Showing papers on "Superparamagnetism published in 2001"

Synthesis and Characterization of Silica-Coated Iron Oxide Nanoparticles in Microemulsion: The Effect of Nonionic Surfactants

Abstract: A water-in-oil microemulsion method has been applied for the preparation of silica-coated iron oxide nanoparticles. Three different nonionic surfactants (Triton X-100, Igepal CO-520, and Brij-97) have been used for the preparation of microemulsions, and their effects on the particle size, crystallinity, and the magnetic properties have been studied. The iron oxide nanoparticles are formed by the coprecipitation reaction of ferrous and ferric salts with inorganic bases. A strong base, NaOH, and a comparatively mild base, NH4OH, have been used in each surfactant to observe whether the basicity has some influence on the crystallization process during particle formation. Transmission electron microscopy, X-ray electron diffraction, and superconducting quantum interference device magnetometry have been employed to study both uncoated and silica-coated iron oxide nanoparticles. All these particles show magnetic behavior close to that of superparamagnetic materials. By use of this method, magnetic nanoparticles ...

Magnetic properties of ultrafine cobalt ferrite particles synthesized by hydrolysis in a polyol medium

Abstract: Fine CoFe2O4 powders with monodisperse, almost equi-axial nanometer-sized particles were synthesised in a polyol medium by forced hydrolysis of ionic Co(II) and Fe(III) salts at 160 °C. K(Co) XANES and 57Fe Mossbauer spectroscopy show that the structure of this ferrite is slightly deviated from an inverse spinel structure: 16% of cobalt atoms are in tetrahedral sites. The particles are superparamagnetic above 300 K and ferrimagnetic below this blocking temperature with, at low temperature, strong coercivity, a saturation magnetisation value close to the bulk value and high reduced remanence. The saturation magnetisation measured at 5 K is clearly enhanced with respect to CoFe2O4 nanometer-sized particles previously prepared by other methods. These magnetic characteristics suggest that these particles have a high crystallinity which may result from this novel synthesis route.

Superparamagnetic Photonic Crystals

Abstract: Photonic crystals consisting of monodisperse superparamagnetic colloidal particles have been synthesized. The particles (see Figure) are obtained by emulsion polymerization of styrene in the presence of freshly precipitated surface-modified iron oxide nanoparticles. A magnetic field self-assembles the particles and controls the diffraction wavelength and crystal parameters of the array.

Compositionally controlled FePt nanoparticle materials

Abstract: High temperature solution phase decomposition of Fe(CO)/sub 5/ and reduction of Pt(acac)/sub 2/ in the presence of stabilizers, oleic acid and oleyl amine, are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)/sub 5/ to Pt(acac)/sub 2/, and the compositions ranging from Fe/sub 30/Pt/sub 70/ to Fe/sub 80/Pt/sub 20/ are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess a disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces a change of internal particle structure and thus of the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe/sub 55/Pt/sub 45/ will lead to the highly ordered fct phase. This Fe/sub 55/Pt/sub 45/ nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications.

Size-Dependent Superparamagnetic Properties of Mn Spinel Ferrite Nanoparticles Synthesized from Reverse Micelles

Abstract: MnFe2O4 nanoparticles in a size range of 4−14 nm have been synthesized from water-in-toluene reverse micelles by using sodium dodecylbenzenesulfonate (NaDBS) as surfactant. The blocking temperature, saturation magnetization, and coercivity of the nanoparticles are clearly size-dependent. The blocking temperature increases from 20 to 250 K when the mean size of the nanoparticles increases from 4.4 to 13.5 nm. The coercivity at 20 K increases from 30 to 300 Oe with increasing nanoparticle size. The field-dependent magnetization hysteresis disappears above the blocking temperature. Due to high saturation field, the saturation magnetization of the nanoparticles has been obtained by the extrapolation of the magnetization vs 1/H plot to 1/H = 0. The saturation magnetization decreases with decreasing nanoparticle size. The high saturation field and the size-dependent saturation magnetization suggest the existence of a magnetically inactive layer on MnFe2O4 nanoparticles. The linear fitting of the saturation magn...

Magnetic and magneto-transport properties of ZnO:Ni films

Abstract: Magnetic semiconductor (MS), Ni doped ZnO film was fabricated by pulsed laser deposition method on sapphire (0 0 0 1) substrates. Ni dissolves until 25 at % into ZnO by low temperature growth technique. Lattice constant c once increases with increasing Ni content and has maximum point at the Ni content of 5 at % , and then it suddenly decreases. In all the Ni content range, the films exhibits n-type conduction. With increasing the Ni content, the carrier density and mobility decrease. ZnO films with the Ni content range from 3 to 25 at % exhibit ferromagnetic behavior at 2 K . At 30 K , the magnetization against applied magnetic field shows superparamagnetic behavior and it maintains at least up to 300 K . To study the effect of carrier density, Al was additionally doped. The effect of carrier density and Ni content on the structure, magnetic and magneto-transport behaviors are described.

Synthesis of Fe3O4 nanoparticles from emulsions

Abstract: A unique oil-in-water emulsion route has been devised to synthesize nanosized magnetite (Fe3O4) particles using a small amount of cyclohexane as the oil phase, NP5 + NP9 as the surfactant phase, and a Fe(II)/Fe(III) salt solution as the aqueous phase. The Fe3O4 powder thus derived from the emulsion containing 88 wt% 0.3 M FeSO4 + Fe(NO3)3 in the aqueous phase possesses an equiaxial morphology and an average particle size of <10 nm. Studies on the electrical conductivity of the emulsions as a function of Fe2+/Fe3+ concentration in the aqueous phase revealed the complexation effect of the NP5 + NP9 surfactant towards Fe2+/Fe3+ ions. The resulting particle size and morphology are therefore dependent on the amounts of surfactant and aqueous phase in the emulsion. A strongly alkaline aqueous phase favors the formation of Fe3O4, while a low alkalinity favors the formation of α-FeOOH. Magnetic property measurements and Mossbauer spectroscopic studies indicate that the emulsion-derived iron oxide powder is superparamagnetic, which becomes ferrimagnetic with decreasing measurement temperature.

Immobilization of yeast alcohol dehydrogenase on magnetic nanoparticles for improving its stability

Abstract: Yeast alcohol dehydrogenase (YADH) was immobilized covalently on Fe3O4 magnetic nanoparticles (10.6 nm) via carbodiimide activation. The immobilization process did not affect the size and structure of magnetic nanoparticles. The YADH-immobilized magnetic nanoparticles were superparamagnetic with a saturation magnetization of 61 emu g−1, only slightly lower than that of the naked ones (63 emu g−1). Compared to the free enzyme, the immobilized YADH retained 62% activity and showed a 10-fold increased stability and a 2.7-fold increased activity at pH 5. For the reduction of 2-butanone by immobilized YADH, the activation energies within 25–45 °C, the maximum specific activity, and the Michaelis constants for NADH and 2-butanone were 27 J mol−1, 0.23 mol min−1 mg−1, 0.62 mM, and 0.43 M, respectively. These results indicated a structural change of YADH with a decrease in affinity for NADH and 2-butanone after immobilization compared to the free enzyme.

Synthesis and Characterization of Soluble Iron Oxide−Dendrimer Composites

Abstract: Carboxylated poly(amidoamine) PAMAM dendrimers (generation 4.5) have been utilized for the synthesis and stabilization of ferrimagnetic iron oxide nanoparticles. Oxidation of Fe(II) at slightly elevated pH and temperature results in the formation of highly soluble nanocomposites of iron oxides and dendrimer, which are stable under a wide range of temperatures and pH's. Size exclusion chromatography indicates aggregates in the 20−30-nm size regime, consistent with the oligomeric nature of the composite material. Transmission electron microscopy reveals small assemblies of mineral cores with electron diffraction and high-resolution TEM data, indicative of the mineral maghemite. SQUID magnetometry demonstrated that this crystalline composition exhibited superparamagnetism at room temperature. NMR relaxation studies of solvent (water) protons revealed unusually high T1 and T2 relaxivities, which make these materials excellent candidates as contrast agents for MR imaging.

Superparamagnetic Latex via Inverse Emulsion Polymerization

Abstract: The structure-directing nature of block copolymers, combined with the “miniemulsion” polymerization process, facilitate synthesis of superparamagnetic latex loaded with nanometric magnetic iron oxide. A “double-hydrophilic” diblock copolymer (polyethylene oxide block-co-polymethacrylic acid), present during the precipitation of magnetic iron oxide, directs nucleation, controls growth, and sterically stabilizes the resulting 5 nm superparamagnetic iron oxide. After drying, the coated particles spontaneously repeptize into a mixture of hydroxyethylmethacrylate and methacrylic acid monomers, creating a ferrofluid-like dispersion. Inverse emulsification of the ferrofluid (magnetic particles plus monomer) into decane, aided by small amounts of diblock copolymer emulsifier along with ultrasonication, creates minidroplets (180 nm) filled with magnetic particles and monomer. Subsequent polymerization generates magnetic latex. Extensive characterization by scanning electron microscopy, transmission electron microscopy, dynamic light scattering, thermogravimetric analysis, magnetic measurements, and x-ray diffraction shows complete and uniform encapsulation of 5 nm magnetic iron oxide into latex of average diameter between 140 and 220 nm depending upon emulsifier concentration. The magnetically active latex contains 18 wt% magnetic iron oxide coated with hydrophilic polymers. In a minimum number of steps, the direct dispersion of magnetic particles in monomer and inverse miniemulsification process generates hydrophilic magnetic latex.

Synthesis and magnetic properties of CoFe2O4 spinel ferrite nanoparticles doped with lanthanide ions

Abstract: Lanthanide ions have been doped into cobalt spinel ferrites using an oil-in-water micellar method to form CoLn0.12Fe1.88O4 nanoparticles with Ln=Ce, Sm, Eu, Gd, Dy, or Er. Doping with lanthanide ions (LnIII) modulates the magnetic properties of cobalt spinel ferrite nanoparticles. In particular cases of Gd3+ or Dy3+ ions, a dramatic increase in the blocking temperature and coercivity is observed. Indeed, the introduction of only 4% of Gd3+ ions increases the blocking temperature ∼100 K and the coercivity 60%. Initial studies on the magnetic properties of these doped nanoparticles clearly demonstrate that the relationship between the modulation of magnetic properties and the nature of doped LnIII ions is interesting but very complex.

Competing interactions in dispersions of superparamagnetic nanoparticles

Abstract: We study the magnetic properties of dispersions of uniformly sized, chemically synthesized cobalt nanoparticles (NP's), observing a crossover from a blocked state to a superparamagnetic one with increasing temperature. By analyzing magnetization data, we determine the distributions of NP volumes and anisotropies, and establish that variations in the shapes of the magnetic cores of the NP's generate the anisotropy governing the crossover. We characterize the frustrated low-temperature state produced by the competition between dipolar interactions and anisotropy at sufficiently high NP density through remanent magnetization measurements, and explain the results through analysis of a simple model.

A novel magnetic affinity support for protein adsorption and purification.

Abstract: A novel magnetic support was prepared by an oxidization-precipitation method with poly(vinyl alcohol) (PVA) as the entrapment material. Transmission electron microscopy indicated that the magnetic particles had a core-shell structure, containing many nanometer-sized magnetic cores stabilized by the cross-linked PVA. The particles showed a high magnetic responsiveness in magnetic field, and no aggregation of the particles was observed after the particles had been treated in the magnetic field. These facts indicated that the particles were superparamagnetic. Cibacron blue 3GA (CB) was coupled to the particles to prepare a magnetic affinity support (MAS) for protein adsorption. Lysozyme was used as a model protein to test the adsorption properties of the MAS. The adsorption equilibrium of lysozyme to the MAS was described by the Langmuir-type isotherm. The capacity for lysozyme adsorption was more than 70 mg/g MAS (wet weight) at a relatively low CB coupling density (3-5 micromol/g). In addition, 1.0 M NaCl solution could be used to dissociate the adsorbed lysozyme. Finally, the MAS was recycled for the purification of alcohol dehydrogenase (ADH) from clarified yeast homogenates. Under proper conditions, the magnetic separation yielded over 5-fold purification of the enzyme with 60% recovery of the enzyme activity.

Intraparticle Magnetic Properties of Co3O4 Nanocrystals

Abstract: We have prepared a diluted system of Co3O4 nanocrystals dispersed in nonmagnetic SiO2 by heating an MCM-41 molecular sieve soaked in 0.075 mol/L CoCl2 aqueous solution in flowing oxygen at 300 °C for 3 h. Transmission electron microscopy showed that fine particles of about 3 nm exhibiting the (220) and (331) lattice fringes of the Co3O4 crystal distribute randomly in amorphous SiO2. Magnetic measurements using a SQUID magnetometer suggested the presence of quantum resonant spin tunneling of intraparticle superparamagnetic moments below the blocking temperature. The maximum points in the ac susceptibility shifted toward higher temperature from ≈3 to ≈5 K with increasing the frequency from ≈10-2 to ≈103 Hz. There were no divergent peaks above 1.7 K in the nonlinear susceptibility. The field dependent magnetization from 0 to ≈20 kG showed a small hysteresis loop at 2 K, whereas there were no hysteresis loops at both 5 and 10 K. The diluted Co3O4 nanocrystals is a new and ideally suitable material for the stu...

Direct Investigation of Superparamagnetism in Co Nanoparticle Films

Abstract: A direct probe of superparamagnetism was used to determine the complete anisotropy energy distribution of Co nanoparticle films. The films were composed of self-assembled lattices of uniform Co nanoparticles of 3 or 5 nm in diameter, and a variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of anisotropy energy distributions of small volume samples will be critical to magnetic optimization of nanoparticle devices and media.

Microstructures of FePt–Al–O and FePt–Ag nanogranular thin films and their magnetic properties

Abstract: The microstructure and magnetic property relationship in L10 ordered FePt–Al–O and FePt–Ag granular thin films have been studied. As-sputtered FePt–Al–O films, composed of isolated fine spherical FePt particles of ∼2 nm diameter with a disordered face-centered-cubic (fcc) structure, exhibit superparamagnetism. Annealing above 650 °C induces a transformation from a disordered fcc structure to ordered L10 phase and the films become magnetically hard. The microstructures of these films change greatly depending on the film compositions and annealing conditions, which are correlated with the magnetic properties. It was found that FePt particles smaller than 5 nm do not order at 500 °C, while the continuous FePt film orders perfectly at the same temperature, suggesting that the ordering temperature, Tc, decreases significantly when the particle size becomes less than 5 nm. In the FePt–Ag granular thin film, when the Ag composition is around 50 at. %, high coercivity (∼10 kOe) and fine uniform microstructure are...