle size and morphology of the product was evaluated. As-synthesized samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy dispersion spectrum (EDS), photoluminescence (PL) spectrum, Inductively coupled plasma (ICP), UV–visible (UV–vis spectroscopy), thermal gravimetric-differential thermal analysis (TG/DTA) and Fourier transform infrared (FTIR)analysis. The optimized sample synthesized with CA:G:TM mole ratio of (1.06:1.06:1) had the average particle size of 30–40 nm with a spherical morphology. Moreover, the optimized sample showed visible photoluminescence, at 400–800 nm.     

Dielectric and magnetic properties of substituted Li–Zn ferrite thick films clouded over a half wavelength microstrip rejection filter

Li_0.35–0.5x Mg _x Zn_0.3Fe_2.35–0.5x O₄ nanoparticles (x = 0, 0.14, 0.28, 0.42, 0.56, and 0.70) were prepared by low-cost combustion synthesis at relatively low temperature. Thick films of Li–Mg–Zn ferrite were prepared by screen printing method and characterized by XRD and microwave characteristics at microwave frequencies (within X- and Ku-bands) by overlay technique using λ/2 rejection filter. The influence of magnesium content x on the resonance frequency, quality factor, effective dielectric constant, complex permittivity, complex permeability, microwave conductivity, and penetration depth was explored.     

Structural and magnetic evaluation of substituted NiZnFe2O4 particles synthesized by conventional sol–gel method

The aim of this study is to evaluate the structural and magnetic properties of Ni–Zn doped ferrite with trivalent Al³⁺ and Cr³⁺ cations substitution in Ni_0.6Zn_0.4Fe_2−xCr_x/2Al_x/2O₄ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) synthesized by employing conventional sol–gel method. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), Mössbauer spectroscopy (MS) and vibrating sample magnetometer (VSM) analysis were carried out in order to characterize the structural and magnetic properties of particles. The XRD results confirmed the formation of single phase of spinel ferrite particles for a whole series of samples. The results of FTIR analysis indicated that the functional groups of Ni–Zn spinel ferrite were formed during the sol–gel process. Furthermore, FE-SEM micrographs revealed that the distribution of particles size is narrow. According to Mössbauer spectra,the doped cations are replaced in iron site occupancy of octahedral sites. It was found that with an increase in substitution contents magnetization decreased due to occupation of Al and Cr cations at low level substitutions in octahedral sites.     

Production of Cr3+ substituted Li–Zn nanocrystalline ferrite by citrate method: Studies on structure,cation occupancy,elastic, optical and magnetic performance

The structural, elastic, optical and magnetic characteristics of Li_0.4Zn_0.2Cr_xFe_2.4-xO₄ (0.0 ≤ x ≤ 0.5; step 0.1) produced using a citrate precursor were studied. X-ray powder diffraction data indicate that all of the generated samples are single-phase spinel structures with no additional phases. The lattice parameter reduces from 8.355 Å to 8.333 Å when the chromium content rises. The crystallite sizes of the compositions were assessed by Scherrer's and Williamson-Hall (W–H) approaches. Infrared (IR) spectroscopy revealed two significant absorption bands generated by vibrations at the tetrahedral and octahedral sites. The elastic moduli (bulk modulus ‘B’, rigidity modulus ‘G’ and Young's modulus ‘E’) as well as the Debye temperature (θ_D) assessed by IR spectroscopy rise as the Cr³⁺ ions concentration increases. Chromium addition inhibits grain growth and enhances the mechanical strength of Li–Zn nanoferrites. Diffuse reflectance spectra (DRS) were utilized to evaluate the optical band gap (E_g) of Li–Zn–Cr nanoferrite, which was found to drop from 1.96 to 1.84 eV. The vibrating sample magnetometer (VSM) was used to perform the magnetic analysis, and various magnetic parameters were derived using the M ? H curves results. Acceptable values of saturation magnetization (78.6–44.05 emu/g) and coercivity (30.87–44.65 G) were found in this system, making these nanoferrites ideal for high-density recording medium and electromagnets applications. Based on the experimental results of lattice parameters, and magnetization, a quite reasonable cation distribution was postulated for all samples. Theoretically predicted lattice parameters and magnetic moments derived from the suggested cation distribution agree with those determined empirically from XRD and VSM results, respectively. The switching field distribution curves were schemed utilizing the first derivative of magnetization data from M ? H loops. The Curie temperature decreases significantly with Cr³⁺ substitution.     

Effects of Y–Co co-substitution on the structural and magnetic properties of M-type strontium hexaferrites

Y³⁺- and Co²⁺-substituted Sr_1-xY_xFe_12-xCo_xO₁₉ (0 ≤ x ≤ 0.50) M-type hexaferrites were synthesized using a traditional oxide ceramic process to study their structural and static magnetic properties. The well-defined M-type phase structures of the pure and Y–Co co-substituted strontium ferrites were verified via XRD analysis. When the Y–Co substitution amount (x) exceeded 0.20, the Fe₂O₃, Y₃Fe₅O₁₂, SrFe₂O₄, and CoFe₂O₄ impurity phases coexisted in the M-type strontium hexaferrite structure. The lattice parameters a and c increased when x ≤ 0.20; however, a further increase in the Y–Co substitution caused them to decrease. The X-ray density d_x initially decreased when x ≤ 0.20, and subsequently increased with a further increase in Y–Co substitution. The density of the sintered samples d_s exhibited a decreasing trend with the increasing Y–Co substitution, inducing the porosity to increase. The saturation magnetization M_s monotonously decreased with the increasing Y–Co substitution amount. The in-plane and out-of-plane coercivities, H_c(ip) and H_c(op), initially increased as x increased from 0 to 0.20. When x > 0.20, however, H_c(ip) exhibited a decreasing trend; particularly, a linear decrease was observed as x increased from 0.30 to 0.50. The squareness ratio S reached its maximum (79.6%) at x = 0.20.     

Sintering behavior of La2O3–B2O3–TiO2 glass powders prepared by spray pyrolysis for low temperature co-fired ceramics

Fine-sized La₂O₃–B₂O₃–TiO₂ glass powders with spherical shape were directly prepared by spray pyrolysis at a temperature of 1500 °C. The optimum flow rate of the carrier gas to prepare the glass powders with dense inner structure and fine size by complete melting was 10 L/min. The ratio of La/Ti was identified to be 2.06:1, which was close to the original starting ratio of La/Ti in mixture of the spray solution. The T_g and T_c of the powders were 614 and 718 °C. The crystal structures within the powders were observed from the sintered disc at 630 °C. The mean sizes of the powders changed from 0.24 to 0.71 μm when the concentrations of the spray solution were changed from 0.025 to 0.5 M. The BET surface areas of the powders changed from 4.4 to 1.6 m²/g. The grain sizes of the sintered discs increased with increasing the sintering temperatures. The main crystal structure of the sintered discs was LaBO₃.     

Crystal structure refinement and the magnetic and electro-optical properties of Er3+–Mn2+-substituted Y-type barium hexaferrites

The influence of Er³⁺–Mn²⁺ substitution on the properties of Y-type hexaferrites (chemical composition: Ba_2–xEr_xZn_0.6Co_0.6Cu_0.8Fe_12?yMn_yO₂₂ (x = 0.0, 0.3, and 0.5 and y = 0.0, 0.4, and 0.6)), which were synthesized by the sol-gel autocombustion method, was investigated. The X-ray diffraction spectra were analyzed by the Rietveld refinement method, and hexaferrite was observed to possess a single-phase crystalline structure, whereas the Fourier-transform infrared spectra clarified the formation of the iron oxide base material. The morphology of the grains revealed that they were hexagonal and without agglomeration. The band gap of the samples decreased as the Er³⁺–Mn²⁺ concentration increased. Dielectric and impedance spectroscopies of the prepared samples indicated the role of polarization in the variation in the dielectric and impedance parameters. Particularly, the occurrence of space-charge polarization increased the dielectric constant at lower frequencies. Further, the Cole–Cole plot revealed a semicircle in the lower frequency region, thereby indicating that the grain boundary contributed the most to the dielectric constants. Modulus spectroscopy revealed that the charge mobility increased as the concentration of Er³⁺–Mn²⁺ increased. Additionally, the magnetic analysis indicated that Mn²⁺ preferably replaces Fe³⁺ at the octahedral site, thereby reducing the magnetization of the prepared samples through a reduced superexchange interaction. Furthermore, increasing the coercivity values thermally stabilized the sample, and this is vital for perpendicular magnetic recording.     

Microstructure and magnetic properties of Ni0·75Zn0·25Fe2O4 ferrite prepared using an electric current-assisted sintering method

Using a Ni_0·75Zn_0·25Fe₂O₄ nanopowder synthesized by means of a hydrothermal method as a raw material, polycrystalline nickel zinc (NiZn) ferrite ceramics composed of sub-micron grains were successfully prepared via an electric current-assisted sintering method. Temperatures ranging from 800 °C to 950 °C and a dwell time of 20 min were employed. The phase composition and microstructure of the samples were characterized via X-ray diffraction and scanning electron microscopy, respectively. Moreover, the magnetic properties of the samples were investigated using a vibrating sample magnetometer and a ferromagnetic resonance system. The results revealed that each sintered sample was mainly composed of a spinel phase. With increasing sintering temperature, the specific saturation magnetization increased from 71.85 emu/g to 74.58 emu/g, owing mainly to the increase in the relative density and the average grain size of the NiZn ferrites. The coercivity and ferromagnetic resonance linewidth of the ferrite ceramics decreased monotonically with increasing sintering temperature, owing mainly to the magnetostriction coefficient, saturation magnetization, and porosity of the sintered ferrites.     

Rietveld refined crystal structure,magnetic, dielectric,and electric properties of Li- substituted Ni–Cu–Zn ferrite and Sm,Dy co-doped BaTiO3 multiferroic composites

The conventional solid-state reaction technique is used to fabricate the multiferroic xLi_0.1Ni_0.3Cu_0.1Zn_0.4Fe_2.1O₄(LNCZFO)+(1-x)Ba_0.95Sm_0.05Ti_0.95Dy_0.05O₃(BSTDO) composites. To determine the ferrite and ferroelectric phases, the Rietveld refinement analysis is used. The excellent fit of experimental diffraction data is confirmed by the low values of reliability factors and the goodness of fit index, and so the crystal structure is perfect. Increasing the LNCZFO phase in the composites causes the formation of more ferrite grains and enhancement of magnetization values. The anisotropy field varies due to compressive stress created by a lattice mismatch between the BSTDO and LNCZFO phases. The dielectric peak shifts to higher temperatures as the ferrite phase increases, indicating that magnetoelectric interaction between the constituent phases exists in composites. At 100 kHz, the diffuseness exponent ranged from 1.01 to 1.79, indicating that a diffuse phase transition (DPT) occurred for some composites. As the ferrite content increases, the DPT effect decreases, resulting a narrower dielectric peak. The small polaron hopping mechanism is responsible for electrical conduction, which followed Jonscher's power law. The magnitude of the angular frequency exponent factor increases with frequency, indicating an increase in charge carrier mobility from long to short range.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

Preparation and self-healing performance of spherical α-Al2O3@MoSi2 particles using the precipitation method

α-Al₂O₃ was coated on the powder surface using the precipitation method to improve the pre-oxidation resistance of molybdenum disilicide in thermal barrier coatings (TBCs). The coating effects of four different aluminum sources (Al(NO₃)₃·9H₂O) to MoSi₂ mass ratios were evaluated by viscosity and micro-morphology. The shell-forming effects on the powder size and post-treatment were thoroughly analyzed to meet the requirements for practical application in TBCs. The results showed that the precipitation method was superior to the sol-gel process in terms of the shell thickness obtained. Optimal shell-forming was generated at 1200 °C in an inert atmosphere. A bonding layer (Al₆Si₂O₁₃) would be formed at the core-shell interface, further preventing oxidation penetration. The self-healing particles MoSi₂@α-Al₂O₃ can effectively seal micro-cracks (width below 300 nm) because of the fluidity of SiO₂ at working temperatures.     

Synthesis of γ-Ga2O3-Al2O3 solid solutions by spray pyrolysis method

Synthesis of the γ-Ga₂O₃-Al₂O₃ solid solutions by spray pyrolysis was examined. Spherical particles were obtained using an aqueous solution of Al(NO₃)₃ and Ga(NO₃)₃ with HNO₃. For Ga-rich composition, γ-phase solid solutions were directly crystallized by the spray pyrolysis. For Al-rich composition, spray pyrolysis gave amorphous products unless a sufficient thermal energy was supplied during the spray pyrolysis. Subsequent calcination of the amorphous products gave γ-Ga₂O₃-Al₂O₃ solid solutions. However, physical properties of the solid solutions were affected by the spray pyrolysis conditions.     

Influence of La3+ additions on the phase behaviour and antibacterial properties of ZrO2–SiO2 binary oxides

The present study reports the influence of lanthanum (La³⁺) content on the phase stability and antibacterial activity of ZrO₂–SiO₂ binary oxides. Four different concentrations of La³⁺ additions in ZrO₂–SiO₂ binary oxides were synthesized using a sol–gel technique. Heat treatment of the synthesized powders resulted in the formation of t-ZrO₂ phase at 1000 °C. Heat treatment beyond 1000 °C resulted in the phase degradation of t-ZrO₂ to yield m-ZrO₂ and ZrSiO₄. Results from antibacterial tests confirmed the potential activity of La³⁺ doped ZrO₂–SiO₂ binary oxides in countering the microbial invasion.     

Effects of Co-substitution on the structural and magnetic properties of NiCoxFe2−xO4 ferrite nanoparticles

The sol–gel auto-combustion method was used to prepare nanocrystalline powders of Co-substituted nickel ferrite with the general formula NiCo_xFe_2−xO₄ (x=0.0, 0.1, 0.25, and 0.5). The effects of Co-doping on the structural, morphological, and magnetic properties of the samples were subsequently evaluated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). Using the MAUD program, the full pattern fitting of Rietveld method was employed to determine the exact coordinate of the atoms, unit cell dimensions, and ion occupancy. X-ray diffraction measurements by Rietveld refinement confirmed the crystalline structure and phase purity of all the ferrites prepared. FTIR results also confirmed the formation of a spinel phase and FE-SEM images showed that the particles were in the nanosize range. Moreover, Rietveld analysis and saturation magnetization (M_s) revealed that Co³⁺ replaced Fe³⁺ in the tetrahedral A-sites up to x=0.1. then, it replaced Fe³⁺ in both A- and B-sites for x≥0.25. Finally, VSM results demonstrated that while M_s remained nearly constant with increasing Co³⁺ substitution, coercivity (H_c) increased significantly. It may be suggested that the larger magneto-crystalline anisotropy of Co³⁺ ions is responsible for the increased H_c observed in the Co-doped Ni ferrite samples.     

Effects of Ca–Gd co-substitution on the structural,magnetic, and dielectric properties of M-type strontium hexaferrite

Sr_1−xCa_xFe_12−xGd_xO₁₉ (x = 0, 0.04, 0.08, 0.12, 0.16, 0.20) hexaferrites were characterized by several techniques, such as X-ray diffraction, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, and vibrating sample magnetometer. Structural results indicate the formation of a pure-phase Sr_1−xCa_xFe_12−xGd_xO₁₉ hexaferrite with space group P6₃/mmc. SEM photography confirms that there are a smaller number of defects due to the reduced porosity and surface area (increased particle size). Magnetic investigations showed a rise of the coercive force from 5069.8 to 5757.4 Oe and saturation magnetization from 79.25 to 80.68 emu/g. The maximum values appear to be for sample x = 0.16, which may be useful in such as permanent magnets, and high-density media for magnetic storage devices. Dielectric parameters, such as conductivity, the real part of permittivity, dielectric loss, dielectric tangent loss, and complex modulus, were studied. Impedance analysis shows that the conduction process is mainly governed by the long-range movement of the charge carriers based on the Debye model for x = 0.12.