Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe₂O₄ and ferrimagnetic oxide/ferromagnetic metal CoFe₂O₄/CoFe₂ nanocomposite. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupled. Two steps were followed to synthesize the bimagnetic CoFe₂O₄/CoFe₂ nanocomposite: (i) first, preparation of CoFe₂O₄ nanoparticles using a simple hydrothermal method, and (ii) second, reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe₂O₄ nanoparticles were investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50 K. The mean diameter of CoFe₂O₄ particles is about 16 nm. Mossbauer spectra revealed two sites for Fe³⁺. One site is related to Fe in an octahedral coordination and the other one to the Fe³⁺ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe₂O₄. TEM measurements of nanocomposite showed the formation of a thin shell of CoFe₂ on the cobalt ferrite and indicate that the nanoparticles increase to about 100 nm. The magnetization of the nanocomposite showed a hysteresis loop that is characteristic of exchange coupled systems. A maximum energy product (BH)_max of 1.22 MGOe was achieved at room temperature for CoFe₂O₄/CoFe₂ nanocomposites, which is about 115% higher than the value obtained for CoFe₂O₄ precursor. The exchange coupling interaction and the enhancement of product (BH)_max in nanocomposite CoFe₂O₄/CoFe₂ are discussed.     

类花状ZnO-CoFe2 O4 复合纳米管束的制备及其电磁波吸收特性

在90 ℃水溶液中采用两步晶体生长法制备出类花状ZnO-CoFe₂O₄复合纳米管束.ZnO纳米管束的管壁厚度大约为60 nm,管的直径大约为350 nm,CoFe₂O₄纳米颗粒连续包覆在ZnO纳米管束的表面,CoFe₂O₄纳米颗粒尺寸小于40 nm, 壳层厚度随着CoFe₂O₄在ZnO-CoFe₂O_{4 关键词： 类花状 2}O₄')" href="#">ZnO-CoFe₂O₄ 纳米管束 微波吸收剂     

Mössbauer and magnetic study of Co x Fe3--x O4 nanoparticles

Magnetic nanoparticles of cobalt ferrites Co _x Fe_3−x O₄ (x = 1 or 2) have been obtained either by mechanical milling or thermal treatment of pre-prepared layered double hydroxide carbonate x-LDH–CO₃. Mechanical milling of the 1-LDH–CO₃ leads to the large-scale preparation of nearly spherical nanoparticles of CoFe₂O₄, the size of which (5 to 20 nm) is controlled by the treatment time. Core-shell structure with surface spin-canting has been considered for the nanoparticles formed to explain the observed hysteresis loop shift (from ZFC–FC) in the magnetic properties. Annealing treatment of the 2-LDH–CO₃ below 673 K results in the formation of nearly spherical pure Co₂FeO₄ nanoparticles. At 673 K and above, the LDH decomposition leads to the formation of a mixture of both spinels phases Co₂FeO₄ and CoFe₂O₄, the amount of the latter increases with annealing temperature. Unusually high magnetic hardness characterized by a 22 kOe coercive field at 1.8 K has been observed, which reflects the high intrinsic anisotropy for Co₂FeO₄.     

Magnetic behavior of core–shell particles

We have prepared composite magnetic core–shell particles using the process of soap-free emulsion polymerization and the co-precipitation method. The shell of the synthesized composite sphere is cobalt ferrite (CoFe₂O₄) nanoparticles and the core consists of poly(styrene-co-methacrylic acid) polymer. The mean crystallite sizes of the coated CoFe₂O₄ nanoparticles were controlled in the range of 2.4–6.7 nm by the concentration of [NH₄⁺] and heated temperature. The magnetic properties of the core–shell spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the crystalline sizes of CoFe₂O₄.     

A facile synthesis of monodisperse CoFe2O4/SiO2 nanoparticles

Aminated-CoFe₂O₄/SiO₂ magnetic nanoparticles (NPs) were prepared from primary silica particles using modified StÖber method. By optimizing the preparation conditions, monodisperse CoFe₂O₄/SiO₂ NPs with high amino groups’ density were obtained, which is necessary for enzyme immobilization. TEM confirm that the sample is a core/shell structure. These aminated-CoFe₂O₄/SiO₂ NPs have narrow size distributions with a mean size of about 60 nm. Moreover, the aminated-CoFe₂O₄/SiO₂ NPs can be easily dispersed in aqueous medium. The experimental results also show that the NPs have superparamagnetism, indicating that the aminated-CoFe₂O₄/SiO₂ NPs can be used as an effective carrier for the enzyme immobilization.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Relaxation phenomena in ensembles of CoFe2O4 nanoparticles

Collective magnetic behavior of CoFe₂O₄ nanoparticles with diameters of 76, 16, 15 and 8 nm, respectively, prepared by different chemical methods has been investigated. Particle composition, size and structure have been characterized by inductive coupled plasma (ICP), transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Basic magnetic properties have been determined from the temperature dependence of magnetization and magnetization isotherms measurements. The three samples exhibit characteristic of a superparamagnetic system with the presence of strong interparticle interactions. Magnetic relaxation phenomena have been examined via frequency-dependent ac susceptibility measurements and aging and memory effect experiments. For the particles coated with oleic acid, it has been demonstrated that the sample reveals all attributes of a super-spin glass (SSG) system with strong interparticle interactions.     

The study of transition on NiFe2O4 nanoparticles prepared by co-precipitation/calcination

In this study, the NiFe₂O₄ nanoparticles have been prepared by co-precipitation and calcination process. Using a vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometer of X-ray (EDX), and X-ray photoelectron spectroscopy (XPS), the samples obtained by co-precipitation and then by further calcination have been analyzed. The experimental results show that the precursor synthesized by co-precipitation is the composite of both amorphous FeOOH and Ni(OH)₂, but has no amorphous NiFe₂O₄. The results of both EDX and XPS revealed that the FeOOH species is wrapped up by Ni(OH)₂ species. In the calcination process, the amorphous composite is dehydrated and transformed gradually into crystalline NiFe₂O₄ nanoparticles, with the metal ions diffusing. The reaction is different from the one used to prepare other ferrite (e.g., CoFe₂O₄, MnFe₂O₄, Fe₃O₄, etc.) nanoparticles directly by co-precipitation. With increasing calcination temperature, the NiFe₂O₄ grains grow and the magnetization is enhanced.     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Preparation of ZnO–Al2O3 Particles in a Premixed Flame

Zinc oxide (ZnO) and alumina (Al₂O₃) particles are synthesized by the combustion of their volatilized acetylacetonate precursors in a premixed air–methane flame reactor. The particles are characterized by XRD, transmission electron microscopy, scanning mobility particle sizing and by measurement of the BET specific surface area. Pure (-)alumina particles appear as dendritic aggregates with average mobile diameter 43–93 nm consisting of partly sintered, crystalline primary particles with diameter 7.1–8.8 nm and specific surface area 184–229 m²/g. Pure zinc oxide yields compact, crystalline particles with diameter 25–40 nm and specific surface area 27–43 m²/g. The crystallite size for both oxides, estimated from the XRD line broadening, is comparable to or slightly smaller than the primary particle diameter. The specific surface area increases and the primary particle size decreases with a decreasing flame temperature and a decreasing precursor vapour pressure. The combustion of precursor mixtures leads to composite particles consisting of zinc aluminate ZnAl₂O₄ intermixed with either ZnO or Al₂O₃ phases. The zinc aluminate particles are dendritic aggregates, resembling the alumina particles, and are evidently synthesized to the full extent allowed by the overall precursor composition. The addition of even small amounts of alumina to ZnO increases the specific surface area of the composites significantly, for example, zinc aluminate particles increases to approximately 150 m²/g. The gas-to-particle conversion is initiated by the fast nucleation of Al₂O₃ or ZnAl₂O₃, succeeded by a more gradual condensation of the excess ZnO with a rate probably controlled by the cooling rate for the flame.     

Morphology Control of Platinum Nanoparticles and their Catalytic Properties

Platinum nanoparticles with different morphology were prepared by reduction of K₂PtCl₄ solution in the presence of different capping polymers. It was found that the shapes and the sizes of the Pt nanocrystals resulted were related to the kind of capping polymer used. When poly(vinylpyrrolidon) (PVP), poly(N-isopropylacrylamide) (NIPA) and sodium poly(acrylate) (SPA) were used as capping agents, the dominant shapes of the Pt nanocrystals observed by transmission electron microscopy were hexagonal (62%), square (67%) and triangular (41%), respectively. The average sizes of Pt nanocrystals were 6.9, 13.6 and 14.6 nm for capping polymers of PVP, NIPA and SPA, respectively. The colloidal Pt nanoparticles with different morphologies were supported on -Al₂O₃ (1 wt.% Pt) and then their catalytic activity for NO reduction by CH₄ was tested in the 350–600°C temperature range. Additionally, the catalytic activities of these alumina-supported Pt nanocrystals were compared with a conventional catalyst having the average size of Pt particles of 2.4 nm. Over the alumina-supported Pt nanocrystals as compared with the conventional Pt/Al₂O₃, it was observed that the NO/CH₄ reaction yields to NH₃ and CO decreased significantly and on the other hand, the yield to N₂O increased. The experimental results are suggesting that the catalytic behavior can be tuned in a convenient way through the morphological control of the metal nanoparticles.     

Effects of applied magnetic field and pressures on the magnetic properties of nanocrystalline CoFe2O4 ferrite

Nanocrystalline CoFe₂O₄ ferrite with crystallite sizes of 30 nm have been successfully prepared by an emulsion method. X-ray diffractometer (XRD) shows that nanocrystalline CoFe₂O₄ ferrite possesses face center cubic structure. Crystal structure of the CoFe₂O₄ nanocrystals will not be changed by the applied magnetic field and pressures. The obtained CoFe₂O₄ nanocrystalline powders were pressed into thin columns with different pressures. Meanwhile, the dependences of the applied pressures and the direction of applied magnetic field on the magnetic properties of the CoFe₂O₄ nanocrystals were investigated in detail using vibrating sample magnetometer (VSM). The pressed CoFe₂O₄ nanocrystal gains the most excellent magnetisms in a parallel applied magnetic field.     

Green synthesis of soya bean sprouts-mediated superparamagnetic Fe3O4 nanoparticles

Superparamagnetic Fe₃O₄ nanoparticles were first synthesized via soya bean sprouts (SBS) templates under ambient temperature and normal atmosphere. The reaction process was simple, eco-friendly, and convenient to handle. The morphology and crystalline phase of the nanoparticles were determined from scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD) spectra. The effect of SBS template on the formation of Fe₃O₄ nanoparticles was investigated using X-ray photoemission spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FT-IR). The results indicate that spherical Fe₃O₄ nanoparticles with an average diameter of 8 nm simultaneously formed on the epidermal surface and the interior stem wall of SBS. The SBS are responsible for size and morphology control during the whole formation of Fe₃O₄ nanoparticles. In addition, the superconducting quantum interference device (SQUID) results indicate the products are superparamagnetic at room temperature, with blocking temperature (T_B) of 150 K and saturation magnetization of 37.1 emu/g.     

Preparation of silica coated cobalt ferrite magnetic nanoparticles for the purification of histidine-tagged proteins

Surface modified cobalt ferrite (CoFe₂O₄) nanoparticles containing Ni–NTA affinity group were synthesized and used for the separation of histidine tag proteins from the complex matrices through the use of imidazole side chains of histidine molecules. Firstly, CoFe₂O₄ nanoparticles with a narrow size distribution were prepared in an aqueous solution using the controlled co-precipitation method. In order to obtain small CoFe₂O₄ agglomerates, oleic acid and sodium chloride were used as dispersants. The CoFe₂O₄ particles were coated with silica and subsequently the surface of these silica coated particles (SiO₂–CoFe₂O₄) was modified by amine (NH₂) groups in order to add further functional groups on the silica shell. Then, carboxyl (–COOH) functional groups were added to the SiO₂–CoFe₂O₄ magnetic nanoparticles through the NH₂ groups. After that Nα,Nα–Bis(carboxymethyl)-l-lysine hydrate (NTA) was attached to carboxyl ends of the structure. Finally, the surface modified nanoparticles were labeled with nickel (Ni) (II) ions. Furthermore, the modified SiO₂–CoFe₂O₄ magnetic nanoparticles were utilized as a new system that allows purification of the N-terminal His-tagged recombinant small heat shock protein, Tpv-sHSP 14.3.     

Characteristics of Fe2O3 Nanoparticles from Doped Low-pressure H2/O2/Ar Flames

A burner stabilized premixed low-pressure flame has been used to generate iron-oxide (Fe₂O₃) nanoparticles with sizes in the range 7–20nm. The H₂/O₂/Ar flames were doped with different amounts of iron-pentacarbonyl (Fe(CO)₅) with concentrations in the range 524–2096ppm. The influence of precursor concentration on composition, structure, morphology, and size have been studied utilizing transmission electron microscopy (TEM), X-ray powder diffraction (XRD), measurements of the specific surface area (BET), and infrared spectroscopy (FT-IR). The product particles consist of both, the - and the -phase of Fe₂O₃. Average particle sizes were measured in the range 7.4–16nm depending on precursor concentration and flame conditions.     

Preparation and characterization of multi-functional CoFe2O4-ZnO nanocomposites

Multi-functional magnetic, photoluminescent and photocatalytic CoFe₂O₄-ZnO nanocomposites were successfully synthesized by a collosol method. The average diameter of the prepared CoFe₂O₄-ZnO nanocomposites was 30±5 nm, and a diffusion layer was formed to link CoFe₂O₄ and ZnO. The saturation magnetization of the CoFe₂O₄-ZnO nanocomposites was 8.99 emu/g. Generation of ZnO from Zn(OH)₂ collosol was nearly complete after thermal decomposition at about 380 °C. A photoluminescence emission peak was observed at 443 nm when excitated at 350 nm. Degradation of methyl orange is performed by CoFe₂O₄-ZnO nanocomposites under ultraviolet radiation, with a degradation rate of up to 93.9%.     

Magnetic properties of CoFe1.9RE0.1O4 nanoparticles (RE=La,Ce, Nd,Sm, Eu,Gd, Tb,Ho) prepared in polyol

Highly crystalline CoFe_1.9RE_0.1O₄ ferrite nanoparticles, where RE=La, Ce, Nd, Sm, Eu, Gd, Tb, and Ho, have been synthesized by forced hydrolysis in polyol. X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), ⁵⁷Fe Mössbauer spectrometry, Co K-edge X-ray absorption spectroscopy and magnetic measurements using a SQUID magnetometer were employed to investigate the effect of the substitution RE³⁺ ions for Fe³⁺ ones on the structure, the microstructure, the chemical homogeneity, and the magnetic properties of the cobalt ferrite system. All the produced particles are superparamagnetic at room temperature. Nevertheless, the substitution causes reduction of the blocking temperature which is mainly ascribed to partial cation exchange among the spinel-like sublattices of CoFe₂O₄ induced by the insertion of the relatively large RE³⁺ ions. The low-temperature saturation magnetization and coercivity appear to be greatly affected by the nature of RE³⁺ ions—maxima values were found for Gd³⁺ and Eu³⁺, respectively.     

Microwave synthesis of magnetic Fe3O4 nanoparticles used as a precursor of nanocomposites and ferrofluids

Methods to synthesize magnetic Fe₃O₄ nanoparticles and to modify the surface of particles are presented in the present investigation. Fe₃O₄ magnetic nanoparticles were prepared by the co-precipitation of Fe³⁺ and Fe²⁺, NH₃·H₂O was used as the precipitating agent to adjust the pH value, and the aging of Fe₃O₄ magnetic nanoparticles was accelerated by microwave (MW) irradiation. The obtained Fe₃O₄ magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The average size of Fe₃O₄ crystallites was found to be around 8–9 nm. Thereafter, the surface of Fe₃O₄ magnetic nanoparticles was modified by stearic acid. The resultant sample was characterized by FT-IR, scanning electron microscopy (SEM), XRD, lipophilic degree (LD) and sedimentation test. The FT-IR results indicated that a covalent bond was formed by chemical reaction between the hydroxyl groups on the surface of Fe₃O₄ nanoparticles and carboxyl groups of stearic acid, which changed the polarity of Fe₃O₄ nanoparticles. The dispersion of Fe₃O₄ in organic solvent was greatly improved. Effects of reaction time, reaction temperature and concentration of stearic acid on particle surface modification were investigated. In addition, Fe₃O₄/polystyrene (PS) nanocomposite was synthesized by adding surface modified Fe₃O₄ magnetic nanoparticles into styrene monomer, followed by the radical polymerization. The obtained nanocomposite was tested by thermogravimetry (TG), differential scanning calorimetry (DSC) and XRD. Results revealed that the thermal stability of PS was not significantly changed after adding Fe₃O₄ nanoparticles. The Fe₃O₄ magnetic fluid was characterized using UV–vis spectrophotometer, Gouy magnetic balance and laser particle-size analyzer. The testing results showed that the magnetic fluid had excellent stability, and had susceptibility of 4.46×10⁻⁸ and saturated magnetization of 6.56 emu/g. In addition, the mean size d (0.99) of magnetic Fe₃O₄ nanoparticles in the fluid was 36.19 nm.     

Microwave synthesis, characterization, and electrochemical properties of α-Fe2O3 nanoparticles

Ultrafine α-Fe₂O₃ nanoparticles with an extremely narrow distribution were synthesized by microwave heating. Transmission electron microscopy (TEM) images showed that most primary particles have ellipsoid shapes, and the average diameter of the primary particles was less than 10 nm. The electron diffraction pattern and fringes in some particles in TEM images showed that these nanoparticles were single crystals. The BET surface area of the freeze-dried product was 217 m²/g. The initial discharge capacity of the α-Fe₂O₃ nanoparticles exceeded 1007 mA/g (cut-off voltage: 0.5 V). This large capacity corresponds to that calculated by assuming the reduction of Fe³⁺ to Fe⁰. The α-Fe₂O₃ nanoparticles also work as a rechargeable electrode material. The charge-discharge test between 4 V and 1.5 V gave a good rechargeable capacity of about 150 mAh/g.     

The modification effect in magnetization behaviors for CoFe2O4–p-NiFe2O4 binary ferrofluids

The magnetization curves of CoFe₂O₄ ferrofluids, p-NiFe₂O₄ paramagnetic fluids and CoFe₂O₄–p-NiFe₂O₄ binary ferrofluids, in which the volume fraction of CoFe₂O₄ particles φ _Co is 0.6% and one of p-NiFe₂O₄ particles φ _Ni is 0.2%, 0.4%, 0.6% and 0.8% respectively, prepared by the Massart method, have been measured at room temperature. Comparison of the experimental data from the CoFe₂O₄ ferrofluids with the Langevin theory curves demonstrates a considerable difference between them, but a curve fitted using a model of a gas-like compression (MGC) agrees with the experimental data very well. The experimental results show that the magnetization of the CoFe₂O₄–p-NiFe₂O₄ binary ferrofluid is not a simple summation of the ferrimagnetic CoFe₂O₄ part and the paramagnetic p-NiFe₂O₄ part. From the fitted results, it was found that the saturation magnetization of the CoFe₂O₄ part of the binary ferrofluid depends non-monotonically on the p-NiFe₂O₄ particle volume fraction, and the CoFe₂O₄ part is a stronger “hard” magnet than CoFe₂O₄ in simple ferrofluids. The magnetization behavior of the binary ferrofluids is explained by the modification of the microstructure of CoFe₂O₄ nanoparticle system by the p-NiFe₂O₄ nanoparticle system.