Study of structural and magnetic properties of (Co–Cu)Fe2O4/PANI composites

The nanocomposites of the polyaniline and Co_1−xCu_xFe₂O₄ (PANI/CoCuFe) were prepared by in-situ oxidative polymerization of aniline. Prepared nanocomposites samples were characterized by using various experimental techniques like X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FT-IR), field emission scanning electron microscope (FE-SEM), vibrating sample magnetometer (VSM), Mössbauer spectroscopy and ultraviolet–visible spectrophotometry (UV–VIS). The elemental analysis as obtained from the energy dispersive X-ray spectroscopy (EDAX) measurement is in close agreement with the expected composition from the stoichiometry of the reactant solutions. XRD result confirms that all the samples have the single phase cubic spinel structure. Unit cell parameter ‘a’ is found to decrease with the increase in copper ion substitution. The crystallite size was investigated by using the Debye–Scherer formula and it was found in the range of ∼28–37 nm. FE-SEM confirmed the homogeneous and well defined surface morphology of the synthesized samples. FT-IR study showed the main absorption bands corresponding to the spinel structure those arose due to the tetrahedral and octahedral stretching vibrations. Cation distribution was estimated using XRD data. Hysteresis measurements revealed that the saturation magnetization decreased with increase in Cu²⁺ substitution. Magnetic environment of ⁵⁷Fe in Cu-doped cobalt ferrite was investigated by using Mössbauer spectroscopy. Mössbauer study evidenced the ferrimagnetic behavior of the synthesized samples.     

Magnetic properties of nanocrystalline CoFe2O4 synthesized by thermal plasma in large scale

The paper reports the large scale synthesis of nanoparticles of CoFe₂O₄ using thermal plasma reactor by gas phase condensation method. The yield of formation was found to be around 15 g h⁻¹. The magnetic properties of CoFe₂O₄, synthesized at different reactor powers, were investigated in view of studying the effect of operating parameters of plasma reactor on the structural reorganization leading to the different cation distribution. The values of saturation magnetization, coercivity and remanent magnetization were found to be influenced by input power in thermal plasma. Although the increase in saturation magnetization was marginal (61 emu g⁻¹ to 70 emu g⁻¹) with increasing plasma power; a significant increase in the coercivity (552 Oe to 849 Oe) and remanent magnetization (16 emu g⁻¹ to 26 emu g⁻¹) were also noticed. The Mössbauer spectra showed mixed spinel structure and canted spin order for the as synthesized nanoparticles. The detailed analysis of cation distribution using the Mössbauer spectroscopy and X-ray photoelectron spectroscopy leads to the conclusion that the sample synthesized at an optimized power shows the different site selective states.     

Soft magnetic amorphous Fe–Zr–Si(Cu) boron-free alloys

Amorphous Fe₈₀Zr_xSi_20−x−yCu_y boron-free alloys, in which boron was completely replaced by silicon as a glass forming element, have been prepared in the form of ribbons by using the melt quenching technique. X-ray diffraction and Mössbauer spectroscopy measurements revealed that the as-quenched ribbons with the compositions with x = 6–10 at.% and y = 0, 1 at.% are fully or predominantly amorphous. Differential scanning calorimetry (DSC) measurements allowed the estimation of crystallization temperatures of the amorphous alloys. Soft magnetic properties have been studied by the specialized rf-Mössbauer technique. Since the rf-collapse effect observed is very sensitive to the local anisotropy fields it was possible to evaluate the soft magnetic properties of the amorphous alloys studied. The rf-Mössbauer studies were accompanied by conventional measurements of hysteresis loops from which the magnetization and coercive fields were estimated. It was found that amorphous Fe–Zr–Si(Cu) alloys are magnetically very soft, comparable with those of the conventional amorphous B-containing Fe-based alloys.     

Structural and magnetic properties of glass-ceramics containing silver and iron oxide

Glass-ceramics with a nominal composition of 25SiO₂–(50 − x)CaO–15P₂O₅–8Fe₂O₃–2ZnO–xAg (where x = 0, 2 and 4 mol%) have been prepared. Structural features of glass-ceramics have been investigated using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Magnetic properties were studied using vibrating sample magnetometer and Mössbauer spectroscopy. Ca₃(PO₄)₂, hematite and magnetite are formed as major crystalline phases. The microstructure reveals the formation of 25–30 nm size particles. Mössbauer spectroscopy has shown the relaxation of magnetic particles. Saturation magnetization value is increased with an increase of Ag content up to 4 mol%, which has been attributed to the formation of magnetically ordered particles. The antibacterial response was found to depend on Ag ions concentration in the glass matrix and samples with 4 mol% Ag in glass matrix have shown effective antibacterial activity against Escherichia coli.     

Structural and magnetic properties of a mechanochemically activated Ti-Fe2O3 solid mixture

The mechanochemical effects on the reactivity and properties of a titanium/hematite powder mixture with molar ratio of 1/2 are investigated. Crystalline-phase structure, composition, hyperfine and magnetic behaviors were analyzed as a function of activation time by means of X-ray diffraction, scanning electron microscopy, Mössbauer spectroscopy and vibrating sample magnetometry. The results showed that at relatively short activation times metallic Ti reduces part of the ferric ions, yielding a complex product formed mainly by a mix of two solid solutions Fe_3−xTi_xO₄ (titanomagnetites), both with very different x values (0 < x < 1). Also metallic iron and superparamagnetic hematite particles were detected by Mössbauer spectroscopy. As the mechanical treatment extends the composition of the reactive mixture changes, prevailing in the end the solid solution with higher x value. In contrast, when these activated samples are thermally treated the fraction of the solid solution which is richer in Ti diminishes. This fact produces a significant variation of the saturation magnetization of the obtained material.     

Synthesis and characterization of Ni1−xZnxFe2O4 spinel ferrites from tailored layered double hydroxide precursors

In this paper, a series of pure Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrites have been synthesized successfully using a novel route through calcination of tailored hydrotalcite-like layered double hydroxide molecular precursors of the type [(Ni + Zn)_{1 − x − y}Fe_y²⁺Fe_x³⁺(OH)₂]^x+(SO₄²⁻)_x/2·mH₂O at 900 °C for 2 h, in which the molar ratio of (Ni²⁺ + Zn²⁺)/(Fe²⁺ + Fe³⁺) was adjusted to the same value as that in single spinel ferrite itself. The physico-chemical characteristics of the LDHs and their resulting calcined products were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. The results indicate that calcination of the as-synthesized LDH precursor affords a pure single Ni_1 − xZn_xFe₂O₄ (0 ≤ x ≤ 1) spinel ferrite phase. Moreover, formation of pure ferrites starting from LDHs precursors requires a much lower temperature and shorter time, leading to a lower chance of side-reactions occurring, because all metal cations on the brucite-like layers of LDHs can be uniformly distributed at an atomic level.     

Impact of transition metal doping on Mössbauer,electrical and dielectric parameters of structurally modified lithium ferrite nanomaterials

Zr–Mn doped spinel lithium ferrites Li_0.5Fe_2.5−2xZr_xMn_xO₄ (0.0 ≤ x ≤ 0.5) are synthesized using the citrate precursor method. The spinel ferrite is formed at a relatively lower annealing temperature (873 K) compared to those synthesized by other conformist methods. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis are carried out to determine the cell parameters, crystallite size and grain growth. Cation distribution and site preferences for the doped ions are determined by Mössbauer spectroscopy at room temperature. The impact of doping of Li_0.5Fe_2.5O₄ with the binary mixtures of transition metals (Mn, Zr) on hyperfine interaction parameters (δ, Δ and H_int), electrical resistivity (ρ), dielectric constant (?) and dielectric loss tangent (tan δ) over the frequency range of 100 Hz to 3 MHz is discussed in details. Zr–Mn doping enhanced the DC electrical resistivity and decreased the loss tangent value which is considered useful for technological application in microwave and telecommunication devices.     

Investigation of phase separation in the Ge x Cu1–x Fe2O4 system

The spinel series Ge _x Cu_1–x Fe₂O₄ (x=0.0 to 0.9) has been studied in detail by means of Mössbauer spectroscopy, X-ray diffraction and magnetization measurements at room temperature (298 K). Analysis of X-ray diffraction intensity data and Mössbauer intensity data suggest that this system remains in single phase up tox=0.4 then it phase separates into two different phases forx=0.5 to 0.9. Lattice constants of this system deviate from Vegard's law. Mössbauer spectra for x=0.0 to 0.4 suggest the existence of two hyperfine fields, one due to the Fe³⁺ tetrahedral ions (A-sites) and the other due to the Fe³⁺ octrahedral ions (B-sites), while forx=0.5 to 0.9 it gives Mössbauer patterns corresponding to two separate phases. The systematic composition dependence of quadrupole interactions and nuclear hyperfine fields of⁵⁷Fe³⁺ ions also support the concept of phase separation forx=0.5 to 0.9. The observed variation of⁵⁷Fe³⁺ hyperfine field on A- and B-sites withx forx=0.0 to 0.4 can be explained qualitatively on the basis of supertransferred hyperfine interactions.     

Mössbauer and magnetic studies of Mg1+2xSbxFe2−3xO4 spinel ferrites

Spinel-related Mg_1+2xSb_xFe_2−3xO₄ samples (x = 0.0, 0.05, 0.10, 0.15, 0.20, and 0.30) prepared using the conventional double sintering technique were investigated using ⁵⁷Fe Mössbauer spectroscopy and magnetic measurements. Mössbauer spectra favor a cationic distribution of the form (Mg_δFe_1−δ)^A[Mg_1+2x−δSb_xFe_1+δ−3x]^BO₄ among the tetrahedral-A and octahedral-B sites of the spinel structure. The cation distribution parameter (δ) was found to vary with the Sb⁵⁺ concentration (x). The Mössbauer hyperfine magnetic fields at both sites and the Curie temperatures of the ferrites decrease as x increases. This was attributed to gradual weakening in the magnetic exchange interaction as more Fe³⁺ ions are substituted by diamagnetic Sb⁵⁺ and Mg²⁺ ones. The sample with x = 0.30 exhibits short range magnetic order due to cationic clustering and/or superparamagnetism. The magnetization of all samples was found to be temperature-dependent implying that δ depends on temperature in addition to x. At low temperatures the substituted ferrites (x ≠ 0.0) unexpectedly exhibit higher magnetization values relative to that of the pure ferrite MgFe₂O₄. This behavior, while at variance with the Néel's model for ferrimagnetism, is explicable in terms of the spin canting mechanism proposed in the Yafet–Kittel model.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Microscopic and Macroscopic Magnetic Properties of the MgAl x Cr x Fe2 − 2x O4 Spinel Ferrite System

To determine the influence of the substitution of Al³⁺ and Cr³⁺ for Fe³⁺ in MgFe₂O₄ ferrites on the structural and magnetic properties, the MgAl _x Cr _x Fe_{2 – 2x} O₄ (x = 0.0–0.8) spinel systems were studied by using the X-ray diffraction analysis, magnetization in strong fields, magnetic susceptibility in weakly variable electric fields, and Mössbauer spectroscopy. Unlike previous investigations, it was discovered that a half of Al³⁺ occupies tetragonal positions. The system forms a noncollinear spin structure and a central paramagnetic doublet is superimposed over the magnetic sextet in the Mössbauer spectrum (0.5 > x > 0.2). The dependence of the magnetic susceptibility on temperature reveals the normal ferromagnetic properties of the material.     

On the substitution of calcium in cobalt ferrite

X-ray, magnetization, susceptibility and Mössbauer spectroscopic measurements have been performed on the Co-Ca ferrite system, Ca _x Co_1–x Fe₂O₄. The measurements show that a solid solution is possible only for x<0.2. Thereafter the system shows a mixture of two phases. The variations in lattice constant and Néel temperature are explained on the basis of calcium substitution. The decrease in magnetization in the mixed phases is due to an increase of paramagnetic phase and decrease of ferrimagnetic phase with concentration. The Mössbauer spectra at 300 and 77 K support the concept of mixed phases.     

Growth of nano-particles of Al2O3, AlN and iron oxide with different crystalline phases in a thermal plasma reactor

The paper presents the experimental results showing that the crystalline phase of the nano-particles, synthesized in a DC transferred arc thermal plasma reactor, critically depend on the operating pressure in the reaction zone. The paper reports about the changes in crystalline phases of three different compounds namely: aluminium oxide (Al₂O₃), aluminium nitride (AlN) and iron oxide (Fe_xO_y) synthesized at 760 Torr and 500 Torr of operating pressures. The major outcome of the present work is that the phases having higher defect densities are more probable to form at the sub-atmospheric operating pressures. The variations in the crystalline structures are discussed on the basis of the change in the temperature during the nucleation process, prevailing at the boundary of the plasma, on account of the ambient pressures. The as-synthesized nano-particles were examined by X-ray diffraction analysis and transmission electron microscopy. In addition, the confirmatory analysis of the crystalline phases of iron oxides was carried out with the help of Mössbauer spectroscopy.     

Extraordinary role of Ce–Ni elements on the electrical and magnetic properties of Sr–Ba M-type hexaferrites

The structural, electrical and magnetic behavior of Sr_0.5Ba_0.5−xCe_xFe_12−yNi_yO₁₉ (where x = 0.00–0.10; y = 0.00–1.00) hexaferrite nanomaterials are reported in this paper. The structural analysis indicates that the Ce–Ni doped Sr–Ba M-type hexaferrite samples synthesized by the co-precipitation method are stoichiometric, single magnetoplumbite phase with crystallite sizes in the range of 35–48 nm. The dc-electrical resistivity of the pure Sr–Ba hexaferrite is enhanced to almost 10² times by doping with Ce–Ni contents of x = 0.06; y = 0.60. The dielectric constant and dielectric loss tangent decrease to values 14 and <0.2, respectively, by increasing the frequency up to 1 MHz. Small relaxation peaks at frequencies >10⁵ Hz are observed for the samples with Ce content of x > 0.06. The values of saturation magnetization increase from 66.32 to 84.33 emu/g and remanance magnetization from 42.64 to 56.01 emu/g but coercivity decreases from 2.85 to 1.59 kOe by substitution of Ce–Ni. Sharp ferri-paramagnetic transition is observed in the samples, which is confirmed by DSC results. Ce–Ni substitution acts to reduce the electron-hopping between Fe²⁺/Fe³⁺ ions and also improves the magnetic properties. These characteristics are desirable for their possible use in microwave and chip devices.     

Dependence of optical properties on the composition of (Ba1−x−ySrxEuy)Si2O2N2 phosphors for white light emitting diodes

BaSi₂O₂N₂: Eu²⁺ is an efficient phosphor because of its high quantum yield and quenching temperature. Partial substitution of Ba²⁺ by Sr²⁺ is the most promising approach to tune the color of phosphors. In this study, a series of (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ (x = 0.0–0.97, y = 0.00–0.10) phosphors are synthesized via high-temperature solid-state reactions. Intense green to yellow phosphors can be obtained by the partial substitution of the host lattice cation Ba²⁺ by either Sr²⁺ or Eu²⁺. The luminescent properties and the relationships among the lowest 5d absorption bands, Stokes shifts, centroid shifts, and the splitting of Eu²⁺ are studied systematically. Then, based on (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors and near-ultraviolet (∼395 nm)/blue (460 nm) InGaN chips, intense green–yellow light emitting diodes (LEDs) and white LEDs are fabricated. (Ba_0.37Sr_0.60)Si₂O₂N₂: 0.03Eu²⁺ phosphors present the highest efficiency, and the luminous efficiency of white LEDs can reach 17 lm/w. These results indicate that (Ba_1−x−ySr_xEu_y)Si₂O₂N₂ phosphors are promising candidates for solid-state lighting.     

Memory effects in exchange coupled Fe/Co3O4 nanocomposites

Nanocomposites comprising a ferromagnet and an antiferromagnet have drawn attention recently because of their interesting physical properties and variety of technological applications. In the present work, structural, hyperfine and magnetic properties of Fe/Co₃O₄ nanocomposites prepared by a chemical route having 10–70 wt.% of Fe, have been investigated. XRD and TEM measurements confirmed polycrystalline nature of the samples having grain size in the nanometer regime. FTIR measurements show the presence of two bands of Co–O corresponding to Co²⁺ and Co³⁺. Mössbauer spectra recorded at room temperature confirm the presence of Fe in the blocked state. Presence of exchange bias at Fe–Co₃O₄ interfaces is confirmed by the magnetization measurements. Irreversibility in temperature dependent FC–ZFC measurements points to interface effect. Frequency dependent ac susceptibility measurements as well as memory effect observed in dc magnetization measurements indicate the superspin glass nature of the nanocomposites.     

Mössbauer studies on Zn-substituted iron molybdate

Mössbauer spectra of Fe_2−yZn_yMoO₄ spinel ferrites were recorded in their paramagnetic state. Paramagnetic Mössbauer spectra of all the samples show broad absorption peaks due to the presence of Fe²⁺ and Fe³⁺ ions on both sites (A and B) of the spinel lattice. All the spectra have been fitted with four doublets, using a least squares fitting program. The isomer shift and quadrupole splitting values show that Fe_A²⁺, Fe_A³⁺, Fe_B³⁺ and Fe_B^2.5+ ions are present. Fe_B^2.5+ represents the presence of the Fe²⁺ and Fe³⁺ ions on the B-sites, which take part in charge hopping. The results of electrical resistivity and magnetic measurements support charge hopping between Fe²⁺ and Fe³⁺ ions on B-sites.     

X-ray Rietveld analysis and Fourier transform infrared spectra of the solid solutions [Eu2−xMx][Sn2−xMx]O7−3x/2 (M = Mg or Zn)

Two series of mixed oxides with formula [Eu_2−xM_x][Sn_2−xM_x]O_7−3x/2 (M = Mg or Zn) have been synthesized. The study by X-ray diffraction and Fourier transform infrared spectroscopy shows that the solids obtained are new non-stoichiometric solid solutions with the pyrochlore type structure. For both series a decrease of the cell parameter is observed when the degree of substitution, x, increases. The structural refinements (X-ray studies) were achieved in the space group Fd-3m, no. 227 (origin at center -3m) by using the Fullprof software. The Rietveld refinements show that the divalent cations M²⁺ (Mg²⁺, Zn²⁺) substitute isomorphically for Eu³⁺ and Sn⁴⁺ ions producing vacancies in the anionic sublattice.     

From Magnetite to Cobalt Ferrite

We synthesized Fe_3–x Co _x O₄ (x = 0–1) using the hydrothermal method in order to demonstrate the compositional modulation of magnetite to cobalt ferrite. Our Mössbauer spectroscopy results provided direct evidence for the presence of the Co substitution in the B sublattice, which was found to be accompanied by a systematic increase of the hyperfine magnetic field at these sites. The mechanism we propose relies on the substitution of Fe²⁺ by Co²⁺ in the B sublattice and is supported by the observed dependence of the populations of the (A) and (B) sites on content x of cobalt substitution. The X-ray diffraction (XRD) determinations demonstrated a linear increase in the lattice parameter when going from magnetite to cobalt ferrite. For the particular value x = 0.1, we report that the two sublattices of magnetite become equally populated with Fe. For this particular value of the cobalt content, we obtained a thin film sample by laser ablation deposition and characterized its properties by XRD and conversion electron Mössbauer spectroscopy (CEMS).     

XRD and Sn Mössbauer spectroscopy characterization of SnSe obtained from a simple chemical route

Crystalline tin selenide semiconductor was synthesized by a chemical route. Selenium powder reacted with potassium boronhydride, giving a soluble selenium species potassium seleniumhydride. The reaction of potassium seleniumhydride with tin chloride produced crystalline tin selenide, which was characterized by X-ray diffraction, ¹¹⁹Sn Mössbauer spectroscopy and scanning electronic microscopy. The material was thermally treated, in nitrogen flow, at 300 and 600 °C for 2 h and the particle size evolution was studied by X-ray diffraction. The X-ray diffraction and ¹¹⁹Sn Mössbauer results showed that a mixture of tin oxides and orthorhombic tin selenide was obtained.