Laudemir C. Varanda

Magnetic properties of spindle-type iron fine particles obtained from hematite

Spindle-type iron fine particles have been prepared by reduction of silica-coated-hematite particles. Hydrogen reduction of the coated-hematite cores yielded uniform spindle-type iron particles, which were stabilized by surface oxidation. Narrow particle distributions are observed from TEM measurements. X-ray, Mossbauer and magnetization data are in agreement with the presence of nanosized α-Fe particles, having surface layer of spinel structure oxide. Mossbauer spectra show that the oxide surface is superparamagnetic at room temperature.

Iron Oxide Versus Fe

In this paper, synthesis of the Fe55Pt45/Fe3O4 core/shell structured nanoparticles using the modified polyol process combined with the seed-mediated growth method is reported. Iron oxide shell thickness was tuned controlling the Fe(acac) 3/FePt seeds in the reaction medium. Annealing of the core/shell structure leads to iron-rich layer formation around the hard FePt phase in the nanoparticle core. However, the 2 nm Fe3O 4 shell thickness seems to be the limit to obtain the enhanced magnetization close to the alpha-Fe and preserving an iron oxide shell after annealing at 500degC for 30 min in a reducing atmosphere. The presence of both the oxide layer on nanoparticle surface and an intermediate iron-rich FePt layer after annealing promote strong decreases in the coercive field of the 2-nm-oxide shell thickness. These annealed nanoparticles were functionalized with dextran, presenting the enhanced characteristics for biomedical applications such as higher magnetization, very low coercivity, and a slightly iron oxide passivated layer, which leads an easy functionalization and decreases the nanoparticle toxicity.

_{55}

Magnetic properties of acicular (/spl sim/60 and /spl sim/200 nm) iron particles, obtained by reduction of alumina-coated goethite particles, are reported. X-ray diffraction and Mossbauer spectroscopy showed that the particles consist of an /spl alpha/-Fe core and a thin surface layer of maghemite. Magnetization data indicated an improvement of /spl sim/28% in the saturation magnetization, coercive field, and squareness for particles with /spl sim/60 nm. This magnetic property enhancement of the present particles, whose size is 40% smaller than those commercially available, could result in a similar decrease of the bit-size for higher density of magnetic media.

Pt

Temperature dependence and uniaxial magnetocrystalline anisotropy properties of the chemically synthesized 4 nm L10-Fe55Pt45 nanoparticle assembly by a modified polyol route are reported. As-prepared nanoparticles are superparamagnetic presenting fcc structure, and annealing at 550 °C converts the assembly into ferromagnetic nanocrystals with large coercivity (HC>1 T) in an L10 phase. Magnetic measurements showed an increasing in the ferromagnetically ordered fraction of the nanoparticles with the annealing temperature increases, and the remanence ratio, S=MR/MS≅0.76, suggests an (111) textured film. A monotonic increase of the blocking temperature TB, the uniaxial magnetocrystalline anisotropy constant KU, and the coercivity HC with increasing annealing temperature was observed. Magnetic parameters indicate an enhancement in the magnetic properties due to the improved Fe55Pt45 phase stabilizing, and the room-temperature stability parameter of 67, which indicates that the magnetization should be stable fo...

_{45}

Synthesis of La/sub 1-x/Sr/sub x/MnO/sub 3/ (x=0.1, 0.2 and 0.3) by homogenous coprecipitation method using urea as precipitant agent is reported. The particles are smaller than 200 nm after heating at 950/spl deg/C. Temperature dependence of the electrical resistivity was found to be similar to the reported value for single crystals of these manganites.

/Fe

UNESP, Inst Quim, Lab Computac Anal Cristalog & Cristalinas, LabCACC, BR-14800900 Araraquara, Brazil

_{3}

The compound obtained via state solid reaction of the La2O3 and SrO oxides and expose the room atmosphere shows the crystallographic data of the compound reported as La2SrOx. However, thermogravimetric, differential thermal analysis and XRD with controlled temperature indicated that the stoichiometry of the compound is 2La(OH)3-SrCO3, which structural parameters were determined by using the Rietveld method. It was verified that when the compound exposed at room atmosphere, the mixture oxide absorbs H2O and CO2 producing hydroxide and carbonate of lanthanum and strontium, respectively, which thermal decomposition occurs by the same steps, producing the La2O3-SrO.

O

Summary form only given. There are several reasons to make to cover magnetic materials with shells of different chemical composition. One is to change physical (optical, magnetic, conductive, etc.) and/or chemical properties of dispersions by choosing the coating material. YIG can show special interest as a magnetic dye, for microwave absorption and as a magnetic fluids when it is coated by another material. Surface and interface magnetic properties are intimately connected with the new properties of the silica on yttrium iron garnet system. Neel first introduced the concept of surface anisotropy and Chen et al. (1998) developed a model that describes the anisotropy effects at the border surface particle, which was applied in this work. Spherical YIG particles were prepared by coprecipitation method and it was coated by silica using the TEOS hydrolysis process. The transmission electron microscopy reveals clearly the edge between silica and yttrium iron garnet. Hysteresis loops shows comparatively the profile of pure and covered YIG. The non-coated samples show the magnetization near to that of the bulk, Ms = 26 emu.g/sup -1/, while covered samples present lower values than those obtained for the naked particle. Surface anisotropies were calculated using the Chen et al. model. Domain walls are unflexible when a thin layer covers the magnetic particle, so it is not a surprise that the Ks estimated is reduced by one order of magnitude, as observed in this work. Theoretically, Neels values of nanoparticle surface anisotropy should be between 0. 1 and 1 erg/cm/sup 2/ and our data agree with this theory. Surface magnetization is generally lower than the inner one and its effect leads to thermodynamic perturbation in exchange interaction near to the surface, which can be estimated. Indeed, in heterocoagulation systems, the surface anisotropy is a result of the symmetry breaking, as observed.

_{4}

Nanoparticles of yttrium iron garnet (YIG) were obtained by coprecipitation. The particles were prepared by hydrolysis in acid medium with addition of ammonia or urea, for homogeneous nucleation, at 90oC. Different compositions and spherical morphologies were achieved by changing reactants concentrations and precipitation agent. X-ray diffractometry, transmission electron microscopy, differential thermal analysis and electrophoretic mobility were carried out on these particles to investigate the obtained phase, phase transition temperature, morphology, particle size and zeta potential, respectively.

: Improved Magnetic Properties of Core/Shell Nanoparticles for Biomedical Applications

We report the synthesis of a new multifunctional nanomaterial based on silica-coated FePt/Fe3O4-CdSe heteronanostructures, combining luminescent and magnetic properties in a promising bifunctional sensor for biomedical applications. Spherical Fe3O4-coated FePt (FePt/Fe3O4) superparamagnetic nanoparticles (10.8 ± 1.5 nm) with high saturation magnetization and controlled size and shape were obtained using thermal decomposition coupled with seed-mediated growth method. Luminescent property was added to the nanomaterial by using the FePt/Fe3O4 magnetic core as seed and growing the CdSe quantum dots (2.7 ± 0.6 nm) onto its surface in a heterodimer-like structure using the hot-injection approach. The FePt/Fe3O4-CdSe luminomagnetic heteronanostructures were coated with silica shell using the reverse-micelle microemulsion route to avoid solvent-quenching effects. After silica coating, the water-dispersible heteronanostructures showed a diameter of 25.3 ± 2 nm, high colloidal stability, magnetic saturation of around 11 emu g−1, and photoluminescence in the blue-green region, as expected for potential bifunctional platform in biomedical applications. The saturation magnetization of heteronanostructures can be increased to 28 emu g−1 by annealing at 550°C due to the presence of the FePt phase.