Sichu Li

Magnetic properties of a series of ferrite nanoparticles synthesized in reverse micelles

Nanoscale particles of the general formula MFe/sub 2/O/sub 4/ (M=Co, Mn, Fe) were synthesized in reverse micelles of twin-tailed anionic bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in isooctane. The size of the particles was controlled by adjusting the AOT/water molar ratio. Particle sizes were confirmed using XRD and uniformity was determined by SEM. Magnetic measurements, carried out using a SQUID Susceptometer, indicated superparamagnetic behavior. FC and ZFC demagnetization experiments indicate blocking temperatures of 46 K, 30 K, and 7 K for for MFe/sub 2/O/sub 4/ M=Co, Mn, Fe respectively. Below blocking temperatures, the nanoparticles demonstrate hysteresis with coercivities of H/sub c/=6000G, 3800G, and 500G for the Co, Mn, and Fe ferrites.

Cobalt-ferrite nanoparticles: Structure, cation distributions, and magnetic properties

Cobalt–ferrite nanoparticles have been synthesized in water-in-oil microemulsions (reversed micelles) with varying cation composition. The microenvironment provides a template effect that controls the size and particle shape. Transmission electron microscopy reveals that the particles are nanospheres with particle size ranging from 12 to 18 nm. X-ray diffraction results indicate that at low Co2+:Fe2+ ratio (1:10 and 1:5) in the precursor, the particles retain an essentially ferrite structure (γ-Fe2O3). However, the cobalt–ferrite phase (CoFe2O4) forms upon further increase of the Co2+ content. The materials are found to exhibit superparamagnetism. The blocking temperatures and coercivities are dependent on the Co2+:Fe2+ ratio in the system.

Cobalt-ferrite nanoparticles: correlations between synthesis procedures, structural characteristics and magnetic properties

Cobalt-ferrite nanoparticles of size 10 to 15 nm have been synthesized in water-in-oil microemulsions (i.e., reverse micelles). When the Co/sup 2+//Fe/sup 2+/ molar ratio in the precursor solution reaches 1:1 and above, the resultant oxides are identified as having the cobalt ferrite structure. By increasing the Co/sup 2+//Fe/sup 2+/ molar ratio in the precursor from 1:1 to 2:1, the atomic ratio of Co:Fe in the resultant ferrite increases from 0.83 to 1.31. The Co site occupancy is also dependent on the Co/sup 2+//Fe/sup 2+/ molar ratio in the precursor. The materials are found to exhibit superparamagnetism. The blocking temperatures and coercivities are dependent upon the Co/sup 2+//Fe/sup 3+/ ratio, reaching values as high as 55 K and 7500 G at 10 K, respectively. The correlation between synthesis procedure, site occupancy and magnetic properties enables the control of materials properties through such synthesis.

Synthesis and magnetic properties of a novel ferrite organogel

A novel magnetic organogel that can be considered a precursor example of a magnetoresponsive gel is reported. The gel is formed by the bridging of ferrite containing anionic bis(2-ethlhexyl) sodium sulfosuccinate reverse micelles with 2,6-dihydroxynaphthalene (2,6-DHN). The addition of 2,6-DHN leads to a room temperature quotes “freezing in” of the liquid solution to a clear organogel. Ferrite particles in the size range 10–15 nm are doped into the gel network and are thus suspended in the optically clear gel media. The magnetic properties of the gel were measured using a superconducting quantum interference device magnetometer. The results reveal that the gel exhibits superparamagnetic behavior with a blocking temperature of 6 K (at an applied field of 1000 G), and a coercivity of 850 G at 2 K. The ferrites introduced into the gel serve the function of magnetic “seeds” via which magnetic properties are acquired by the gel.

Synthesis and reactivity of nanophase ferrites in reverse micellar solutions

Abstract Self assembly preparative techniques in confined media that lead to magnetic materials with nanometer dimensions are described. Synthesis of nanoparticles using the restricted environments offered by surfactant systems such as water-in-oil microemulsions (reverse micelles) provide excellent control over particle size, inter-particle spacing, and particle shape. These environments have been used in the synthesis of γ-Fe2O3, Fe3O4, MnFe2O4, and CoFe2O4 with particle sizes ranging from 10–20 nm. The controlled environment of the reverse micelle also allows sequential synthesis which can produce a core-shell type structure, for example Fe3O4 nanoparticles with MnO coatings. Lyotropic liquid crystal media also offer template effects for the synthesis of magnetic nanostructures. The nanoscale ordering of magnetic particles when synthesized in lyotropic liquid crystal gels is characterized. The structures, theory and modeling concepts, and novel physical properties of these materials are discussed with emphasis given to the differences between course and fine grained magnetic materials.

Hydrothermal synthesis and characterization of mixed-valence hexatungstates: crystal structures of [(C2H5)4N]3[WVW5VIO19]·0.5H2O and [H3N(CH2)2NH3]2 [WVW5VIO19]·[H2N(CH2)2NH2]Cl·8H2O

Abstract The hydrothermal reaction of a mixture of Na 2 WO 4 ·2H 2 O, MoO 3 , Mo, Et 4 NCl, and H 2 O at 160°C gives bluish-purple crystals of [(C 2 H 5 ) 4 N ] 3 [W V W 5 VI O 19 ] · 0.5H 2 O ( 1 ) in 60% yield. Under analogous reaction conditions, a mixture of Na 2 WO 4 · 2H 2 O, V, H 2 N(CH 2 ) 2 NH 2 ·2HCl, and H 2 O yields brown crystals of [H 3 N(CH 2 ) 2 NH 3 ] 2 [W V W 5 VI O 19 ] · [H 2 N(CH 2 ) 2 NH 2 ]Cl·8H 2 O ( 2 ). 1 and 2 are the first one-electron reduced mixed valence hexatungstatesto be crystallized and fully characterized. The crystal structures of 1 and 2 consist of discrete [W 6 O 19 ] 3− anions, cations, and water molecules of crystallization. Crystal data. 1 : Orthorhombic space group Pbcn (No. 60). a = 19.063(4), b = 22.257(5), c = 19.631(4) A , V = 8329(3) A 3 , Z = 8 . A total of 6617 reflections (2 θ max = 45°) were collected, of which 4434 unique reflections with F 0 > 4 σ ( F 0 ) were used for structural elucidation. The structure was solved by direct methods and least-squares refinement converged at R = 0.0834. 2 : Monoclinic, space group, C 2/ m (No. 12), a = 15.7339(10), b = 11.497(9), c = 9.0934(5) A , β = 100.855 (4)°, V = 1615.5(2) A 3 , Z = 2 . A total of 2473 reflections (2 θ max = 30°) were collected. The structure was solved by direct methods and least-squares refinement converged at R = 0.0600.

Studies of the oxovanadium(IV)—oxalate system: syntheses and crystal structures of binuclear (Ph4P)2[(VO)2(H2O)2(C2O4)3]·4H2O and of the one-dimensional chain material, (Ph4P)[VOCl(C2O4)]

Abstract The hydrothermal reactions of (Ph4P)[VO2Cl2] and H2C2O4 at 150 and 125°C yield (Ph4P)2[V2O2(H2O)2(C2O4)3]·4H2O (1) and (Ph4P)[VOCl(C2O4)] (2), respectively. The structure of the molecular anion of 1 consists of a binuclear unit of oxovanadium(IV) octahedra bridged by a bisbidentate oxalate group. The VO6 coordination geometry at each vanadium site is defined by a terminal oxo group, an aquo ligand, and four oxygen donors — two from the bisbidentate bridging oxalate and two from the terminal bidentate oxalate. The structure of 2 consists of discrete Ph4P+ cations occupying regions between [VOCl(C2O4)]−∞ spiral chains. The structure of the one-dimensional anionic chain exhibits V(IV) octahedra bridged by bisbidentate oxalate groups. Crystal data: 1·4H2O, monoclinic P2 1 /n, a = 12.694(3), b = 12.531(3), c = 17.17(3) A , β = 106.32(2)°, V = 2621.3(13) A 3 , Z = 2, D calc = 1.501 g cm −3 , structure solution and refinement converged at a conventional residual of 0.0518; 2, tetragonal P4 3 , a = 12.145(2), c = 15.991(3) A , V = 2358.7(12) A 3 , Z = 4, R = 0.0452 .

Ferrite synthesis in microstructured media: Template effects and magnetic properties

Inverse micelles and organogels provide novel environments to synthesize ferrite particles. The fluid microstructure provides a template for the synthesis. Our experiments with ferrite synthesis in inverse micelles indicate the formation of superparamagnetic nanoparticles. Of interest is the encapsulation of these particles in polymer microspheres. The encapsulation is done using simple polymer precipitation in the micellar nonsolvent. The process results in a polymer-ferrite composite exhibiting supermagnetism. Low temperature spin glass properties of the composite are characterized through SQUID measurements. These composites have a superparamagnetic blocking temperature of 16 K and follow Curie–Weiss law at temperatures above 60 K with the fitted parameters: C=0.941 emu/g K, θ=−287 K, and TIP=0.0001 emu/g. Since the polymer used is polyphenol, a highly functionalizable material, the composite is well suited for applications in magnetic bioseparations and magnetic coatings.

Magnetoresistance of a (γ-Fe2O3)80Ag20 nanocomposite prepared in reverse micelles

The magnetic and transport properties of a (γ-Fe2O3)80Ag20 nanocomposite, prepared by a reverse micelle technique, have been studied. γ-Fe2O3 nanoparticles and Ag particles were individually synthesized in reverse micelles. The nanocomposite material was then prepared by mixing the two different particles in a γ-Fe2O3/Ag molar ratio 80/20. The morphology of the nanoparticles was examined with transmission electron microscopy. Mossbauer spectra revealed no obvious presence of any divalent iron. Zero field cooled and field cooled magnetic susceptibilities indicated a blocking temperature of about 40 K. Negative magnetoresistance was observed resembling that in ball milled γ-Fe2O3/Ag nanocomposites. However, the magnitude of the negative magnetoresistance is smaller and is ∼2.2% at 220 K and 9 T. Two possible mechanisms, spin-dependent hopping and tunneling across magnetic barriers, are discussed.

Superparamagnetism and transverse susceptibility in magnetic nanoparticle systems

The temperature (T) and field (H) dependence of the transverse susceptibility (/spl chi//sub T/) of a magnetic nanoparticle system has been investigated. The temperature dependence is found to exhibit characteristic features that can be related to relaxation behavior in nanoscale magnetic systems. To the best of our knowledge, this is the first time the full temperature and field dependence of the experimentally determined transverse susceptibility is presented for a magnetic nanoparticle system, The experimental data are interpreted using a modified coherent magnetization rotation model based on a two level approach.