Dielectric and magnetic properties of bulk and layered tape cast CoFe2O4–Pb(Fe1/2Ta1/2)O3 composites

The microstructure, dielectric and magnetic properties of bulk and layered CoFe₂O₄–Pb(Fe_1/2Ta_1/2)O₃ composites were studied. Ceramic samples based on previously mixed ferrite and relaxor powders were sintered at 950 °C. Ferrite and relaxor green tapes 150 μm thick were prepared by tape casting, then cut, stacked alternately, laminated and co-sintered at 950 °C. High and broad maxima of dielectric permittivity reaching 2000 at 1 kHz were found for bulk CoFe₂O₄–Pb(Fe_1/2Ta_1/2)O₃ ceramic. Measurements of the magnetization of the investigated composites as a function of magnetic field and temperature exhibited behavior typical of hard magnetic materials. The layered composites showed lower coercivity, higher saturation magnetization and a higher magnetoelectric coefficient than the bulk ceramics. Distinct changes in field- and zero field-cooled magnetization curves at −200 °C could be ascribed to the antiferromagnetic transition of the PFT relaxor phase. Multilayer CoFe₂O₄–Pb(Fe_1/2Ta_1/2)O₃ composites exhibited a magnetoelectric coefficient of 200 mV/(cm Oe) at a frequency of the modulation magnetic field equal to 5 kHz.     

Hysteresis loops of polarization and magnetization in (BiNd0.05)(Fe0.97Mn0.03)O3/Pt/CoFe2O4 layered epitaxial thin film grown by pulsed laser deposition

(BiNd_0.05)(Fe_0.97Mn_0.03)O₃ (BNFM)/Pt/CoFe₂O₄ (CFO) layered thin film was fabricated on (100) SrTiO₃ substrate by pulsed laser deposition. BNFM, Pt, and CFO layers were epitaxially grown on the substrate. Almost no increase of leakage current due to the formation of heteroepitaxial structure was found, and well-saturated hysteresis loops in the polarization vs electric field and magnetization vs magnetic field curves coexist at room temperature. The remnant polarization and remnant magnetization values were 55 μC/cm², and 70-145 mA/m, respectively.     

Structure and magnetic property of CoFe2−xSmxO4 (x = 0–0.2) nanofibers prepared by sol–gel route

CoFe_2−xSm_xO₄ (x = 0–0.2) nanofibers with diameters about 100–300 nm have been prepared using the organic gel-thermal decomposition method. The composition, structure and magnetic properties of the CoFe_2−xSm_xO₄ nanofibers were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, inductive coupling plasma mass analyzer and vibrating sample magnetometer. The CoFe_2−xSm_xO₄ (x = 0–0.2) nanofibers obtained at 500–700 °C are of a single spinel structure. But, at 800 °C with a relatively high Sm content of 0.15–0.2 the spinel CoFe_2−xSm_xO₄ ferrite is unstable and the second phase of perovskite SmFeO₃ occurs. The crystalline grain sizes of the CoFe_2−xSm_xO₄ nanofibers decrease with Sm contents, while increase with the calcination temperature. This grain reduction effect of the Sm³⁺ ions doping is largely owing to the lattice strain and stress induced by the substitution of Fe³⁺ ions with larger Sm³⁺ ions in the ferrite. The saturation magnetization and coercivity increase with the crystallite size in the range of 8.8–57.3 nm, while decrease with the Sm content from 0 to 0.2 owing to a smaller magnetic moment of Sm³⁺ ions. The perovskite SmFeO₃ in the composite nanofibers may contribute to a high coercivity due to the interface pinning, lattice distortion and stress in the ferrite grain boundary fixing and hindering the domain wall motion.     

Flat epitaxial ferromagnetic CoFe2O4 films on buffered Si(001)

Ferromagnetic films of spinel CoFe₂O₄ have been grown epitaxially on Si(001) using CeO₂/YSZ double buffer layers. The heterostructures were built in a single process by pulsed laser deposition with real-time control by reflection high-energy electron diffraction. YSZ and CeO₂ grow cube-on-cube on Si(001) and CoFe₂O₄ grows with (111) out-of-plane orientation, presenting four in-plane crystal domains. The interface with the buffer layers is smooth and the CoFe₂O₄ surface is atomically flat, with roughness below 0.3 nm. The films are ferromagnetic with saturation magnetization around 300 emu/cm³. The properties signal that CoFe₂O₄ is a good candidate for monolithic devices based on ferromagnetic insulating spinels.     

Plasmachemical synthesis of maghemite nanoparticles in atmospheric pressure microwave torch

The powder of γ − Fe₂O₃ nanoparticles was synthesized in microwave torch at atmospheric pressure from 0.05 sccm of Fe(CO)₅ vapors in 670 sccm of argon. The optimization of the torch reactor design and deposition conditions allowed continual synthesis of γ − Fe₂O₃ nanoparticles at low power consumption. The synthesized powder was collected at the reactor walls and analyzed by TEM, X-ray diffraction and Raman spectroscopy without any further purification or treatment. The mean diameter of NPs, as observed by TEM, was 12 nm with a 90% confidence interval 5.5-22 nm.     

High‐Performance Flexible Organic Nano‐Floating Gate Memory Devices Functionalized with Cobalt Ferrite Nanoparticles

Nano‐floating gate memory (NFGM) devices are transistor‐type memory devices that use nanostructured materials as charge trap sites. They have recently attracted a great deal of attention due to their excellent performance, capability for multilevel programming, and suitability as platforms for integrated circuits. Herein, novel NFGM devices have been fabricated using semiconducting cobalt ferrite (CoFe₂O₄) nanoparticles (NPs) as charge trap sites and pentacene as a p‐type semiconductor. Monodisperse CoFe₂O₄ NPs with different diameters have been synthesized by thermal decomposition and embedded in NFGM devices. The particle size effects on the memory performance have been investigated in terms of energy levels and particle–particle interactions. CoFe₂O₄ NP‐based memory devices exhibit a large memory window (≈73.84 V), a high read current on/off ratio (read I_on/I_off) of ≈2.98 × 10³_, and excellent data retention. Fast switching behaviors are observed due to the exceptional charge trapping/release capability of CoFe₂O₄ NPs surrounded by the oleate layer, which acts as an alternative tunneling dielectric layer and simplifies the device fabrication process. Furthermore, the NFGM devices show excellent thermal stability, and flexible memory devices fabricated on plastic substrates exhibit remarkable mechanical and electrical stability. This study demonstrates a viable means of fabricating highly flexible, high‐performance organic memory devices.     

Solvothermal synthesis of Co/CoFe2O4 nanobelts

A facile solvothermal route is used for the synthesis of Co/CoFe₂O₄ nanobelts by rationally manipulating the dosages of a surfactant Poly(vinylpyrrolidone) (PVP, MW 40000). PVP plays a pivotal role in preparing the Co/CoFe₂O₄ nanobelts. The optimal dosage for the synthesis of the Co/CoFe₂O₄ nanobelts is 0.5 g. Lower than this value, octahedral particles and nanobelts were coexistent; higher than this value, octahedral particles were obtained. Furthermore, the possible formation mechanism of Co/CoFe₂O₄ nanobelts was proposed. A small quantity of Co²⁺ ions are reduced by glycerol, which is the reason for the presence of metallic Co in the CoFe₂O₄ ferrite. The Co/CoFe₂O₄ nanobelts may be very attractive for potential applications because of their outstanding magnetic properties (Ms = 110 emu/g, Hc = 387 Oe).     

Structure, properties and gas sensing behavior of Cr2 − xTixO3 films fabricated by electrostatic spray assisted vapour deposition

Single-phase eskolaite crystalline Cr_2 − xTi_xO₃ films (CTO) with a uniform porous microstructure were fabricated via an electrostatic spray assisted vapour deposition (ESAVD) method. The sensing behavior upon exposure to ammonia and ethanol was characterized in a CTO film-based sensor device in terms of response, reproducibility, humidity constraints and sensor stability. The ESAVD process has been shown to be capable of producing CTO films at low temperature (650 °C) and more importantly, it results in a more uniform titanium distribution and better microstructural control than processes based on solid-state chemical reactions. The material with a nominal composition of Cr_1.7Ti_0.3O₃ exhibited the highest sensitivity among the different Cr_2 − xTi_xO₃ compositions examined towards ammonia over the temperature range of 200-500 °C with a peak sensitivity of 2.90 at 200 °C. The CTO materials, when used as sensors, also exhibit excellent responses to ethanol concentration in air. The sensitivity was 0.64 for 10 ppm ethanol, 0.85 for 25 ppm, and 0.92 for 50 ppm, respectively.     

Structural and magnetic properties of CoFe2O4 thin films deposited by laser ablation on Si (001) substrates

Cobalt ferrite (CoFe₂O₄) thin films have been deposited on Si (001) substrates, with different substrate temperatures (T_dep = 25 °C − 600 °C). The films were prepared by pulsed laser ablation with a KrF excimer laser (wavelength λ = 248 nm). The oxygen pressure during deposition was 2 × 10⁻² mbar. The films structure was studied by X-ray diffraction (XRD) and their surface was examined by scanning electron microscopy (SEM). The magnetic properties were measured with a vibrating sample magnetometer (VSM). For low deposition temperatures, the films presented a mixture of a CoFe₂O₄ phase, with the cubic spinel structure, and cobalt and iron antiferromagnet oxides with CoO and FeO stoichiometries. As the deposition temperature increased, the CoO and FeO relative content strongly decreased, so that for T_dep = 600 °C the films were composed mainly by polycrystalline CoFe₂O₄. The magnetic hysteresis cycles measured in the films were horizontally shifted due to an exchange coupling field (H_exch) originated by the presence of the antiferromagnetic phases. The exchange field decreased with increasing deposition temperature, and was accompanied by a corresponding increase of the coercivity and remanence ratio of the cycles. This behavior was due to the strong reduction of the CoO and FeO content, and to the corresponding dominance of the CoFe₂O₄ phase on the magnetic properties of the thin films.     

High temperature stability of surfactant capped CoFe2O4 nanoparticles

We investigate the effect of adsorbed surfactant on the structural stability of CoFe₂O₄ nanoparticles during vacuum thermal annealing. In-situ high temperature X-ray diffraction studies show a reduction of oleic acid coated CoFe₂O₄ nanoparticles into α-Fe and CoO under annealing at 800 °C. On the contrary, the uncoated CoFe₂O₄ nanoparticles remains stable, with its cubic phase intact, even at 1000 °C. Thermo-gravimetric analysis coupled mass spectra reveals that the evolved carbon from the surfactant aids the removal of oxygen atom from CoFe₂O₄ lattice thereby reducing it to α-Fe and CoO phases. These results are important in tailoring stable CoFe₂O₄ nanostructures for various applications.     

Size Controlled Synthesis of CoFe2O4 Nanoparticles with Polyethylene Glycol

Nanoparticles (NPs) of CoFe₂O₄, which is a well-known spinel ferrite and hard magnetic material with very high cubic magnetocrystalline anisotropy, were synthesized by co-precipitation method. The effect of different molecular weights of polyethyleneglycol (PEG) on the magnetic properties and crystallite size of CoFe₂O₄ NPs was investigated by using PEG-400, PEG-10.000 and, PEG-20.000. Crystalline structure and size of CoFe₂O₄ NPs were studied using X-ray diffraction (XRD). The VSM studies showed that the saturation magnetization (M _s ) and coercivity (H _c ) of the CoFe₂O₄ NPs depend on molecular weight of PEG.     

Self-controlled growth of Fe3BO6 crystallites in shape of nanorods from iron-borate glass of small templates

Fe₃BO₆ can be an ideal compound for devising functional magnetic and dielectric properties in a single material for multiple applications such as electrodes, gas sensors, or medical tools. Useful to tailor such properties, here we report on a self-controlled Fe₃BO₆ growth in a specific shape of nanorods from a supercooled liquid precursor (an inorganic polymeric liquid or glass) of an initial composition (100 − x)B₂O₃ − xFe₂O₃, x = 40–50 mol%. B₂O₃ as a strong glass former co-bridges the Fe³⁺ ions in oxygen polygons primarily in a 2-D interconnected polymer network so that it dictates preferably a 1-D directional growth on the reaction Fe³⁺ species in form of a compound Fe₃BO₆, a favorable phase to nucleate and grow when annealing a precursor at 500–800 °C in ambient air. Distinct nanorods with a diameter ∼200 nm and 40–100 μm length have been formed on 10–15 min annealing a sample in microwave at moderate temperature 550 °C. A bonded surface B₂O₃ layer (15–25 nm thickness) has grown on the Fe₃BO₆ of the nanorods in situ in a specific structure. XPS bands in the Fe³⁺, B³⁺ and O²⁻ species confer this model structure. A local BO₃ → BO₄ conversion has incurred in the boroxol (B₃O_4.5)_n, n → ∞, rings in the surface layer, showing three distinct IR bands at 1035, 1215 and 1425 cm⁻¹.     

Amperometric hydrogen peroxide biosensor based on cobalt ferrite-chitosan nanocomposite

A novel H₂O₂ biosensor based on horseradish peroxidase (HRP) immobilized into CoFe₂O₄-chitosan nanocomposite has been developed for the detection of hydrogen peroxide. The nanocomposite films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). HRP has been entrapped into CoFe₂O₄-chitosan nanocomposite film and the immobilized enzyme could retain its bioactivity. This biosensor exhibited a fast amperometric response to hydrogen peroxide. The linear range for H₂O₂ determination was from 3 × 10^− 2 to 8 mM, with a detection limit of 2 × 10^− 3 mM based on S/N = 3. The response time of the biosensor was 4 s. The effects of the pH and the temperature of the immobilized HRP electrode were also studied.     

Preparation and characterization of polyoxometalate-Ag nanoparticles composite multilayer films

The multifunctional thin films (BW₁₂/Ag NPs)_n (BW₁₂ = BW₁₂O₄₀, NPs = nanoparticles) were prepared by layer-by-layer self-assembly method. The (BW₁₂/PEI-Ag⁺)_n (PEI = polyethylenimine) composite films were achieved through alternately depositing anionic BW₁₂ and cationic PEI-Ag⁺ complex. The deposition process of (BW₁₂/PEI-Ag⁺)₁₀ multilayer is linear layer-by-layer self-assembly. Under UV irradiation, Ag ions in (BW₁₂/PEI-Ag⁺)_n multilayer films were reduced photochemically into Ag NPs and (BW₁₂/Ag NPs)₁₀ films were obtained. Through UV-vis measurements, the presence of surface plasma absorption peak at 445 nm demonstrated the formation of silver NPs. The electrochemical and antibacterial activities of (BW₁₂/Ag NPs)_n films were investigated. The electrochemical results indicate that the glassy carbon electrode modified with (BW₁₂/Ag NP)_n film exhibits the electroreduction toward O₂. Moreover, the (BW₁₂/Ag NP)₁₀ multilayer films exhibit long-lasting antibacterial properties toward Escherichia coli (E. coli).     

Synthesis,structural and electrical properties of triethylene glycol (TREG) stabilized Mn0.2Co0.8Fe2O4 NPs

Triethylene glycol (TREG) stabilized Mn_0.2Co_0.8Fe₂O₄ NPs was synthesized by a glycothermal reaction. XRD analysis identified the product as Mn_0.2Co_0.8Fe₂O₄ with a high phase purity. Nano-sized particles with an average size of about 6–8 nm were obtained with nearly single crystalline nature with TEM analysis. Superparamagnetic-like behavior of TREG stabilized Mn_0.2Co_0.8Fe₂O₄ NPs was observed by VSM. The binding between TREG and Mn_0.2Co_0.8Fe₂O₄ NPs was investigated with FT-IR and found to be via O on the TREG and NP surface. TG analysis indicated that the Mn_0.2Co_0.8Fe₂O₄ NP content was about 40%, with a TREG-shell content to be around 60%. Overall conductivity of the nanocomposite is in the range of 10^?10 to 10^?7 S cm^?1 with a strong dependence on temperature and frequency, indicating ionic conductivity. The nanocomposite exhibited lower ?’ and ?″ compared to TREG stabilized Mn_0.2Co_0.8Fe₂O₄ NPs due to the doping of co-doping of manganese and cobalt.     

Preparation and characterization of hollow glass microspheres-cobalt ferrite core-shell particles based on homogeneous coprecipitation

Hollow glass microspheres-CoFe₂O₄ (HGMs-CF) core-shell particles were successfully synthesized directly by using the homogeneous coprecipitation method at 90 °C without calcination. The morphology, composition, microstructure and the magnetic property of the samples were characterized by SEM, XRD, EDX and VSM, respectively. The results showed that the HGMs-CF core-shell particles exhibited smooth, compact and continuous CoFe₂O₄ coating on the surface of the HGMs. The Fe/Co atom ratio of the CoFe₂O₄ coating was 2.2, saturation magnetization (M_s) and coercivity (H_c) of the samples were 46 emu/g and 612 Oe, respectively. It was suggested that this method could be applied to the scale industry production for high purity products.     

Hydrothermal preparation of high saturation magnetization and coercivity cobalt ferrite nanocrystals without subsequent calcination

In this work, CoFe₂O₄ nanocrystals with high saturation magnetization (M_s) and high coercivity (H_c) have been fabricated via a simple hydrothermal method and without subsequent calcination. The resulting CoFe₂O₄ nanocrystals are characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray spectrometry, differential scanning calorimetry and vibrating sample magnetometry. The results indicate that CoFe₂O₄ nanocrystals are single crystal and the average crystallite size is increasing with the hydrothermal temperature. The electron micrographs show that the nanocrystals are well-dispersed and possess uniform size. The shape of CoFe₂O₄ nanocrystals is transformed from spherical into rod by increasing the hydrothermal temperature. The nanocrystals show relatively high M_s of 74.8 emu g⁻¹ and H_c of 2216 Oe, as compared to previous reported results. The obtained results reveal the applicability of this method for efficiently producing well crystallized and relatively high magnetic properties CoFe₂O₄ nanocrystals as compared to other methods. More importantly, it does not require further calcination processes.     

Zirconia-based mixed potential sensors using precursor Bi2W2O9 and tungsten nanoplate sensing electrode for detection of NO2

Sensing electrodes with two different materials (precursor Bi₂W₂O₉ and the tungsten oxide) were synthesized. The tungsten oxide with high specific surface area (78 m² g^?1) was prepared by acid leaching of Bi₂W₂O₉ under microwave processing and the ultrasonic exfoliation reaction. The structure and morphology of the precursor Bi₂W₂O₉ and the tungsten oxide were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and the Brunauer–Emmett–Teller method. The mixed potential NO₂ sensors based on yttria-stabilized zirconia and sensing electrodes with the two materials were fabricated and compared and then their sensing properties were examined. All sensors showed good response to 30–500 ppm NO₂ at their best operation temperature. The sensor equipped with exfoliated tungsten oxide exhibited the better sensing performance than the sensor equipped with precursor Bi₂W₂O₉, which shows the sensitivity of 64.61 mV decade^?1 to NO₂ at the minimum operation temperature of 450 °C. In addition, the stability, repeatability, and cross-sensitivity of the two sensors were analyzed in details. Furthermore, the mixed potential sensing mechanism of NO₂ for the tungsten oxide was also discussed.

     

Magnetic properties of SBA-15 mesoporous nanocomposites with CoFe2O4 nanoparticles

A novel mesoporous nanocomposite based on SBA-15 with CoFe₂O₄ and Fe₂O₃ magnetic nanoparticles was prepared via the hydrothermal treatment and impregnation method. We showed that Fe₂O₃ nanoparticles were anchored in the frame and CoFe₂O₄ nanoparticles were confined in the mesopores of SBA-15 in the prepared nanocomposite. Our results indicated that the magnetic properties could be adjusted by the addition of CoFe₂O₄ and Fe₂O₃ magnetic nanoparticles. The presence of couple exchange interaction was confirmed with Kelly-Hankel (δM) curves, which enhanced the magnetic properties of the CoFe₂O₄/Fe₂O₃-SBA-15 mesoporous nanocomposite.     

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.