Structural properties and hyperfine interactions in Co–Zn Y-type hexaferrites prepared by sol–gel method

Y-type barium hexaferrites Ba₂Co_2−xZn_xFe₁₂O₂₂ (0.0≤x≤2.0) were prepared using sol–gel method and then sintering at temperatures between 900 and 1100 °C. The properties of the prepared samples were investigated using X-ray diffraction, scanning electron microscopy, and Mössbauer spectroscopy. XRD patterns revealed the presence of a single Y-type hexaferrite phase in the samples sintered at temperatures above 1000 °C. Mössbauer data indicated that Co²⁺ ions occupied octahedral sites in the T blocks, while Zn²⁺ ions were distributed between the two tetrahedral sites. This trend for cationic distribution resulted in weakening the superexchange interactions between spin-up and spin-down sublattices with increasing Zn content, and a consequent reduction in the hyperfine fields in Zn rich compounds.     

Structural development and magnetic phenomenon in Zn–Cr–Fe multi oxide nano-crystals

The Cr³⁺ ions doped multi-oxide ZnFe_2−xCr_xO₄ ferrite nanoparticles have been synthesized by chemical co-precipitation method. Site occupancies of Zn²⁺, Cr³⁺ and Fe³⁺ ions were analyzed using X-ray diffraction data and Buerger's method. The effect of the constituent phase variation on the magnetic hysteresis behavior was examined by saturation magnetization which decreases with the increase in Cr³⁺ content in place of Fe³⁺ ions at octahedral B-site. Typical blocking temperature (T_B) around 90 K was observed by zero field cooling and field cooling magnetization study. Room temperature Mössbauer spectra show two paramagnetic doublets (tetrahedral and octahedral sites). The isomer shifts of both doublets decrease whereas quadrupole splitting and relative area of tetrahedral A-site increases with increasing Cr³⁺ substitution. The dielectric constant (measured on compositions x=0, 0.4, 0.8 and 1.0) increases when the temperature increases as in the semiconductor. This behavior is attributed to the hopping of electrons between Fe²⁺ and Fe³⁺ ions with a thermal activation.     

Magnetic and thermal expansion properties of chromium-substituted lithium ferrite

The structure, magnetic, and thermal expansion properties of chromium-substituted lithium ferrite have been investigated. The lattice constant (Å) decreases linearly as a (x) = 8.32366 − 0.04338x for Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0). When increasing Cr content, the initial permeability decreased gradually. The average thermal expansion coefficient of Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0) varied from 15.34 to 17.77 ppm/°C, with increasing Cr content, the average thermal expansion coefficient decreased. The average thermal expansion coefficient (ppm/°C) in the range of 25–850 °C give the polynomial correlation as follows, TEC (x) = 1 7.775 − 0.216x − 0.723x² − 1.493x³ for Li_0.5Fe_2.5−xCr_xO₄ (x = 0.0–1.0).     

Structural,morphological, optical,cation distribution and Mössbauer analysis of Bi3+ substituted strontium hexaferrite

Single-phase M-type hexagonal ferrites, SrBi_xFe_12−xO₁₉ (0.0≤x≤1.0), were prepared by a co-precipitation assisted ceramic route. The influence of the Bi³⁺ substitution on the crystallization of ferrite phase has been examined using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and Mössbauer spectroscopy. The XRD data show that the nanoparticles crystallize in the single hexagonal magnetoplumbite phase with the crystallite size varying between 65 and 82 nm. A systematic change in the lattice constants, a=b and c, was observed because of the ionic radius of Bi³⁺ (1.17 Å) being larger than that of Fe³⁺ ion (0.64 Å). SEM analysis indicated the hexagonal shape morphology of products. From ⁵⁷Fe Mössbauer spectroscopy data, the variation in line width, isomer shift, quadrupole splitting and hyperfine magnetic field values on Bi substitutions have been determined.     

Gadolinium ferrite nanoparticles: Synthesis and morphological,structural and magnetic properties

In this study we report on the successful synthesis of Gd_xFe_3−xO₄ nanoparticles with nominal Gd-content (x) in the range 0.00≤x≤0.50. The effect of the nominal Gd-content on morphological, structural and magnetic properties was investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and Mössbauer spectroscopy. We found the actual inclusion of Gd³⁺ ions into cubic ferrite structure lower than the nominal values, though no extra phase was observed in the whole range of our investigation. Moreover, from Mössbauer data we found evidences of Gd³⁺ ions replacing both Fe³⁺ and Fe²⁺ ions, the latter leading to iron vacancies in the cubic ferrite crystal structure. As the nominal Gd-content, the lattice parameter and the average crystallite size increases monotonically. We found that in the same range of nominal Gd-content the lattice parameter decreases with the increase of iron vacancy content.     

Study of cation distribution for Cu–Co nanoferrites synthesized by the sol–gel method

Nano sized polycrystalline soft ferrite particles with composition Cu_1−xCo_xFe₂O₄ (x =0.1, 0.3, 0.5, 0.7, 0.9) were synthesized by the sol–gel technique. The existence of well-defined single cubic spinel structure was confirmed in all the samples by X-ray diffraction. The crystallite size found by XRD varied from 14.8 to 34.0 nm. The microstructure was also characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Slight expansion of the unit cell was detected with the increase of Cobalt concentration, which may be attributed due to larger ionic radius of Co²⁺. Lattice parameter ranged from 8.34 Å to 8.37 Å for Co²⁺ from 0.1–0.9. The distribution of cations amongst A- and B-sites of the lattice was estimated by X-ray diffraction by using the R-factor technique. The results showed that both Cu²⁺ and Co²⁺ ions occupy mainly the B-site while Fe³⁺ ions were equally distributed among A- and B-sites. The data obtained from cation distribution analysis was used to determine the magnetic moment for each sample and VSM studies were also carried out to validate these calculations. Magnetic measurements showed that the saturation magnetization (Ms) and coercivity (Hc) increased with increasing cobalt content.     

Structure,magnetic and electrical transport properties of the perovskites Sm1−xCaxFe1−xMnxO3

The structure of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.0 ≤ x ≤ 1.0) compounds was investigated. The lattice parameters increase with coupled substitution Sm³⁺ by Ca²⁺ and Mn⁴⁺ for Fe³⁺. The variation of parameter, c, is larger than that of a and b, respectivly. The detailed analysis of magnetic properties of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.1 ≤ x ≤ 0.9) shows that local magnetic interaction between Fe³⁺ and Fe³⁺ and Mn⁴⁺ and Mn⁴⁺ at below magnetic transition temperature is antiferromagnetic. Above magnetic transition temperature the presence of large magnetic cluster is proposed and the sizes of magnetic clusters decrease with Mn⁴⁺. The electrical transport behaviors related with small polaron hopping and variable range hopping models.     

Fe doping and magnetic properties of zincblende SiC ceramics

Fe-doped SiC bulk ceramics were fabricated by hot-pressing, and their magnetic and electronic properties were investigated. Si_1−xFe_xC (x ≤ 0.04) samples having a zincblende crystal structure exhibited ferromagnetic hysteresis at room temperature with the saturation magnetization increasing with x. X-ray diffraction measurements revealed the creation of a Fe₃Si phase in the samples with its density increasing with x. The samples were found to be p-type semiconductors with a hole concentration (electrical resistivity) of ∼10¹⁹ cm⁻³ (∼10⁰ Ω cm) at room temperature. The observed magnetic properties of the samples are mainly ascribed to the presence of ferromagnetic Fe₃Si crystallites. The high carrier concentration of the samples likely implies the existence of acceptors due to individual Fe³⁺ occupation of the Si sites in the lattice. The randomly distributed Fe³⁺ ions represent a minor contribution to the magnetization of the samples through the formation of magnetic polarons with the carriers.     

Structural and magnetic properties of Co1−xZnxFe2O4 nanorods prepared by the solvothermal annealing method

Co_1−xZn_xFe₂O₄ (0.1≤x≤0.9) nanorods have been prepared by the thermal decomposition of the corresponding oxalate precursor, which was synthesized by the template-, surfactant-free solvothermal method. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM). The obtained Co_1−xZn_xFe₂O₄ (0.1≤x≤0.9) nanorods were built by many nanoparticles with average sizes around 20 nm to form one-dimensional arrays. Vibrating sample magnetometry measurements show that the coercivity of the ferrite nanorods decreases with increasing Zn content, whereas the specific saturation magnetization initially increases and then decreases with the increase of Zn content. The maximum saturation magnetization value of the as-prepared sample (Co_0.5Zn_0.5Fe₂O₄) reaches 43.0 emu g⁻¹.     

Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

The linear thermal expansion coefficients (TECs) of perovskite-type La_0.3Sr_0.7Fe_1−xGa_xO_3−δ (x=0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10⁻⁶ K⁻¹ at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10⁻⁴ to 1 atm, the isothermal chemically induced expansion of La_0.3Sr_0.7Fe(Ga)O_3−δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, (ε_C/Δδ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10⁻². The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La_0.3Sr_0.7Fe_1−xGa_xO_3−δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

Catalytic combustion of methane on substituted strontium ferrites

Sr-hexaferrites prepared by co-precipitation method and calcined at 700-1000 °С have been characterized by thermogravimetric and differential thermal analysis (TG-DTA), Fourier transformed infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H₂-TPR), and Ar adsorption techniques. It has been shown that hexaferrite phase formed after calcination at 700 °С is amorphous and its crystallization occurs at 800 °С. Specific surface area (S_BET) of the samples calcined at 700 °С is 30-60 m²/g. Reduction in hydrogen proceeds in several steps, Fe(III) in the hexaferrite structure being practically reduced to Fe⁰. Amount of hydrogen necessary for the reduction of the samples decrease in the order: SrMn₂Fe₁₀O₁₉ > SrFe₁₂O₁₉ > SrMn₆Fe₆O₁₉ > SrMn₂Al₁₀O₁₉. Surface composition of the ferrites differs from bulk. According to XPS data, the surface is enriched with strontium. Sr segregation is most probably explained by the formation of surface carbonates and hydroxocarbonates. The main components on the surface are in oxidized states: Mn³⁺ and Fe³⁺. Maximum activity in the methane oxidation is achieved for the SrMn_xFe_12−xO₁₉ (0 ? x ? 2) catalysts. These samples are characterized by highest amount of the hexaferrite phase, which promotes change of oxidation state Mn(Fe)³⁺ ↔ Mn(Fe)²⁺.     

Synthesis and characterization of ZnxMg1 − xGa2O4:Co fabricated by low-temperature burning method

A series of Zn_xMg_1 − xGa₂O₄:Co²⁺ spinels (x = 0, 0.25, 0.5, 0.75, and 1.0) was successfully produced through low-temperature burning method by using Mg(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Ga(NO₃)₃·6H₂O, CO(NH₂)₂, NH₄NO₃, and Co(NO₃)₂·6H₂O as raw materials. The product was characterized by X-ray diffraction, transmission electron microscopy, and photoluminescence spectroscopy. The product was not merely a simple mixture of MgGa₂O₄ and ZnGa₂O₄; rather, it formed a solid solution. The lattice constant of Zn_xMg_1 − xGa₂O₄:Co²⁺ (0 ≤ x ≤ 1.0) crystals has a good linear relationship with the doping density, x. The synthesized products have high crystallinities with neat arrays. Based on an analysis of the form and position of the emission spectrum, the strong emission peak around the visible region (670 nm) can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴A₂(⁴F)] of Co²⁺ in the tetrahedron. The weak emission peak in the near-infrared region can be attributed to the energy level transition [⁴T₁(⁴P) → ⁴T₂(⁴F)] of Co²⁺ in the tetrahedron.     

Effect of holmium on the magnetic and electrical properties of barium based W-type hexagonal ferrites

A series of BaHo_xFe_16−xO₂₇ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) W-type hexagonal ferrites were prepared by co-precipitation technique at high annealing temperature of 1320 °C. XRD reveals single W-type hexagonal phase in these ferrites. The grain size is measured by SEM analysis using line intercept method. Saturation magnetization, retentivity and coercivity were measured from MH-loops taken on VSM. It was observed that magnetization increases with the increase of Ho content due to difference in ionic radii of Ho³⁺ (0.901 Å) and Fe³⁺ (0.67 Å) ions. Room temperature dc resistivity increases as a function of Ho³⁺ that may be due to separation between grains. The dc electrical resistivity decreases as a function of temperature which indicates the semi-conducting behavior of the samples.     

Influence of manganese on the structure and magnetic properties of YFeO3 nanocrystal

High purity Mn doped YFeO₃ nanocrystals were readily prepared by sol-gel combustion method, with citric acid as the fuel. The doping effect on the structure and magnetic properties was systematically studied. X-ray diffraction and TEM results indicate that the samples are well crystallized. Orthorhombic YFeO₃ crystallites can be obtained when calcined at 800 °C. In YFe_1−xMn_xO₃ system for x ≤ 0.2, hexagonal structure exists as metastable phase and the calcined temperature has to be elevated to obtain pure orthorhombic phase. Distortion of the crystal structure was also observed, with changes in lattice parameters. The antiferromagnetic coupling was effectively strengthened with increased Mn content. This is possibly originated from the structural distortion which gives rise to enhanced antiferromagnetic superexchange interaction between Fe³⁺-Fe³⁺ and Fe³⁺-Mn³⁺.     

Removal of entrapped iron compounds from isothermally treated catalytic chemical vapor deposition derived multi-walled carbon nanotubes

Compositional changes of the residual iron compounds in isothermally treated catalytic chemical vapor deposition derived multi-walled carbon nanotubes have been monitored using ⁵⁷Fe transmission Mössbauer spectroscopy and X-ray fluorescence. The iron phases entrapped in the as-synthesized carbon nanotubes consist of γ-iron, α-iron, Fe₃C and Fe_1−xS. The Fe_1−xS phase decomposes completely around 1500 °C while the iron carbide phase decomposes in the temperature range of 1500-2400 °C. The obtained apparent activation energy of ca. 76 kcal/mol suggests that the entrapped iron was removed via a diffusion process during thermal treatment.     

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.     

Structural, morphological, magnetic and optical properties of chromium substituted strontium ferrites, SrCrxFe12 − xO19 (x = 0.5, 1.0, 1.5, 2.0 and 2.5) annealed with potassium halides

Chromium substituted strontium ferrites SrCr_xFe_12 − xO₁₉ (x = 0.5, 1.0, 1.5, 2.0 and 2.5) have been synthesized via sol gel method and the dry gels obtained have been annealed with various inorganic template agents (KCl, KBr and KI). The powder X ray diffraction studies reveal a crystallite size of ~ 40-45 nm. The use of KCl as inorganic template agent leads to an increase in the crystallite size. This may be attributed to the fact that the coordination ability of Cl⁻ is maximum due to its larger charge to size ratio, which promotes crystal growth in one dimension leading to needle-like morphology. On the other hand, KI undergoes sublimation to form I₂ which gets entrapped in the strontium ferrite crystal leading to a bubble-like morphology. A systematic change in the lattice constants, a and c, is not observed because the radius of Cr³⁺ ion (0.63 Å) is similar to that of Fe³⁺ ion (0.64 Å). The saturation magnetization decreases with increase in the chromium concentration from 43.03 emu/g to 17.40 emu/g due to the substitution of Fe³⁺ ions by less magnetic Cr³⁺ ions in 2a and 12k sites of the lattice. The coercivity decreases with increase in the chromium concentration due to decrease in magnetocrystalline anisotropy. In the presence of KCl and KBr, both saturation magnetization and coercivity increase and the saturation magnetization has the maximum value in case of samples annealed with KBr. However, with KI, the values of both saturation magnetization and coercivity decrease sharply which may be due to lower crystallinity due to bubble-like morphology because of the decomposition of KI to I₂. The energy band gap for all the ferrite compositions is found to be ~ 2.2 eV and its value increases in the samples annealed with KI.     

Structural, physical, magnetic and electrical properties of La-substituted W-type hexagonal ferrites

Lanthanum doped W-type hexaferrites BaZn₂La_xFe_16−xO₂₇ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were synthesized by co-precipitation and sintered at 1320 °C. The X-ray diffraction reveals W-type hexagonal structure with few traces of secondary phase. The decrease in grain size as a function of La-concentration is attributed to the fact that La acts as a grain inhibitor. The saturation magnetization and remanance decrease due to spin canting on B-sites. The increase in coercivity follows 1/r behavior where r is the radius of grain. The DC resistivity was observed to increase from 0.59 × 10⁷ to 8.42 × 10⁷ Ω cm with increasing La-contents due to the unavailability of Fe³⁺ ions. This enhancement in resistivity makes these materials promising candidates for use at high frequencies in order to reduce eddy current losses.     

Effect of Ga3+ substitution on the microwave dielectric properties of 0.67CaTiO3–0.33LaAlO3 ceramics

0.67CaTiO₃–0.33La(Al_1−xGa_x)O₃ (0≤x≤0.4) (CTLAG) ceramics with pure perovskite structure were prepared by a conventional two-step solid-state reaction process. The effect of Ga³⁺ substitution for Al³⁺ on the microwave dielectric properties of the ceramics was subsequently investigated. As Ga content increased, the ionic polarizability increased and led to an increase of the dielectric constant (ε_r). Meanwhile, both the tolerance factor (t) of CTLAG ceramics and A-site bond valence were considered to have effect on the temperature coefficient of the resonant frequency (τ_f) with the increase of Ga content. Results also showed that the quality factor (Q×f ) varied with increasing Ga³⁺ content because of not only intrinsic factor but also extrinsic factors such as the bimodal grain size distribution, the variation of relative density, and the packing fraction. Excellent microwave dielectric properties with ε_r≈45.81, Q×f≈34,152 GHz, and τ_f≈3.09 ppm/°C were achieved for 0.67CaTiO₃–0.33La(Al_0.9Ga_0.1)O₃ ceramics sintered at 1420 °C for 4 h.