Influence of Co content on the characterization and magnetic properties of magnetite

Preparation of nanosized Co_xFe_3−xO₄; 0.05 ≤ x ≤ 0.20 particles from metal nitrates solution through citrate–precursor method was performed. XRD pattern of all prepared systems showed single phase with cubic spinel structure. The crystallite size was determined from TEM and found to agree with that calculated from Sherrer's equation (60–76 nm) The magnetic constants such as molar magnetic susceptibility (χ_M), Curie temperature (T_C) and saturation magnetization (M_S) were measured and the results indicated that, at x = 0.2 the values of χ_M, M_S, remanent magnetization (M_r,) and coercive field (H_c) are 23 emu/g mol, 77.62 emu/g, 33.17 emu/g and 574.5 Oe, respectively_.     

Synthesis and characterization of pure single phase Ni-Zn ferrite nanopowders by oxalate based precursor method

Series of single-phase Ni_1 − xZn_xFe₂O₄ (x = 0.20, 0.35, 0.50 and 0.60) nanopowders with average particle size of ∼ 35 nm have been synthesized by using oxalate based precursor method. Precursor powders were synthesized by reacting aqueous solutions of metal nitrates and oxalic acid by using different total metal ions: oxalic acid molar ratios and then evaporating them to dryness. Pure, single-phase Ni-Zn ferrite nanopowder was formed by calcining the precursor with total metal ion: oxalic acid ratio of 1:0.125 at a temperature of 850 °C. The synthesized nanopowders were characterized by using X-ray diffraction, Thermo-gravimetric and Differential Scanning Calorimetric analysis, Transmission Electron Microscope and Scanning Electron Microscope. Room temperature DC resistivity of the nanopowders was measured with respect to temperature by the two-probe method and was of the order of ∼ 10⁷ Ωcm. Room temperature saturation magnetization was measured by using Vibrating Sample Magnetometer and it varied between 34-49 emu/g depending on the composition. This aqueous solution based method provides a simple and cost-effective route to synthesize single phase, Ni-Zn ferrite nanopowders.     

Synthesis of carbon-Fe3O4 coaxial nanofibres by pyrolysis of ferrocene in supercritical carbon dioxide

Carbon-Fe₃O₄ coaxial nanofibres with a diameter around 100 nm were synthesized by pyrolysis of ferrocene in supercritical CO₂ at 400 °C. The single crystal Fe₃O₄ cores of the coaxial nanofibres are continuous, and the carbon shell is rich in H. The M-H hysteresis loop for the nanorodes measured at room temperature shows a saturation magnetization of 27.5 emu/g, much lower than 92 emu/g of the corresponding bulk material. This is attributed the considerable mass of the carbon shell and the nanoscale and anisotropy of the magnetite nanorods. A possible mechanism was proposed.     

On some properties of PZT-NZF composite films manufactured by hybrid synthesis route

Composite magnetoelectric films using ferroelectric lead zirconate titanate (PZT) and ferromagnetic nickel zinc ferrite (NZF) were prepared using the combination of sol-gel and hydrothermal process on Pt/Ti/SiO₂/Si substrates. The thickness was estimated ∼2 μm using cross-sectional SEM. Structure, morphology and electro-magnetic characterization were assessed using XRD, XPS, SEM, dielectric, leakage current, ferroelectric, and magnetic property analyze. The composite films exhibit coexistence of ferroelectric and ferromagnetic ordering at room temperature with a remnant polarization (P_r), and coercive field (E_c) of 1.2 μC/cm² and 7.8 kV/cm, respectively, and saturation magnetization (M_s) ∼20 emu/cm³. Polarization improved ∼5.2% upon poling the composite film using a magnetic field of 1 T.     

Synthesis and characterization of nano-crystalline strontium hexaferrite using the co-precipitation and microemulsion methods with nitrate precursors

Nano-crystalline strontium hexaferrite (SrFe₁₂O₁₉) powder was synthesized using the classical co-precipitation and microemulsion methods. The precursors were obtained by precipitating Sr²⁺ and Fe²⁺ ions using tetramethylammonium hydroxide and calcinating at different temperatures ranging from 400 °C to 1000 °C in air. The influence of the Sr²⁺/Fe³⁺ mol ratio and the calcination temperature on the product formation and magnetic properties were studied. The formation of nanosized particles of SrFe₁₂O₁₉ with a relatively high saturation magnetization M_s = 64 Am²/kg, remanent magnetization of M_r = 39 Am²/kg and a coercitivity of H_c = 5.5 kOe was achieved at a Sr²⁺/Fe³⁺ mol ratio of 1:8 calcined at 900 °C. The formation of the SrFe₁₂O₁₉ was inspected using XRD analysis, thermogravimetric analysis (TGA), differential thermal analysis (DTA), TEM, and magnetic measurements.     

Magnetic core-bilayer shell nanoparticle: A novel vehicle for entrapmentof poorly water-soluble drugs

We are herein reporting a synthesis of indomethacin-loaded bilayer-surface magnetite nanoparticles and their releasing behavior. The particles were first stabilized with oleic acid as a primary surfactant, followed by poly(ethylene glycol) methyl ether-poly(?-caprolactone) (mPEG-PCL) amphiphilic block copolymer as a secondary surfactant to form nanoparticles with hydrophobic inner shell and hydrophilic corona. mPEG-PCL copolymers with systematically varied molecular weights of each block (2000-2000, 2000-10,000, 5000-5000 and 5000-10,000 g/mol, respectively) were synthesized via a ring-opening polymerization of ?-caprolactone using mPEG as a macroinitiator. The particles were 9 nm in diameter and exhibited superparamagnetic behavior at room temperature with saturation magnetization (M_s) about 35 emu/g magnetite. Percent of magnetite and the copolymers in the complexes were determined via thermogravimetric analysis (TGA). The effect of mPEG and PCL block lengths in the copolymer-magnetite complex on the properties of the particles, e.g. particle size, magnetic properties, stability in water, drug entrapping and loading efficiency and its releasing behavior were investigated. This novel magnetic nanocomplex might be suitable for use as an efficient drug delivery vehicle with tunable drug-released properties.     

Improvement of the magnetic properties of highly porous open-celled magnets

In the present study, we have investigated the influence of B₂O₃ addition on structural and magnetic properties of hard magnetic BaFe₁₂O₁₉ foams. In the presence of B₂O₃ open-celled foams were successfully fabricated at a calcination temperature of 1300 °C. Magnetization values have been improved by 50% with B₂O₃-addition. Remanence magnetization (M_R), specific magnetization at 1.5 T (M_S) and coercive field (H_c) values were obtained to be 32.7 emu/g, 63.0 emu/g and 2100 Oe, respectively for the 0.5 wt% B₂O₃ containing foams having 30 pores/in. Foams with these magnetic properties have the potential to be used in different areas of technology as permanently magnetic materials.     

Effect of oxygen substitution by nitrogen on magnetic and transport properties in Sr2FeMoO6 compound

The result of Fullprof_suite refinement shows that the structure of Sr₂FeMo(O_0.889N_0.111)₆ is assigned to tetragonal system with space group I4/m and its cell parameters are a = 0.559250(8) nm and c = 0.789842(19) nm. The M-T curve demonstrates the magnetic transition temperature being over 300 K. Isothermal magnetization shows that the saturation and spontaneous magnetization are 1.29 and 0.87 μ_B/fu at 300 K, respectively. The sample shows a large magnoresistance in a 0.5 T magnetic field. The electrical transport behavior in the temperature range from 108 to 274 K is dominated by electron-phonon scattering under a zero magnetic field. However, under a magnetic field of 0.5 T the electron-electron and electron-magnon scattering control electrical transport behavior in the temperature range from 145 to 267 K and electrical transport property is governed by electron-phonon scattering in the temperature region of 97-130 K.     

A novel bithiazole oligomer containing C60 and its ferro-complexes: syntheses and magnetic properties

A novel bithiazole oligomer (PCBT) was synthesized from C₆₀ and the diazo salt of 2,2′-diamino-4,4′-bithiazole (DABT). Its ferro-complex (PCBT-Fe²⁺) was prepared from PCBT and FeSO₄ in DMSO solution under a purified nitrogen atmosphere. The magnetic behavior of PCBT and PCBT-Fe²⁺ was measured as a function of magnetic field strength (0-60 kOe) at 5 K and as a function of temperature (5-300 K) at a magnetic field strength of 30 kOe. PCBT-Fe²⁺ complex exhibits a hysteresis cycle at 5 K, the observed coercive field (H_C) and remnant magnetization (M_r) are 690 Oe and 0.12 emu/g, respectively. The results show that PCBT is an anti-ferromagnet and its Fe²⁺-complex is a soft ferromagnet.     

Effects of PVA content on the synthesis of LaFeO3 via sol-gel route

LaFeO₃ were synthesized via a sol-gel route based on polyvinyl alcohol (PVA). Differential scanning calorimetry (DSC), Thermogravimetric (TG), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Raman spectroscopy and field emission scanning electron microscopy (FESEM) techniques were used to characterize precursors and derived oxide powders. The effect of the ratios of positively charged valences to hydroxyl groups of PVA (Mⁿ⁺/-OH) on the formation of LaFeO₃ was investigated. XRD analysis showed that single-phase and well-crystallized LaFeO₃ was obtained from the Mⁿ⁺/-OH = 4:1 molar ratio precursor at 700 °C. For the precursor with Mⁿ⁺/-OH = 2:1, nanocrystalline LaFeO₃ with average particle size of ∼50 nm was formed directly in the charring procedure. With increase of PVA content to Mⁿ⁺/-OH = 1:1, phase pure LaFeO₃ was obtained at 500 °C.     

Temperature dependent magnetic properties of CoFe2O4/CTAB nanocomposite synthesized by sol–gel auto-combustion technique

A CoFe₂O₄/cetyl trimethylammonium bromide (CTAB) nanocomposite has been fabricated by a sol–gel auto-combustion method. Characterization of the material revealed the composition of the crystalline phase as CoFe₂O₄ while FT-IR confirmed the presence of CTAB on the nanoparticles. From X-ray line profile fitting, average crystallite size was estimated to be 22±6 nm. SEM analysis showed a porous sheet-like morphology with internal nanosize grains of about 30 nm. The room temperature coercive field (H_c) of the CoFe₂O₄/CTAB nanocomposite was found to be 1045 Oe which is close to the previously reported room temperature values for bulk CoFe₂O₄. The H_c was observed to decrease almost linearly with the square root of the temperature (√T) according to Kneller's law. From the linear fit of H_c versus √T, the zero-temperature coercivity (H_c0) and superparamagnetic blocking temperature (T_B) of the CoFe₂O₄/CTAB nanocomposite were found to be ∼9.1 kOe and ∼425 K, respectively. The remanence magnetization (M_r), the reduced remanent magnetization (M_r/M_s), and the effective magnetic anisotropy (K_eff) decrease with increasing temperature. The M_r/M_s value of 0.6 at 10 K higher than the theoretical value of 0.5 for non-interacting single domain particles with the easy axis randomly oriented suggests the CoFe₂O₄/CTAB nanocomposite to have cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model.     

Effect of BNBTKNN on the electrical properties of bismuth ferrite thin films

BiFeO₃/[0.93(Bi_0.50Na_0.50TiO₃)-0.05BaTiO₃-0.02K_0.50Na_0.50NbO₃] (BFO/BNBTKNN) bilayered thin films were fabricated on Pt/TiO₂/SiO₂/Si substrates without any buffer layers by a combined sol-gel and radio frequency sputtering route. Effect of BNBTKNN on electrical properties of BFO/BNBTKNN thin films was investigated. A higher phase purity and a denser microstructure are induced for the BFO/BNBTKNN bilayered thin film by using the bottom BNBTKNN layer, resulting in its lower leakage current density. Moreover, the enhancement in dielectric behavior is also demonstrated for such a bilayer, where a high dielectric constant and a low dielectric loss are obtained. The BFO/BNBTKNN bilayered thin film has an improved multiferroic behavior: 2P_r ∼ 76.8 μC/cm², 2E_c ∼ 378.1 kV/cm, 2M_s ∼ 52.6 emu/cm³, and 2H_c ∼ 453.6 Oe, together with a low fatigue rate up to ∼1 × 10⁹ switching cycles.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.     

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.     

Mechanosynthesis,crystal structure and magnetic characterization of M-type SrFe12O19

M-type strontium hexaferrite was prepared by mechanosynthesis using high-energy ball milling. The influence of milling parameters, hematite excess and annealing temperature on magnetic properties of SrFe₁₂O₁₉ were investigated. Commercial iron and strontium oxides were used as starting materials. It was found that mechanical milling followed by an annealing treatment at low temperature (700 °C) promotes the complete structural transformation to Sr-hexaferrite phase. For samples annealed at temperatures from 700 to 1000 °C, saturation magnetization values (M_s) are more sensitive to annealing temperature than coercivity values (H_c). The maximum M_s of 60 emu/g and H_c of 5.2 kOe were obtained in mixtures of powders milled for 5 h and subsequently annealed at 700 °C. An increase in the annealing temperature produces negligible changes in magnetic saturation and coercivity. An excess of hematite as a second phase produces a slight decrease in the saturation magnetization but leads to a significant increase in coercive field, reaching 6.6 kOe.     

Magnetic and Optical Properties of Mn1−xZnxFe2O4 Nanoparticles

Mn_1?xZn_xFe₂O₄ (x = 0.0?1.0) NPs (MZF NPs) were synthesized by a citric acid assisted sol–gel process. MZF NPs show superparamagnetic characteristics at room temperature. Saturation magnetization (M_s) of MnFe₂O₄ NPs is 70.52 emu/g is very close to the bulk saturation magnetization value of 80 emu/g. The observed M_s = 35.90 emu/g value for ZnFe₂O₄ particles is much greater than the bulk M_s value of 5 emu/g. This case is attributed to cation distribution change from normal spinel to mixed structure. The small M_r/M_s ratios (the maximum 0.147) specify uniaxial anisotropy in the Mn_1-xZn_xFe₂O₄ NPs. The average crystallite diameter (D _mag) was evaluated from magnetic analyses. The obtained D _mag values are between 27.67 and 33.60 nm and this range is in great accordance with the results calculated from XRD measurements. Among the NPs, the samples with more zinc content show higher diffuse reflectance. The optical direct band gap of MZF NPs is found to decrease from 2.1 to 1.90 eV as the zinc content rises.     

Silver doped lanthanum chromites by microwave combustion method

Considering the advantages of microwave combustion technique with the possibility of utilizing cheap precursors, short reaction time and nanocrystalline products, the present work reports the synthesis of silver doped lanthanum chromites. Structural and physicochemical properties were investigated with the help of various characterization techniques. The FTIR spectrum reveals the characteristic metal oxygen bands for Cr-O stretching at 604 cm⁻¹, O-Cr-O bending mode at 419 cm⁻¹ and Ag-O bands at 561 cm⁻¹ and 443 cm⁻¹. The powder X-ray diffraction patterns exhibit the formation of hexagonal structure with the dopant peaks at 2θ values of 38.3°, 44.1° and 64.4° apart from the peaks corresponding to lanthanum chromite. TGA analysis of the samples shows stable behavior of the product. Nanosized particles with size as small as ∼7-8 nm and larger ones ∼20-26 nm are observed from transmission electron micrographs. Room temperature magnetic study exhibits hysteresis loop formation during magnetization of samples.     

Chemical vapor deposition of carbon on particulate TiO2 from CH4 and subsequent carbothermal reduction for the synthesis of nanocrystalline TiC powders

The present study aims to investigate chemical vapor deposition of carbon from CH₄ on TiO₂ particles and to establish optimal conditions for the synthesis of nanocrystalline TiC powders. Mass measurements, XRD, HR-TEM and SEM were used to characterize the products at various stages of the reactions. Oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere and were coated with thin pyrolytic carbon layers of 10-20 nm. XRD analysis showed that the coated powders consisted of C, TiO₂ and titanium sub-oxides. The powders containing ∼33 ± 2 wt% carbon were used for the carbothermal reduction of TiO₂ to TiC at 1600-1800 K under Ar flow. Lattice constant and mass loss of the samples increased to the levels of TiC with temperature and time. Nearly pure TiC powders with a mean particle size of ∼125 nm were synthesized at 1750-1800 K within 30 min.     

Suppression of the memory effect observed in alkaline secondary batteries under partial charge-discharge conditions

Under partial charge-discharge cycling conditions at a state of charge (SoC) of 50-70%, a greater effect on the suppression of the memory effect, which occurs in rechargeable alkaline batteries containing a Ni-electrode, was observed using conductive materials such as nano-sized Co(OH)₂ and CoO powders in the Ni-electrode. The nano-sized Co(OH)₂ powder, prepared by the reverse micelle method, consisted of approximately 15-60 nm spherical-shaped particles. The Ni-Cd cells containing approximately 15- or 20-nm-sized Co(OH)₂ powder and 18 nm CoO powder in the Ni-electrode did not show any memory effect after a 150-cycle partial charge-discharge cycling test, and γ-NiOOH, which is mainly the origin of the memory effect, was not detected in the Ni-electrode, whereas the Ni-Cd cell containing the same content of 500 nm Co(OH)₂ powder and 18 nm CoO in the Ni-electrode showed the memory effect. The nano-sized Co(OH)₂ powder could be more homogeneously dispersed in the Ni-electrode when compared to the 500 nm powders and might develop a superior electro-conductive network between the Ni(OH)₂ particles.     

Preparation and properties of poly(ethylene glycol)-block-poly(acrylic acid)-coated cobalt nanocrystals

Reported in this paper are the preparation and properties of ?-Co nanocrystals coated by poly(ethylene glycol)-block-poly(acrylic acid) (PEG-b-PAA). These particles were prepared via the thermal decomposition of Co₂(CO)₈ at 185 °C in 1,2-dichlorobenzene, in the presence of the surfactant PEG-b-PAA and the co-surfactant trioctylphosphine oxide. At a given initial Co₂(CO)₈ concentration, the size of the particles increased with increasing Co₂(CO)₈-to-PEG-b-PAA molar ratio, and could be tuned between ∼5 and ∼20 nm. The size distribution of the particles narrowed as the Co₂(CO)₈ concentrations increased. The resultant particles were dispersible in a wide range of solvents, including chloroform, N,N-dimethylforamide, and water, which solubilized PEG. Magnetic measurements revealed that the particles possessed saturation magnetization close to that of bulk Co, suggesting high purity of the particles.