Magnetic colloidal properties of ionic ferrofluids

Magnetic properties of colloidal suspensions of γ-Fe₂O₃ particles, obtained through a chemical synthesis, are investigated. Using an optical technique it is verified that these ionic aqueous ferrofluids are stable in high fields. The magnetization saturation of the particles is found independent of their size. Electron microscopy, magnetization and birefringence measurements allow us to separate the two superparamagnetic processes existing in such ferrofluid solutions: Bulk and Néel rotations. The Néel process is investigated through remanent magnetization of frozen solution.     

A magnetic field-dependent modulation effect tends to stabilize light transmission through binary ferrofluids

In binary ferrofluids composed of ferromagnetic γ?Fe₂O₃/Ni₂O₃ composite nanoparticles (A particles) and noncrystalline Fe₂O₃ nanoparticles (B particles), the A particles alone will form chain-like aggregates upon application of a magnetic field. Due to both the long-range ‘magnetic convergent force’ (F_C) and the short-range ‘magnetic divergent force’ (F_D), the A-particle chains immersed in the B-particle ‘sea’ will move in a manner similar to the process of vibrational damping. The apparent damping of the ferrofluids will vary from weak to overdamping according to the motion of the chains, so that the intensity of light transmitted through a ferrofluid film along the direction of the field would tend to stabilize after a period of rapid decrements and increments. In binary ferrofluids, the B-particle system can produce a modulation effect on both the damping and the driving force, further stabilizing the behavior of the transmitted light. At low fields (e.g., 500 Gs, 900 Gs) only the modulation of the viscosity drag force (F_v) is considerable, so that overdamping increases linearly with B-particle volume fraction (Ф_B), and the variation in the transmitted light is much slower during the process tending towards stability as Ф_B increases. However, at high fields (e.g., 1300 Gs) the polarization of the B-particle ‘sea’ is enhanced, so that F_D is modulated as well as F_v (i.e., both the practical damping and driving forces are modulated simultaneously). Thus, the apparent overdamping of the binary ferrofluids system will vary non-linearly as Ф_B increases, and the transmitted light will tend to stabilize faster for ferrofluids with high Φ_B than for those with low Ф_B at an applied magnetic field of 1300 Gs.     

Rheological properties of a γ-Fe2O3 paraffin-based ferrofluid

A stable γ-Fe₂O₃ paraffin-based ferrofluid was prepared via high energy milling. The magnetic particles were characterized using X-ray diffraction, dynamic light scattering and vibrating sample magnetometer techniques. The rheological properties of the ferrofluid were studied using a standard rotating rheometer. The magnetoviscous effect and thixotropy in the ferrofluid were studied. The formation and destruction of magnetically induced structures and the interactions of nanoparticles and aggregates are discussed.     

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.     

Gummic acid stabilized γ-Fe2O3 aqueous suspensions for biomedical applications

Biomedical applications of magnetic nanoparticles depend critically on their preparation as aqueous colloidal suspensions, or ferrofluids, with long term stability under physiological conditions. Dispersion of the magnetic nanoparticles is generally achieved by the use of protein cages, polysaccharide, polypeptide and charged macromolecular coatings, which minimize interparticle magnetic interactions, particle agglomeration and precipitation. The synthesis and characterization of gummic-acid stabilized maghemite ferrofluids is reported. X-ray diffraction, transmission electron microscope and dynamic light scattering measurements give a γ-Fe₂O₃ magnetic core diameter of 8 nm and a nanocomposite particle hydrodynamic diameter of 50 nm. Mössbauer and magnetization measurements indicate the presence of isolated, sterically stabilized superparamagnetic nanoparticles resistant to aging, and thus, promising agents for the production of novel magneto-pharmaceuticals.     

A study of modified Fe3O4 nanoparticles for the synthesis of ionic ferrofluids

XRD and XPS analyses revealed that a Fe(NO₃)₃·9H₂O layer formed outside γ-Fe₂O₃ particles when Fe₃O₄ nanoparticles were treated with ferric nitrate. The particle density differed for untreated and treated particles and was not uniform for the latter. The specific saturation magnetization of both treated and untreated particles was used to estimate the thickness of the Fe(NO₃)₃·9H₂O layer and the average density of the treated particles. The density of the treated particles was used to calculate the density of ferrofluids of different particle volume fractions. These values are in agreement with measured results. Therefore, the particle volume fraction can be designed to synthesize acid ionic ferrofluids based on Fe₃O₄ nanoparticles using Massart's method.     

The properties of acicular particles of (γ-Fe2O3)x(Fe3O4)1-x

It is known that acicular particles, initially of Fe₃O₄, which have been partially oxidised, show anomalous properties e.g. in the variation of H_c with composition. It is shown that these can be explained by considering the particles to comprise an inner core of Fe₃O₄ with an outer layer of γ-Fe₂O₃. Owing to the large change in volume which occurs on oxidation, these mutually stress each other. Over a certain range of composition, the stress field interacts with the magnetization to increase H_c, and it is also responsible for H_c increasing with time after the oxidation process. A similar situation applies to partially reduced γ-Fe₂O₃.     

Novel carbon nanotube iron oxide magnetic nanocomposites

A novel magnetic nanocomposite of γ-Fe₂O₃ nanoparticles decorated multiwalls carbon nanotubes (MWNTs) was synthesized for the first time by a simple chemistry precipitation method. The structure and morphology of the composite was characterized by X-ray powder diffractometer (XRD), TEM and EDS. The results of XRD and TEM show that γ-Fe₂O₃ nanoparticles is immobilized on the side wall of the MWNTs, the size of most of the particle is <5 nm.The EDS analysis shows that the atomic ratio of Fe to O is 2:3. The magnetization curves of the MWNTs and γ-Fe₂O₃ decorated MWNTs were measured by VSM at room temperature, which indicate that the saturated magnetization (Ms), remanence (Mr) and coercivity (Hc) of the decorated MWNTs are much larger than those of MWNTs, and the decorated MWNTs exhibit well magnetic properties.     

Saturation Magnetization and Law of Approach to Saturation for Self-formed Ionic Ferrofluids Based on MnFe2O2 Nanoparticles

测量了MnFe₂O₄纳米微粒及其磁性液体在室温下的磁化曲线.微粒的中值粒径为13.67 nm. 磁性液体的比饱和磁化强度小于理论值.在高场范围(5～10 kOe)下,磁性液体趋于饱和时,其体积分数越大,磁化曲线的斜率越大. 这种饱和磁化强度性质和趋饱和律分别源自于无场时的环状自组装团聚体和场致团聚体. 场致团聚体是耗散结构,以致于其趋饱和磁化律不同于顺磁理论所描述的趋饱和律. 磁性液体中的大微粒导致了表观磁滞现象.     

Synthesis and magnetic properties of platelet γ-Fe2O3 particles for medical applications using hysteresis-loss heating

Platelet γ-Fe₂O₃ particles of particle size less than 100 nm were prepared for medical applications that use the hysteresis-loss heating of ferromagnetic particles. The γ-Fe₂O₃ particles were obtained through the dehydration, reduction, and oxidation of platelet α-FeOOH particles, which were synthesized by the precipitation of ferric ions in an alkaline solution containing ethanolamine, and the crystals grown using a hydrothermal treatment. The γ-Fe₂O₃ particles contained dimples formed by the dehydration of α-FeOOH particles. The coercive force and the saturation magnetization of the γ-Fe₂O₃ particles were in the ranges 11.9 to 12.7 kA/m (150 to 160 Oe), and 70 to 72 Am²/kg (70 to 72 emu/g), respectively. The specific loss power of the γ-Fe₂O₃ particles, estimated from their temperature-raising property measured under a peak magnetic field of 50.9 kA/m (640 Oe) and at a frequency of 117 kHz, was 590 W/g. This value is higher than that of spherical cobalt-containing iron oxide particles having equivalent coercive force and saturation magnetization, reflecting the larger area of the minor hysteresis loop measured under a peak magnetic field of 50.9 kA/m (640 Oe).     

Apparently enhanced magnetization of Cu(I)-modified <Emphasis Type="Italic">γ</Emphasis>-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> based nanoparticles

Using a chemically induced transition method in FeCl₂ solution, γ-Fe₂O₃ based magnetic nanoparticles, in which γ-Fe₂O₃ crystallites were coated with FeCl₃?6H₂O, were prepared. During the synthesis of the γ-Fe₂O₃ nanoparticles Cu(I) modification of the particles was attempted. According to the results from both magnetization measurements and structural characterization, it was judged that a magnetic silent “dead layer”, which can be attributed to spin disorder in the surface of the γ-Fe₂O₃ crystallites due to breaking of the crystal symmetry, existed in the unmodified particles. For the Cu(I)-modified sample, the CuCl thin layer on the γ-Fe₂O₃ crystallites incurred the crystal symmetry to reduce the spin disorder, which “awakened” the “dead layer” on the surface of the γ-Fe₂O₃ crystallites, enhancing the apparent magnetization of the Cu(I)-modified nanoparticles. It was determined that the surface spin disorder of the magnetic crystallite could be related to the coating layer on the crystallite, and can be modified by altering the coating layer to enhance the effective magnetization of the magnetic nanoparticles.     

包钴型γ-Fe2O3磁粉矫顽力的研究

包钴型γ-Fe₂O₃磁粉分为包钴γ-Fe₂O₃(简记为Co-γ-Fe₂O₃)和包钴包亚铁γ-Fe₂O₃(简记为CoFe-γ-Fe₂O₃)两种,它们的矫顽力可比γ-Fe₂O₃磁粉的提高100至400Oe左右,本工作对这两种磁粉矫顽力增大的原因作了探讨,认为它们矫顽力增大的机制不同:CO-γ-Fe₂O₃矫顽力增大是由于表面包覆一层Co(OH)₂使表面各向异性增大,而CoFe-γ-Fe₂O₃则是由于表面包覆的是钴铁氧体,γ-Fe₂O₃与钴铁氧体之间发生耦合作用,使矫顽力增大。     

Mössbauer studies of ɛ-Fe3−x Ni x N and γ′-Fe4−y Ni y N nanoparticles

Nanocrystalline ?-Fe_3?x Ni _x N (0.0?≤?×?≤?0.8) particles are synthesized by precursor technique and nitridation of decomposed products in NH₃ (g) in the temperature range 673 K-823 K. For x?=?0.1–0.4 compositions, single phase ?-Fe_3?x Ni _x N hexagonal structure with space group P6₃/mmc is formed, while for x?=?0.5–0.8, fcc γ′-Fe_4?y Ni _y N phase is also precipitated. The room temperature Mössbauer spectrum for all the compositions shows the presence of superparamagnetic doublet, which is attributed to ?-Fe_3?x Ni _x N phase. For x?=?0.5–0.8 compositions, two additional sextets are observed corresponding to two different iron sites, the corner position (Fe^c) and the fcc position (Fe^f), in γ′-Fe_4?y Ni _y N. The added Ni atoms preferentially substitute the corner Fe^c positions. The isomer shift, quadrupole splitting and hyperfine field values are found to change with the Ni content.     

Crystal growth of Co ferrite on fine acicular γ-Fe2O3 particles

Highly coercive magnetic powder was obtained by growing cobalt ferrite on the surface of γ-Fe₂O₃ particles in highly alkaline suspensions containing cobalt and ferrous ions in a Co/Fe molar ratio = $\text{1}\text{2}$. The mechanism of the growth and the structure of cobalt ferrite on γ-Fe₂O₃ were studied by X-ray diffraction and electron diffraction techniques. Results show that crystals of cobalt ferrite CoFe₂O₄ with a spinel type crystal structure of lattice constant 8.415 Å grew epitaxially on γ-Fe₂O₃. The acicular direction of the epitaxially grown Co-γ-Fe₂O₃ as well as γ-Fe₂O₃ was in the [$\text{1}$01] direction. It was found that from the lattice constant value and the half width of X-ray diffraction peaks, the lattice constant epitaxially grown Co γ-Fe₂O₃ may be attributed to two kinds of crystals, viz., seed γ-Fe₂O₃ ($\text{a = 8.35 ～ 8.37}\text{A}\text{?}$) which was partly reduced to Fe₃O₄, and surface layer CoFe₂O₄ ($\text{a = 8.415}\text{A}\text{?}$). The crystal growth in the interface between the seed crystals and the growth layer was affected by the crystal structure of the seed crystals. The lattice constant of CoFe₂O₄ which was located in the vicinity of the interface was almost equal to that of the seed crystals.     

γ-Fe2O3中的氢含量

利用热中子透射法测定γ-Fe₂O₃的氢含量。利用差热分析、磁分析以及穆斯堡尔效应研究γ-Fe₂O₃的相变,实验结果表明在γ-Fe₂O₃结构中确实含有一定量的氢,当γ-Fe₂O₃结构中的阳离子空位被H¹⁺,Co²⁺,Si⁴⁺,P⁵⁺等离子占据时,将 关键词：     

Pressure induced phase transition of nanocrystalline and bulk maghemite (γ-Fe2O3) to hematite (α-Fe2O3)

Phase transition and bulk moduli of bulk and nanocrystalline γ-Fe₂O₃ were studied using synchrotron X-ray diffraction under high pressure. Contrary to most other nanomaterials, nanocrystalline γ-Fe₂O₃ begins to transform into α-Fe₂O₃ at the same pressure as bulk γ-Fe₂O₃, which is caused by a special structure of γ-Fe₂O₃, in which there exist vacancies of crystal. It is believed that phase transition starts from a certain site of vacancy because of the stress concentration at vacancy sites. Compared to bulk material, nanocrystalline γ-Fe₂O₃ has a larger bulk modulus, which is ascribed to the large ratio of surface to volume.     

Stabilizing the triclinic structure of Mn2V2O7 via isovalent cationic substitution

The thermal expansion of Mn₂V₂O₇ in the temperature range of ?190 to 1030°C is studied. The volumetric thermal expansion coefficients for triclinic (α) and monoclinic (β) modifications are 2.57 × 10^?5 and 3.86 × 10^?5 1/deg, respectively. It is shown that as the Ni²⁺ concentration in Mn_2?2x Ni_2x V₂O₇ rises, the point of the α → β phase transition point moves from room temperature for Mn₂V₂O₇ to 155 ° 5°C for Mn_1.46Ni_0.54V₂O₇ (27 mol % Ni₂V₂O₇).     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Magnetic and microwave absorbing properties of polyaniline/γ-Fe2O3 nanocomposite

The conducting protonated polyaniline (ES)/γ-Fe₂O₃ nanocomposite with the different γ-Fe₂O₃ content were synthesized by in-situ polymerization. Its morphology, microstructure, DC conductivity and magnetic properties of samples were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), four-wire-technique, and vibrating sample magnetometer (VSM), respectively. The microwave absorbing properties of the nanocomposite powders dispersing in wax coating with the coating thickness of 2 mm were investigated using a vector network analyzers in the frequency range of 7–18 GHz. The pure ES has shown the absorption band with a maximum absorption at approximately 16 GHz and a width (defined as frequency difference between points where the absorption is more than 8 dB) of 3.24 GHz, when 10% γ-Fe₂O₃ by weight is incorporated , the width is broadened to 4.13 GHz and some other absorption bands appear in the range of 7–13 GHz. The parameter dielectric loss tan δ_e (=ε″/ε′) in the 7–18 GHz is found to decrease with increasing γ-Fe₂O₃ contents with 10%, 20%, 30%, respectively, but magnetic loss tan δ_m (=μ″/μ′) increases with increasing γ-Fe₂O₃ contents. The results show that moderate content of γ-Fe₂O₃ nanoparticles embedded in protonated polyaniline matrix may create advanced microwave absorption properties due to simultaneous adjusting of dielectric loss and magnetic loss.     

包钴型r-Fe2O3磁粉各向异性研究

包钴型γ-Fe₂O₃磁粉的矫顽力可比原γ-Fe₂O₃磁粉提高8000—32000A/m。木文研究探讨了两种包钴型γ-Fe₂O₃磁粉(包钴γ-Fe₂O₃和包钴包亚铁γ-Fe₂O₃)的单轴各向异性的起源和矫顽力增大的机制。包钴γ-Fe₂O₃磁粉矫顽力 关键词：