Preparation and magnetic properties of SrFe12O19/Ni0.5Zn0.5Fe2O4 nanocomposite ferrite microfibers via sol–gel process

SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrite microfibers with diameters of 1–2 μm have been prepared by the sol–gel process. The SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrites are formed after the precursor calcined at 850 °C for 2 h, fabricating from nanosized particles with a uniform phase distribution. The ferrite grain size increases with the calcination temperature. The magnetic properties for the nanocomposite ferrite microfibers are mainly influenced by the chemical composition and grain size. The nanocomposite ferrite microfibers obtained at 900 °C show the enhanced specific saturation magnetization (M_sh) of 64.8 Am² kg⁻¹, coercivity (H_c) of 146.5 kA m⁻¹ and remanence (M_r) of 33.6 Am² kg⁻¹ owing to the exchange–coupling interaction. This exchange–coupling interaction in the SrFe₁₂O₁₉/Ni_0.5Zn_0.5Fe₂O₄ nanocomposite ferrite microfibers has been discussed.     

Microstructural and Magnetic Features of SrFe<Subscript>12</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>19</Subscript> Materials Synthesized from Different Fuels by Sol-Gel Auto-Combustion Method

The nanocrystalline SrFe₁₂ O ₁₉ materials were prepared by a sol-gel auto-combustion method using different fuels such as citric acid, dextrose, aniline, and hexamine. The combustion product obtained from all the fuels except from that of aniline show a single phase of SrFe₁₂ O ₁₉ materials upon annealing at 1000 ^°C/2 h. The combustion product obtained from aniline as fuel shows SrFe₁₂ O ₁₉ as the main phase with α-Fe₂ O ₃ as impurity. No notable change in lattice parameters is observed due to variation in fuels for SrFe₁₂ O ₁₉ materials. With a little change in the NIR relative reflectance (72–85 %) on fuels, the different SrFe₁₂ O ₁₉ materials display high NIR reflectance in the wavelength range, 1500–2500 nm. The photoluminescence emission spectra of SrFe₁₂ O ₁₉ materials reveal a broad emission peak at ～350 nm which is reminiscent to the Ba-based hexaferrite, BaFe₁₂ O ₁₉. The FESEM images expose quite dissimilar morphology for the various fuels used in the synthesis of SrFe₁₂ O ₁₉ materials. Hysteresis loops for all the nanocrystalline SrFe₁₂ O ₁₉ materials observed under the applied field of ±1.5 T at room temperature exhibit hard ferromagnetic property. The SrFe₁₂ O ₁₉ materials produced from glycine and aniline as fuels exhibit highest and lowest M _s values of 61.3 and 50.5 emu/g, respectively.     

Phase equilibria and crystal structures of solid solutions in the Sr-Fe-Ni-O system at 1100°C in air

The phase equilibria in the Sr-Fe-Ni-O system at 1100°C in air have been studied by x-ray diffraction, and the corresponding phase diagram at constant temperature and pressure has been constructed. The system has been shown to contain two solid-solution series at 1100°C in air: SrFe_{1 ? x} Ni _x O_{3 ? δ} (0 < x ≤ 0.075, sp. gr. Cmmm) and Sr₃(Fe_{1 ? y} Ni _y )₂O_{7 ? δ} (0 < y ≤ 0.15, sp. gr. I4/mmm). Neither Sr₄(Fe_{1 ? z} Ni _z )₆O₁₃ nor Sr(Fe_{1 ? z} Ni _z )₁₂O₁₉ solid solutions have been identified. The lattice constants and structural parameters of single-phase samples have been refined by the full profile Rietveld analysis method.     

Synthesis and properties of SrFe<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>M<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 − <Emphasis Type="Italic">z</Emphasis></Subscript> (M = Mo,W) perovskites

We have synthesized SrFe_{1 ? x} M _x O_{3 ? z} (M = Mo, W; 0 < x ≤ 0.5) solid solutions. Our results indicate that the introduction of stable MO₆ octahedra narrows the range of oxygen stoichiometries of the material and suppresses the perovskite-brownmillerite structural phase transition at low temperatures and oxygen partial pressures. We have studied the thermal stability of the synthesized materials in a reducing atmosphere and the effect of oxygen stoichiometry on their electronic and oxygen-ionic conductivity and phase transformations.     

Synthesis,structure, and magnetic properties of rare-earth-doped Ni<Subscript>0.75</Subscript>Zn<Subscript>0.25</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> nickel zinc ferrite

We have developed processes for the synthesis of Ni_0.75Zn_0.25Fe_2–xLn_xO₄ ferrite solid solutions with the spinel structure and investigated the effect of the rare-earth elements Nd, Gd, Yb, and Lu on the chemical composition, extent, lattice parameters, and magnetic properties of the solid solutions. The results demonstrate that rare-earth solubility in the parent spinel reaches ≈2.5 at %, which leads to changes in the magnetic characteristics of the material, in particular in its saturation magnetization M_s, T_C, and coercive force H_c.     

Effect of Fe content on the properties of perovskite-related oxides

A series of SrFe _x Co_0.5O_{3 ? δ} (x = 0.5, 0.75, 1.0, 1.25) powders have been synthesized by the citrate method. The effect of Fe content on the properties of the SrFe _x Co_0.5O_{3 ? δ} was investigated by X-ray diffraction (XRD), thermo-gravimetric analysis and differential scanning calorimeters (TGA/DSC). XRD results reveal that the phases of the SrFe _x Co_0.5O_{3 ? δ} powders depend upon the content of Fe. Variations in Fe-content and temperature play important effects on the oxygen loss of the SrFe _x Co_0.5O_{3 ? δ} samples. The phase transition from brownmillerite to perovskite structure in helium is strongly dependent on the content of Fe by TGA/DSC analysis. The phase transition temperature is 795, 853, 828 and 874°C for SrFe_0.5Co_0.5O_{3 ? δ}, SrFe_0.75Co_0.5O_{3 ? δ}, SrFeCo_0.5O_{3 ? δ} and SrFe_1.25Co_0.5O_{3 ? δ}, respectively. The oxygen sorption capacity of the products is closely related to the Fe content and temperature. The oxygen sorption capacity of the SrFe_1.25Co_0.5O_{3 ? δ} sample increases rapidly from 790°C with the highest oxygen sorption capacity of 6.83 × 10^?3 mmol/g at 870°C exhibiting the good oxygen sorption capacity from 860 to 880°C.     

Influence of cerium substitution on structural,magnetic and dielectric properties of nanocrystalline Ni–Zn ferrites synthesized by combustion method

Nickel–Zinc (Ni–Zn) ferrites substituted by cerium (Ce) and having the chemical composition Ni_0.5Zn_0.5Ce _x Fe_2?x O₄ (x?=?0.00, 0.02, 0.04, 0.06, 0.08 and 0.10) were synthesized by combustion method using the organic fuel urea as reducing agent. The effects of cerium substitution on the structural, magnetic and dielectric properties of Ni–Zn compounds have been evaluated. X-ray diffraction patterns indicate that the Ni_0.5Zn_0.5Ce _x Fe_2?x O₄ crystallizes into cubic spinel structure initially and secondary phase emerged along with main spinel phase when the Ce³⁺ content is increased. The elemental composition analysis confirms the stoichiometric presence of expected elements in the samples. Scanning electron microscope and high-resolution transmission electron microscope images reveal the nature of grain growth and the particle size of the synthesized samples. Fourier transform infrared spectroscopy confirms the formation of spinel phase and predicted the shifting of bands corresponding to Fe–O vibrations towards higher wavenumbers compared to undoped Ni–Zn ferrite. Magnetic characterization studies reveal that the substitution of Ce³⁺ into the Ni–Zn ferrite leads to a significant change in saturation magnetization and coercivity values. A plot of dielectric constant (?′) versus applied electrical frequency measured at room temperature shows the normal dielectric behavior of the spinel ferrites. The introduction of Ce³⁺ rare earth ions into Ni–Zn ferrites samples is found to affect the values of both dielectric constant and AC conductivity.     

Enhanced antibacterial activity of Ag-doped ZnO/polyaniline nanocomposites

Ag _x Zn_1?x O_1?0.5x and Ag_0.02Z_0.98O_0.99/polyaniline (AZO/PANI) nanocomposites were prepared by citrate sol–gel method and in situ inverse microemulsion method, respectively. The composition, structure and morphology of the samples were characterized by means of modern testing techniques. The antibacterial activities of the as-prepared samples against Staphylococcus aureus, Escherichia coli and Candida albicans were carried out using inhibition zone, minimum inhibitory concentration and minimal bactericidal concentrations methods under the irradiation of sunlight. Results showed that the antibacterial activity of Ag _x Zn_1?x O_1?0.5x was better than that of ZnO, and it was optimum when the mole ratio of Ag was 0.02. The AZO/PANI composites had preferable antibacterial effect than AZO, and shown the strongest antibacterial activity when the mass fraction of AZO was up to 60 %.     

Influence of the Preparation Conditions and Percolation Threshold on the Properties of Lead Zirconate Titanate/Cobalt Nickel Ferrite Magnetoelectric Composites

We have prepared magnetoelectric (ME) composite ceramics, free of foreign phases, in the lead zirconate titanate–cobalt nickel ferrite two-phase system: xPZT-36 + (100–x)Ni_0.9Co_0.1Fe₂O₄. The sol–gel derived ferrite powder used in our preparations seems to be doped with titanium cations from the PZT-36. The ceramics have a percolation threshold at x = 50–70 wt %, which is due to the increased electrical conductivity of Ni_0.9Co_0.1Fe₂O₄. As a consequence, the piezoelectric parameters of the ME ceramics drop sharply at x < 50–70 wt %: the piezoelectric moduli |d_ij| and piezoelectric voltage coefficients |g_ij| decrease by a factor of 3–5 in this composite range. The piezoelectric parameters |d_ij| and |g_ij| of the composites produced using the fine ferrite powder exceed those of the materials prepared using macrocrystalline Ni_0.9Co_0.1Fe₂O₄ powder by more than a factor of 2. The piezoelectric voltage coefficient g33 correlates with the ME coefficient ΔE/ΔH. The highest ME conversion efficiency (up to 45 mV/(cm Oe)) is offered by the 80 wt % PZT-36 + 20 wt % Ni_0.9Co_0.1Fe₂O₄ composites, whose composition lies in a subpercolation region. Even though the composites produced using the fine ferrite powder possess improved piezoelectric properties, they have smaller ΔE/ΔH coefficients (no greater than 25 mV/(cm Oe)), which can be tentatively attributed to the degradation of the properties of the ferrite as a consequence of doping with Ti⁴⁺ cations during the sintering of the composite ceramics.     

Lattice Energy Determination for Polycrystalline Oxide Ceramics and Single-Crystalline Counterparts

The compositional dependence of lattice energies for polycrystalline specimens of spinel ferrite systems, Zn _x Co_1?x Fe₂ O ₄ (x = 0.0–0.6); slowly cooled and quenched systems of CuAl _x Fe_2?x O ₄ (x = 0.0–0.6); high-energy ball milled mixed ferrite composition, Ni_0.5Zn_0.5Fe₂ O ₄ (0–9 h); garnet system, Y_3?x Fe_{5 + x} O ₁₂ (x = 0.0–0.5); manganite perovskite system, La_1?x Ca _x MnO₃ (x = 0.0–1.0); and superconducting systems, Bi_1.7?x Pb_0.3Al _x Sr₂Ca₂Cu₃ O ₁₀ (x = 0.0–0.3), Bi_1.7?x Pb_0.3Ga _x Sr₂CaCu₂ O ₈ (x = 0.0–0.3), and Bi₂Sr₂CaCu₂ O ₈+0??5 % Ag ⁺ addition has been evaluated, making use of mean sound velocity data and employing Kudriavtsev’s approach. It is found that for all the systems, lattice energy decreases, and it is explained based on the change in structural and microstructural parameters as a function of substitution. The lattice energies for single-crystalline counterparts have been computed using four different estimation models based on Kapustinskii method, molecular volume and X-ray density, connectivity indices, and chemical hardness. The observed difference between the two has been discussed in the light of grain and grain boundary contributions and presence of pores and microcracks in polycrystalline materials. A simple model suggested for lattice energy determination for complex oxide compositions based on the oxide additivity rule was found to be quite satisfactory.     

Solid-state synthesis,characterization, and properties of Ni<Subscript>4</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript>-based solid solutions

In this paper, we report the solid-state synthesis, characterization, and physicochemical properties of Ni_4–x Zn _x Nb₂O₉ (x = 0, 0.1, 0.3, 0.5, 0.75, 1.0) zinc-containing nickel niobates and Ni₄Nb_2–x Ta _x O₉ (x = 0.1, 0.3, 0.5, 1.0, 2.0) solid solutions. The materials were characterized by X-ray diffraction. We determined the particle size composition of the synthesized powders, assessed their chemical stability in acid media, obtained IR spectra of the solid solutions, and measured their electrical conductivity as a function of temperature. Some of the solid solutions were used to fabricate and characterize nickel-selective electrodes, which were successfully tested in ion-selective measurements.     

Phase equilibria and crystal structures of phases in the La-Fe-Ni-O system at 1370 K in air

The phase equilibria in the La-Fe-Ni-O system have been studied at 1370 K in air, and the La-Fe-Ni-O phase diagram at constant temperature and pressure has been constructed. Based on x-ray diffraction results for samples prepared by standard solid-state reactions and via citrate and nitrate routes, the following solid solutions have been shown to exist at 1370 K in air: LaFe_{1 ? x} Ni _x O_{3 ? δ} (0 < x ≤ 0.4, sp. gr. Pbnm; 0.6 ≤ x ≤ 0.8, sp. gr. R-3 c), La₄(Ni_{1 ? y} Fe _y )₃O_{10 ? δ} (0 < y ≤ 0.3), La₃(Ni_{1 ? z} Fe _z )₂O_{7 ? δ} (0 < z ≤ 0.05), La₂Ni_{1 ? v} Fe _v O_{4 + δ} (0 < v ≤ 0.05), Ni_kFe_{3 ? k} O₄ (0.81 ≤ k ≤ 1.05), Ni_{1 ? m} Fe _m O (0 < m ≤ 0.05), and Fe_{2 ? p} Ni _p O₃ (0 < p ≤ 0.04). The lattice constants and structural parameters of single-phase samples have been refined by the Rietveld profile analysis method.     

Controllable preparation of large-area arrays of Al-substituted CoCuNi ferrite rods with improvement of saturation magnetization and initial permeability

Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ (x = 0, 0.07, 0.14, and 0.21) rods of large-area arrays are synthesized by a solvothermal method, followed by calcination in air. The samples are characterized by powder X-ray diffraction, FT-IR spectra, scanning electron microscope, and vibrating sample magnetometer. The effect of diamagnetic Al³⁺ ion substitution and calcination temperature on the structure, morphology, and magnetic properties of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ has been investigated. The results indicate that high-crystallized cubic Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ rods of large-area arrays are obtained when the precursors are calcined at 750 °C in air for 3 h. The crystallite size of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ increases with the increase in Al³⁺ content, attributed to the decrease in lattice strain in Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ with the increase in Al³⁺ content. The lattice parameters of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ slightly increase with the increase in Al³⁺ content. This is due to the transformation from cubic NiFe₂O₄ phase to cubic CoFe₂O₄ phase after doping Al³⁺ ion. Al³⁺ substitution can improve the magnetic properties of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄. Co_0.5Cu_0.3Ni_0.2Al_0.14Fe_1.86O₄, calcined at 950 °C, has the highest specific saturation magnetization (86.36 ± 2.25 emu/g) and magnetic moment (3.586 ± 0.093 μ _B ). Co_0.5Cu_0.3Ni_0.2Al_0.21Fe_1.79O₄, calcined at 950 °C, has the highest initial permeability (17.216 ± 0.448). The results are explained by Neel’s two sublattices.     

Magnetically Recyclable Spinel Mn x Ni1−x Fe2 O 4 (x= 0.0–0.5) Nano-photocatalysts: Structural,Morphological and Opto-magnetic Properties

Manganese doped nickel ferrite (Mn _x Ni_1?x Fe₂ O ₄: x = 0.0–0.5) spinel nanoparticles (NPs) were successfully prepared by a facile microwave combustion method (MCM) using urea as the fuel. The prepared samples were characterized by different techniques. Powder X-ray diffraction (XRD) analysis was confirmed the formation of a single-phase NiFe₂ O ₄ spinel structure. The average crystallite sizes of the samples were in the range of 11.49 to 17.24 nm, which was confirmed by Sherrer’s formula. The morphology of the samples showed a nanoparticle-like structure with smaller agglomeration, which was confirmed by high-resolution scanning electron microscopy (HR-SEM). The particle size diameter ranges from 15 to 20 nm, which was confirmed by high-resolution transmission electron microscopy (HR-TEM). Energy dispersive X-ray (EDX) analysis confirmed the elemental composition, which was also evidence for the formation of single pure phase. Selected area electron diffraction (SAED) analysis showed well crystalline nature. UV-visible diffuse reflectance spectra (DRS) and photoluminescence (PL) spectrum analysis was used to calculate the optical band gap, and the values are slightly increased (2.02 to 2.42 eV) with increasing the Mn-dopant, due to the decreasing of particle size, which may be due to the quantum confinement effect. Magnetic properties of the samples were analyzed by vibrating sample magnetometer (VSM) technique, which showed the magnetization (M _s ) value of the samples are increased with increasing Mn content and reach a maximum value of 67.82 emu/g for Mn_0.5Ni_0.5Fe₂ O ₄ sample. Photo-catalytic activity of the samples was measured and showed the photocatalytic degradation (PCD) of methylene blue dye with good results. The catalyst was magnetically recycled and reused five consecutive cycles and showed good reproducibility without change of catalytic activity.     

Microwave dielectric properties of ZnO–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–<Emphasis Type="Italic">x</Emphasis>TiO<Subscript>2</Subscript> ceramics prepared by reaction-sintering process

The ZnO–Nb₂O₅–xTiO₂ (1 ≤ x ≤ 2) ceramics were fabricated by reaction-sintering process, and the effects of TiO₂ content and sintering temperature on the crystal structure and microwave dielectric properties of the ceramics were investigated. The XRD patterns of the ceramics showed that ZnTiNb₂O₈ single phase was formed as x ≤ 1.6 and second phase Zn_0.17Nb_0.33Ti_0.5O₂ appeared at x ≥ 1.8. With the increase of TiO₂ content and sintering temperature, the amount of the second phase Zn_0.17Nb_0.33Ti_0.5O₂ increased, resulting in the increase of dielectric constant, decrease of Q × f value, and the temperature coefficient of resonant frequency (τ _f ) shifted to a positive value. The optimum microwave dielectric properties were obtained for ZnO–Nb₂O₅–2TiO₂ ceramics sintered at 1075 °C for 5 h: ε _r  = 45.3, Q × f = 23,500 GHz, τ _f  = +4.5 ppm/°C.     

Low temperature sintering and microwave dielectric properties of Zn<Subscript>3</Subscript>Mo<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramic

Crystal structure and dielectric properties of Zn₃Mo₂O₉ ceramics prepared through a conventional solid-state reaction method were characterized. XRD and Raman analysis revealed that the Zn₃Mo₂O₉ crystallized in a monoclinic crystal structure and reminded stable up to1020 °C. Dense ceramics with high relative density (~ 92.3%) were obtained when sintered at 1000 °C and possessed good microwave dielectric properties with a relative permittivity (ε _r ) of 8.7, a quality factor (Q?×?f) of 23,400 GHz, and a negative temperature coefficient of resonance frequency (τ _f ) of around ??79 ppm/°C. With 5 wt% B₂O₃ addition, the sintering temperature of Zn₃Mo₂O₉ ceramic was successfully lowered to 900 °C and microwave dielectric properties with ε _r ?=?11.8, Q?×?f?=?20,000 GHz, and τ _f = ??79.5 ppm/°C were achieved.     

Structural and Magnetic Studies of Cu-Substituted Nanocrystalline Ni–Zn Ferrites Obtained Via Hexamine–Nitrate Combustion Route

Ni\({}_{0.6 -_x}\)Cu _x Zn_0.4Fe₂ O ₄ ferrite compositions with x = 0.1, 0.2, 0.3, 0.5, and 0.6 were synthesized by a hexamine–nitrate combustion route. The phase purity and nanocrystalline nature was confirmed from XRD studies. The structural parameters such as lattice constant, X-ray density, bulk density, and porosity were calculated and compared for the effect of Cu inclusion in Ni–Zn ferrites. The lattice constant, X-ray density, and bulk density increase while the porosity exhibits a decreasing trend with increasing copper content. The FTIR spectra display two characteristic bands in the region 574–548 and 421–395 cm^?1 assigned to M–O stretching vibrations in tetrahedral and octahedral sites, respectively. The AC susceptibility studies indicate the presence of superparamagnetic as well as single-domain particles and the decrease in Curie temperature with Cu substitution. The saturation magnetization, remanence, and coercivity were found to decrease with increasing Cu concentration and attributed to the lower magnetic moment of Cu ions than Ni ions.     

Effect of Barium on Morphological Transition and Magnetic and Dielectric Properties in Ferrite Nanoparticles

Barium ferrite (BaFe₂O₄) nanoparticles were synthesized by auto combustion method under different weight percentages of barium. The role of barium in the behavior of spinel ferrite property is identified from this study. XRD exhibits prominent orientation of (212) for BaFe₂O₄ has confirmed especially in 20 and 30 wt% of barium. The addition of barium metal induced the specific vibration in FTIR spectra and such changes coincide well with the particle size. Further, the EDX spectrum reflects the atomic percentage of elemental presence in the samples of barium ferrite. Addition of barium on ferrite nanoparticles reduces the intensity of fluorescence. The morphological changes occurred due to increasing doping concentration of barium and is visualized from the FESEM and TEM images. The formation of different morphologies such as spherical, hexagonal platelets and small rectangular bar shape are observed only due to inclusion of barium at surfactant medium. The magnetic properties of the barium ferrite samples are studied by VSM. It reveals that 35.11 emu/g saturation magnetization (M _s ) with 3775.08 Oe coercivity. The change in values of coercivity (H _c ) from 3775.08 to 1572.95 Oe due to the variation of barium levels confirmed that the role of barium induced the hard magnetic behavior. The dielectric study also indicates the significance of barium ferrite in the variation of dielectric constant.     

Influence of mechanical milling and thermal annealing on electrical and magnetic properties of nanostructured Ni-Zn and cobalt ferrites

The present article reports some of the interesting and important electrical and magnetic properties of nanostructured spinel ferrites such as Ni_0.5Zn_0.5Fe₂O₄ and CoFe₂O₄. In the case of Ni_0.5Zn_0.5Fe₂O₄, d.c. electrical conductivity increases upon milling, and it is attributed to oxygen vacancies created by high energy mechanical milling. The real part of dielectric constant (?′) for the milled sample is found to be about an order of magnitude smaller than that of the bulk nickel zinc ferrite. The increase in Néel temperature from 538 K in the bulk state to 611 K on the reduction of grain size upon milling has been explained based on the change in the cation distribution. The dielectric constant is smaller by an order of magnitude and the dielectric loss is three orders of magnitude smaller for the milled sample compared to that of the bulk. In the case of cobalt ferrite, the observed decrease in conductivity, when the grain size is increased from 8–92 nm upon thermal annealing is clearly due to the predominant effect of migration of some of the Fe³⁺ ions from octahedral to tetra-hedral sites, as is evident from in-field Mössbauer and EXAFS measurements. The dielectric loss (tan δ) is an order of magnitude smaller for the nano sized particles compared to that of the bulk counterpart.     

The Effect of Annealing on Phase Stability and Magnetic Properties of Hexaferrite BaFe<Subscript>12</Subscript><Emphasis Type="Italic">O</Emphasis><Subscript>19</Subscript>

The hexaferrite BaFe₁₂ O ₁₉ phase was synthesized through the mechanical alloying process followed by subsequent annealing. Rietveld refinements of as-milled powder annealed at 700 ^°C confirm the formation of the BaFe₁₂ O ₁₉ phase with the presence of an important amount of the α-Fe₂ O ₃ phase. Thus, prior mechanical milling shows much lower reaction temperature and less reaction time compared to conventional methods. Further annealing up to 900 and 1100 ^°C could not enable the formation of a single BaFe₁₂ O ₁₉ phase, reaching an optimum phase composition ratio close to BaFe₁₂ O ₁₉/ α-Fe₂ O ₃ 70/30. The crystallite size was found to be in the nanoscale level but increases with increasing temperature (BaFe₁₂ O ₁₉ = 20–62 nm; α-Fe₂ O ₃ = 31–74 nm). SEM micrographs show that as the annealing temperature rises, the particles become more regular with sharp edges and hexagonal-like shapes. Magnetic measurements reveal that both M _s and M _r increase with annealing temperature to reach maximum values at 900 ^°C then remain unchanged, associated with phase composition. The coercivity H _c increases upon annealing up to 700 ^°C to a much higher value, from 1.7 kOe for as-milled powder to 4.8 kOe. Its value then decreases, attributed to grain (particle) growth (formation of larger particles) due to high annealing temperatures: 900–1100 ^°C. The obtained composites show very interesting magnetic properties and can be considered for potential applications, such as hyperthermia, heavy metal and dye removal, and hard/soft magnetic composites.