Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

Self-propagating high temperature synthesis and magnetic properties of Ni0.35Zn0.65Fe2O4 powders

Ni-Zn ferrite powders were synthesized by self-propagating high temperature synthesis (SHS) method. X-ray diffraction, TEM and vibrating sample magnetometry (VSM) were used to characterize the phase composition, microstructure and magnetic properties of the combustion products. The effect of the combustion temperature (T _c), the major parameter of the SHS process, on particle size, phase composition and magnetic properties of the products was also studied. The results showed that particle size grew with the increasing combustion temperature. The maximum saturation magnetization,M _s, increased with combustion temperature indicating the growth of grain size and high degree of ferritization, while residual magnetization,M _r, and coercive force,H _c, decreased. Compared with other methods, Ni_0.35Zn_0.65Fe₂O₄ ferrite powders with improved magnetic properties can be obtained by SHS at 1000°C.     

Fabrication of aligned ferrite nanofibers by magnetic-field-assisted electrospinning coupled with oxygen plasma treatment

This paper describes a simple and effective approach to fabrication of aligned magnetic ferrite nanofibers by magnetic-field-assisted electrospinning coupled with oxygen plasma treatment. Large and flexible magnetic hybrid membranes of aligned Fe₃O₄/PVP composite nanofibers were fabricated readily by electrospinning mixtures of Fe₃O₄/PVP in a magnetic field. The PVP matrix could be removed either by calcination or by oxygen plasma treatment. Oxygen plasma treatment retained the original crystalline phase of Fe₃O₄, and large inorganic membranes of aligned ferrite nanofibers were obtained. The ferrite nanofibers showed ferromagnetic behaviors, and are promising in flexible magnetic membranes, magnetic separation, drug delivery, and magnetic sensors.     

Phase transformation and the mechanism of combustion synthesis of ZnFe2O4 ferrite powders

The reaction mechanism of combustion synthesis of zinc ferrite, which belongs to the complex oxide combustion reaction was investigated using a combustion front quenching method (CFQM). Phase transformation and microstructural evolution of the quenched samples were observed by XRD, SEM, and Mössbauer spectroscopy. The results showed that the combustion proceeded by a dissolution-precipitation mechanism, viz the iron was burned to form Fe₂O₃ in an oxygen atmosphere and melting of the Fe₂O₃ led to the dissolving of the ZnO particles, then ZnFe₂O₄ precipitated out. In addition, a model of the mechanism was drawn.     

The effect of calcining temperature on the magnetic properties of the ultra-fine NiCuZn-ferrites

In this study, the ultra-fine NiCuZn-ferrite was prepared by a coprecipitation method. The magnetic properties were investigated in terms of calcining temperature. The ferrite powders, Ni_0.206Cu_0.206Zn_0.618Fe_1.94O_4−δ, were initially heat treated at various temperatures of 300-750 °C, and then sintered at the final temperature of 900 °C. The average particle size calculated by a XRD pattern and confirmed by a transmission electron microscope (TEM) micrograph was 7.5 nm. The calcining temperature was an important factor for microstructures and magnetic properties of the sintered ferrite. Scanning electron microscope (SEM) micrographs showed a uniform grain growth with small pores and high densification at the calcining temperature of 450 °C. From the results of magnetic property measurements, the ferrite calcined at 450 °C showed higher initial permeability (170) and quality factor (72) than those of other calcining temperatures.     

Synthesis of single-phase Sr3Co2Fe24O41 Z-type ferrite by polymerizable complex method

Synthesis of single-phase Sr₃Co₂Fe₂₄O₄₁ Z-type (Sr₃Co₂Z) ferrite was realized by adopting the polymerizable complex method. Crystal structure of samples has been investigated by powder X-ray diffraction (XRD). Single-phase Sr₃Co₂Z ferrite was obtained by heating at 1473 K for 5 h in air. Magnetic properties were discussed by measurements of M-H curves with vibrating sample magnetometer (VSM). Sr₃Co₂Z ferrite prepared by polymerizable complex method showed typical M-H curve of soft ferrite, with a saturation magnetization of 21.5μ_B/formula unit (50.5 emu/g) and a coercive force of 0.014 T at room temperature.     

Synthesis and characterization of Bi2Fe4O9 powders

Bi₂Fe₄O₉ have been successfully prepared using ethylenediaminetetraacetic (EDTA) acid as a chelating agent and ethylene glycol as an esterification agent. Heating of a mixed solution of EDTA, ethylene glycol, and nitrates of iron and bismuth at 140 °C produced a transparent polymeric resin without any precipitation, which after pyrolysis at 250 °C was converted to a powder precursor for Bi₂Fe₄O₉. The precursors were heated at 400–800 °C in air to obtain Bi₂Fe₄O₉ powder and differential scanning calorimetry (DSC), thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) techniques were used to characterize the precursors and the derived oxide powders. XRD analysis showed that well-crystallized and single-phase Bi₂Fe₄O₉ with orthorhombic symmetry was obtained at 700 °C for 2 h and BiFeO₃ and Fe₂O₃/FeCO₃ were intermediate phases before the formation of Bi₂Fe₄O₉. Bi₂Fe₄O₉ powders show weak ferromagnetism at room temperature.     

Development of perovskite in Fe-substituted Pb(Zn1/3Ta2/3)O3 and dielectric characteristics

Pb(Zn_1/3Ta_2/3)O₃ ceramics, compositionally modified by the incorporation of Fe to the octahedral lattice sites, were prepared and characterized in terms of perovskite development, dielectric properties, as well as microstructure evolution. The powders of the B-site precursor compositions were synthesized separately and reacted with PbO to form Pb[(Zn_1/3Ta_2/3),(Fe_1/2Ta_1/2)]O₃. The perovskite contents increased continuously with the Fe concentration. The maximum dielectric constant values of the ceramics increased tremendously with the fraction of Fe, whereas the dielectric maximum temperatures were rather insensitive to the compositional change.     

Crystal and magnetic study of the disordered perovskites Ca(Mn0.5Sb0.5)O3 and Ca(Fe0.5Sb0.5)O3

We have investigated the double perovskites Ca₂MSbO₆ (M = Mn, Fe) that have been prepared by solid-state reaction (M = Fe) and wet chemistry procedures (M = Mn). The crystal and magnetic structures have been studied from X-ray (XRD) and neutron powder diffraction (NPD) data. Rietveld refinements show that the crystal structures are orthorhombic (space group Pbnm) with complete disorder of M and Sb cations, so the formula should be rewritten as Ca(M_0.5Sb_0.5)O₃. Due to this disorder no evidences of Jahn-Teller distortion can be observed in the MnO₆ octahedra of Ca(Mn_0.5Sb_0.5)O₃, in contrast with the ordered double perovskite Sr₂MnSbO₆. Ca(Fe_0.5Sb_0.5)O₃ behaves as an antiferromagnet with an ordered magnetic moment for Fe³⁺ of 1.53(4)μ_B and a propagation vector k = 0, as investigated by low-temperature NPD. The antiferromagnetic ordering is a result of the high degree of Fe/Sb anti-site disorder of the sample, which originates the spontaneous formation of Fe-rich islands, characterized by the presence of strong Fe-O-Fe antiferromagnetic couplings with enough long-range coherence to produce a magnetic contribution perceptible by NPD. By contrast, the magnetic structure of Ca(Mn_0.5Sb_0.5)O₃ cannot be observed by low-temperature NPD because the magnitude of the ordered magnetic moments is below the detection threshold for neutrons.     

Fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composite ceramics synthesized by novel powder-in-sol precursor hybrid processing route

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via sol-gel wet chemistry process. Uniformly distributed slurry is obtained after ball milling and used in the fabrication of the ceramics with low sintering temperatures. The ceramics show coexistence of ferromagnetic and ferroelectric phases with obvious ferromagnetic and ferroelectric hysteresis loops at room temperature, besides exhibiting excellent magnetic and dielectric properties in a wide range of frequency. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

High frequency magnetic mechanism of Ni-substituted Co2Z hexagonal ferrite

The magnetic properties, especially the high frequency magnetic mechanism, of Ni-substituted Co₂Z hexagonal ferrite were studied. The polycrystalline Z-type hexagonal ferrite of Ba₃Ni_xCo_2−xFe₂₄O₄₁ (0 ≤ x ≤ 2) were prepared by solid-state reaction. The results indicate that Ni-substituted Co₂Z samples all exhibit typical soft magnetic character. Substitution of Ni for Co will turn the planar magnetocrystalline anisotropy of Co₂Z to uniaxial anisotropy when x ≥ 1, so that the permeability drops dramatically and domain wall resonance appears in the frequency spectra. With the rise of Ni amount or sintering temperature, domain wall resonance strengthens gradually.     

Grain size-dependent electrical transport properties in La0.75Sr0.25Mn1−xMgxO3±δ ceramics

The role of GB in Mg-substituted lanthanum-strontium manganite ceramics is studied with microstructural details. At higher concentrations of Mg (x>0.05), where the average grain size is ∼1 μm, the M-I transition is shifted from 348 to 110 K. Annealing in lower p_O2 (10⁻⁶ atm) at 1375 K for 1 h obliterates the M-I transition and brings in insulating behaviour throughout the temperature of measurement. Re-annealing in oxygen atmosphere for 10-25 min reintroduces the M-I transition, indicating that the electrical transport properties depend on the chemical inhomogeneity introduced by the in- or out-diffusion of oxygen through the GB regions. Samples with larger grain size (∼35 μm) do not exhibit major modifications in electrical resistivity on annealing in different p_O2. The insulating manganites display non-linear J-E characteristics below the magnetic transition temperature at electric field strengths <50 V/cm. The non-linear behaviour is explained on the basis of the inelastic tunnelling through the multiple localised states in the insulating GB regions. The external magnetic field lowers the voltage at which the non-linearity sets in. The tunnelling therefore may be not only through independent defect centres of oxygen vacancies (V_O), but possibly from defect complexes such as Mn³⁺-V_O or Mn²⁺-V_O, where spin-dependent tunnelling can take place.     

Temperature dependence of magnetic property and photocatalytic activity of Fe3O4/hydroxyapatite nanoparticles

Fe₃O₄/hydroxyapatite (HAP) nanoparticles have been developed as a novel photocatalyst support, based on the embedment of magnetic Fe₃O₄ particles into HAP shell via homogeneous precipitation method. The resultant nanoparticles were characterized by transmission electron microscope (TEM) and X-ray diffraction (XRD). These particles were almost spherical in shape, rather monodisperse and have a unique size of about 25 nm in diameter. The effect of calcination temperature on magnetic property and photocatalytic activity of Fe₃O₄/HAP nanoparticles was investigated in detail. The obtained results showed that the Fe₃O₄/HAP nanoparticles calcined at 400 °C possessed good magnetism and photocatalytic activity in comparison with that calcined at other temperatures.     

Mechanosynthesis and magnetic properties of nanocrystalline LaFeO3 using different iron oxides

The synthesis of the orthoferrite LaFeO₃ using high-energy ball-milling of La₂O₃ and Fe₃O₄ or α-Fe₂O₃ oxides and subsequent thermal treatments of resulting powders was studied. The phase evolution during the mechanical treatment was analyzed by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). Also, magnetic properties of the obtained materials were measured at room temperature by vibrating sample magnetometry (VSM). From 30 min of mechanochemical activation the gradual disappearance of reactants and the formation of LaFeO₃ were observed. For both reactive mixtures the reaction was completed after 3 h of milling. Magnetic hysteresis loops of these mechanoactivated samples showed a significant ferromagnetic component in LaFeO₃. This behavior was interpreted on the basis of a spin-canting effect induced by the mechanochemical treatment. Thermal treatments allowed the relaxation of the distorted structure, resulting in the formation of the conventional antiferromagnetic LaFeO₃ phase.     

Nb NMR and Fe EPR study of local magnetic properties of magnetoelectric Pb(Fe1/2Nb1/2)O3

The ⁹³Nb NMR and Fe³⁺ EPR spectra of a magnetoelectric Pb(Fe_1/2Nb_1/2)O₃ (PFN) single crystal as well as of PFN ceramics and nanopowder have been measured. The relation between the chemical disorder and the local magnetic structure has been determined. The data show the occurrence of an antiferromagnetic (AFM) phase transition at 145 and 135 K in both single crystal and ceramic samples, respectively. The AFM phase transition is however essentially smeared out in the nanopowder. Two different Nb sites exist which have different local magnetic fields which are essentialy distributed in value. These two Nb sites correspond to different types of “chemical” order: Fe rich, Nb poor and Fe poor, Nb rich regions. The temperature dependences of the sublattice magnetizations and the values of the superhyperfine fields at the Nb ions are obtained. NMR data suggest that a spin-glass like state of PFN below T = 20 K arises from the Fe poor, Nb rich regions.     

The effect of annealing on the photoluminescence of layered perovskite H1.77[Sr0.8Bi0.21Ta2O7] powders

The microstructural and luminescent properties of H_1.77[Sr_0.8Bi_0.21Ta₂O₇] powders under different dehydrating conditions were investigated. The powders were obtained by protonating Bi₂SrTa₂O₉ powders in 3 M aqueous HCl solution, and were dehydrated by a post-annealing in air from 250 to 700 °C. The photoluminescence was enhanced for all dehydrated powders, especially for the sample annealed at 400 °C. The thermogravimetric analysis showed that there is an obvious dehydration temperature range from 200 to 400 °C, which was corresponding to the changing of the microstructure and photoluminescence in the same temperature range. The mechanism behind the experimental results was discussed.     

Multiferroic properties of Pb2Fe2O5 ceramics

Pb₂Fe₂O₅ (PFO) powders in monoclinic structure have been synthesized using lead acetate in glycerin and ferric acetylacetonate as the precursor. The powders were pressed into pellets, which were sintered into ceramics at 800 °C for 1 h. The morphology and structure have been determined by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). Polarization was observed in Pb₂Fe₂O₅ ceramics at room temperature, exhibiting a clear ferroelectric hysteresis loop. The remanent polarization of Pb₂Fe₂O₅ ceramic is estimated to be Pr ∼ 0.22 μC/cm². The origin of the polarization may be attributed to the off-centers of shifted Pb²⁺ ions as well as the FeO₆ octahedra in the perovskite-based structure of Pb₂Fe₂O₅. Magnetic hysteresis loop was also observed at room temperature. The Pb₂Fe₂O₅ ceramic shows coexistence of ferroelectricity and ferromagnetism. It provides a new field of research for complex oxides with multiferroic properties.     

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.     

Dependence of the SHS reaction behavior and product on B4C particle size in Al-Ti-B4C and Al-TiO2-B4C systems

Dependence of the combustion reaction behavior and product on the B₄C particle size in the Al-Ti-B₄C and Al-TiO₂-B₄C systems was investigated. The results show that in both systems, the SHS reaction kinetics is primarily controlled by diffusion. Increasing the B₄C particle size not only makes the ignition and self-sustainment of the SHS reaction difficult but also decreases the wave velocity and the degree of conversion. In comparison, the dependence of the SHS reaction behavior and the resultant product on the B₄C particle size is much more significant in the Al-Ti-B₄C system than that in the Al-TiO₂-B₄C system. The reason for the different dependence behaviors was investigated based on the SHS reaction mechanisms.