Showing papers on "Superparamagnetism published in 2002"

Finite-size effects in fine particles: magnetic and transport properties

Abstract: Some of the most relevant finite-size and surface effects in the magnetic and transport properties of magnetic fine particles and granular solids are reviewed. The stability of the particle magnetization, superparamagnetic regime and the magnetic relaxation are discussed. New phenomena appearing due to interparticle interactions, such as the collective state and non-equilibrium dynamics, are presented. Surface anisotropy and disorder, spin-wave excitations, as well as the enhancements of the coercive field and particle magnetization are also reviewed. The competition of surface and finite-size effects to settle the magnetic behaviour is addressed. Finally, two of the most relevant phenomena in the transport properties of granular solids are summarized namely, giant magnetoresistance in granular heterogeneous alloys and Coulomb gap in insulating granular solids.

Self-assembled magnetic matrices for DNA separation chips.

Abstract: When a constant, homogenous magnetic field is applied to a suspension of superparamagnetic particles confined in a thin gap perpendicular to the field, the particles self-organize into a fixed, quasi-regular array of columns ([Fig. 1][1]A) ([1][2]). The array returns to a liquid suspension

Atom Transfer Radical Polymerization Synthesis and Magnetic Characterization of MnFe2O4/Polystyrene Core/Shell Nanoparticles

Abstract: An atom transfer radical polymerization route is developed for the coating of MnFe2O4 nanoparticles with polystyrene yielding the core−shell nanoparticles with size <15 nm. Magnetic studies show a decrease in coercivity after the formation of polystyrene shell, which is considered due to the reduction of magnetic surface anisotropy upon polymer coating. The MnFe2O4 nanoparticles as the magnetic core were separately prepared by a reverse micelle microemulsion method. Polymerization initiators are chemically attached onto the surface of nanoparticles. The modified nanoparticles are then used as macro-initiators in the subsequent polymerization reaction. This approach provides great flexibility in the selection of magnetic core. Consequently, magnetic tunability is able to be introduced into these core/shell nanoparticulate systems to achieve the desired superparamagnetic response.

Interaction of Anionic Superparamagnetic Nanoparticles with Cells: Kinetic Analyses of Membrane Adsorption and Subsequent Internalization

Abstract: Cell targeting with magnetic nanoparticles raises a growing interest in the fields of both cellular biology and medical imaging. We have investigated the nonspecific interactions of superparamagnetic negatively charged iron oxide nanoparticles with HeLa tumor cells and mouse RAW macrophages, qualitatively by electron microscopy and quantitatively following the particle cell uptake by two magnetic based quantification assays (magnetophoresis and electron spin resonance). The analyses of particle uptake kinetics at 4 and 37 °C led us to modelize the observed endocytosis as a two-step process: we distinguish the binding of anionic magnetic nanoparticles onto the cell surface (described as a Langmuir adsorption) from the subsequent step of cell internalization (also described as a saturable mechanism). Fits of experimental uptake kinetics result in the quantitative determination of binding parameters (adsorption rate, desorption rate, and density of binding sites) as well as internalization rate and internali...

Synthesis and utilization of monodisperse superparamagnetic colloidal particles for magnetically controllable photonic crystals

Abstract: We demonstrate fabrication of novel magnetically controllable photonic crystals formed through the self-assembly of highly charged, monodisperse superparamagnetic colloidal spheres. These superparamagnetic monodisperse charged polystyrene particles containing nanoscale iron oxide nanoparticles were synthesized through emulsion polymerization. They self-assemble into crystalline colloidal arrays (CCAs) in deionized water and Bragg diffract visible light. The diffraction from these superparamagnetic CCAs can be controlled by imposition of magnetic fields, which readily alter the CCA lattice constant. We also observe magnetically induced self-assembly of these superparamagnetic particles into CCAs in media such as NaCl aqueous solutions and organic polar solvents, which normally do not permit spontaneous CCA self-assembly. We also find that magnetic fields can strain the face-centered cubic lattice of superparamagnetic CCAs polymerized within hydrogels.The lattice symmetry of this photonic crystal becomes tetragonal. The observed magnetically induced CCA self-assembly enables the development of novel photonic crystal materials and devices.

Reduction of the fcc to L10 Ordering Temperature for Self-Assembled FePt Nanoparticles Containing Ag

Abstract: [Fe49Pt51]88Ag12 nanoparticles were prepared by the simultaneous polyol reduction of platiunum acetylacetonate and silver acetate and the thermal decomposition of iron pentacarbonyl, giving 3.5 nm diameter FePt particles with Ag atoms substituted in the lattice. The addition of Ag promoted the fcc to tetragonal phase transition, thereby reducing the temperature required for this transition by some 100 to 150 °C compared with pure FePt nanoparticles. After heat treatment at 400 °C for 30 min, the coercivity of the films containing [Fe49Pt51]88Ag12 nanoparticles was more than 3400 Oe, while the films containing FePt nanoparticles were superparamagnetic. This decrease in phase transformation temperature allowed us to decrease the particle coalescence and loss in particle positional order seen when FePt nanoparticles were transformed at temperatures above 550 °C.

Preparation and characterization of a novel magnetic nano-adsorbent

Abstract: A novel magnetic nano-adsorbent has been developed using Fe3O4 nanoparticles (13.2 nm) as cores and polyacrylic acid (PAA) as ionic exchange groups. The Fe3O4 magnetic nanoparticles were prepared by co-precipitating Fe2+ and Fe3+ ions in an ammonia solution and treating under hydrothermal conditions. PAA was covalently bound onto the magnetic nanoparticles via carbodiimide activation. Transmission electron micrographs showed that the magnetic nanoparticles remained discrete and had no significant change in size after binding the PAA. The X-ray diffraction patterns indicated the magnetic nanoparticles were pure Fe3O4 with a spinel structure, and the binding of PAA did not result in a phase change. Magnetic measurement revealed the magnetic nanoparticles were superparamagnetic, and their saturation magnetization was reduced only slightly after PAA binding. Fourier transform infrared spectroscopy, thermogravimetric and differential thermal analyses, and X-ray photoelectron spectroscopy confirmed the binding of PAA to the magnetic nanoparticles, suggested a binding mechanism for the PAA, and revealed the maximum weight ratio of PAA bound to the magnetic nanoparticles was 0.12. In addition, the ionic exchange capacity of the resultant magnetic nano-adsorbents was estimated to be 1.64 mequiv g−1, much higher than those of commercial ionic exchange resins. When the magnetic nano-adsorbents were used for the recovery of lysozyme, the adsorption/desorption of lysozyme was completed within 1 min due to the absence of pore-diffusion resistance. Also, the adsorption/desorption efficiency could reach almost 100% under appropriate conditions, and the recovered lysozyme retained 95% activity.

Application of the static dephasing regime theory to superparamagnetic iron-oxide loaded cells.

Abstract: The relaxation rates of iron-oxide nanoparticles compartmentalized within cells were studied and found to satisfy predictions of the static dephasing (SD) regime theory. THP-1 cells in cell culture were loaded using two different iron-oxide nanoparticles (superparamagnetic iron-oxide (SPIO) and ultrasmall SPIO (USPIO)) with four different iron concentrations (0.05, 0.1, 0.2, and 0.3 mg/ml) and for five different incubation times (6, 12, 24, 36, and 48 hr). Cellular iron-oxide uptake was assessed using a newly developed imaging version of MR susceptometry, and was found to be linear with both dose and incubation time. R(2)* sensitivity to iron-oxide loaded cells was found to be 70 times greater than for R(2), and 3100 times greater than for R(1). This differs greatly from uniformly distributed nanoparticles and is consistent with a cellular bulk magnetic susceptibility (BMS) relaxation mechanism. The cellular magnetic moment was large enough that R(2)' relaxivity agreed closely with SD regime theory predictions for all cell samples tested [R(2)'=2 pi/(9 x the square root of 3) x gamma LMD] where the local magnetic dose (LMD) is the sample magnetization due to the presence of iron-oxide particles). Uniform suspensions of SPIO and USPIO produced R(2)' relaxivities that were a factor of 3 and 8 less, respectively, than SD regime theory predictions. These results are consistent with theoretical estimates of the required mass of iron per compartment needed to guarantee SD-regime-dominant relaxivity. For cellular samples, R(2) was shown to be dependent on both the concentration and distribution of iron-oxide particles, while R(2)' was sensitive to iron-oxide concentration alone. This work is an important first step in quantifying cellular iron content and ultimately mapping the density of a targeted cell population.

Magnetic Nanocomposites: Preparation and Characterization of Polymer-Coated Iron Nanoparticles

Abstract: Nanoparticles bearing a strongly bound polymer coating were formed by the thermal decomposition of iron pentacarbonyl in the presence of ammonia and polymeric dispersants. The dispersants consist of polyisobutylene, polyethylene, or polystyrene chains functionalized with tetraethylenepentamine, a short polyethyleneimine chain. Polystyrene-based dispersants were prepared with both graft and block copolymer architectures. Inorganic−organic core−shell nanoparticles were formed with all three types of dispersants. In addition, more complex particles were observed in the case of the polystyrene-based dispersants in 1-methylnaphthalene. The core material was identified as metallic iron, while the particle shells are formed from the polymeric dispersant which binds to the core. High-resolution TEM revealed evidence for crystallization within the polymer shell, possibly facilitated by chain alignment upon binding. The nanocomposites display room-temperature magnetic behavior ranging from superparamagnetic to ferr...

Synthesis of nickel nanoparticles in aqueous cationic surfactant solutions

Abstract: The synthesis of nickel nanoparticles by the reduction of nickel chloride with hydrazine in an aqueous solution of cationic surfactants CTAB/TC12AB was studied. It was found that an appropriate amount of NaOH, trace acetone, and an elevated temperature were necessary for the formation of pure nickel nanoparticles. Also, it was not necessary to perform the reaction under a nitrogen atmosphere. X-ray diffraction revealed the resultant particles were pure nickel crystalline with a face-centered cubic (fcc) structure. Their mean diameter was 10–36 nm, increasing with increasing nickel chloride concentration or decreasing hydrazine concentration. When the concentration ratio of hydrazine to nickel chloride was above 40, the mean diameter approached a constant value. The magnetic measurement on a typical sample with a mean diameter of 12 nm showed that the resultant nickel nanoparticles were superparamagnetic due to their extremely small size. The saturation magnetization, remanent magnetization, and coercivity were 32 emu g−1, 5.0 emu g−1, and 40 Oe, respectively. Also, the magnetization was observed to increase with decreasing temperature. All the observed magnetic properties essentially reflected the nanoparticle's nature.

Superparamagnetism and magnetic properties of Ni nanoparticles embedded in SiO 2

Abstract: We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a SiO2 amorphous matrix A modified sol-gel method was employed which resulted in Ni particles with average radius ;3 nm, as inferred by TEM analysis Above the blocking temperature TB’20 K for the most diluted sample, magnetization data show the expected scaling of the M/ MS vs H/T curves for superparamagnetic particles The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer For T,TB the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation HC(T)5HC0@12(T/TB) 1/2 # below TB with HC0;780 Oe The obtained values of HC0, considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles

Magnetic behavior of nanostructured films assembled from preformed Fe clusters embedded in Ag

Abstract: We have observed the magnetic behavior of nanostructured magnetic materials produced by co-depositing pre-formed Fe nanoclusters from a gas aggregation source and Ag vapor from a Knudsen cell. Films containing particle volume fractions from $l1%$ (isolated clusters) to $100%$ (pure clusters with no matrix) have been prepared in UHV conditions and, after capping with a thin Ag layer for removal from the deposition chamber, have been studied at temperatures in the range 2--300 K by magnetometry and field-cooled/zero-field-cooled measurements. The results have been interpreted with the help of a Monte Carlo simulation of the cluster-assembled films that includes exchange and dipolar interactions. At elevated temperatures $(g50\mathrm{K})$ the lowest concentration films display ideal superparamagnetism with an $H/T$ scaling of the magnetization. With increasing cluster density the films pass through an interacting superparamagnetic phase in which the effective blocking temperature and the initial susceptibility above the blocking temperature increase, in contrast to predictions of nanoparticle systems interacting via dipolar forces only. It is concluded that the exchange interaction becomes important even at volume fractions of $10%$ as clusters that are in contact behave as a single larger particle. This is confirmed by the theoretical model. At high volume fractions, well above the percolation threshold, the cluster assemblies form correlated superspin glasses (CSSG's). At 2 K, the magnetization curves in all films, irrespective of cluster concentration, have a remanence of $\ensuremath{\approx}50%$ and an approach to saturation that is characteristic of randomly oriented, particles with a uniaxial anisotropy, in agreement with the remanence. In the most dense Ag-capped films there appears to be a ``freezing out'' of the interparticle exchange interaction, which is attributed to temperature-dependent magnetoelastic stress induced by the capping layer. An uncapped $100%$ cluster film measured in UHV remains in the CSSG state at all temperatures and does not show the low-temperature decoupling of particles evident in the Ag-capped samples.

Preparation and characterization of YADH-bound magnetic nanoparticles

Abstract: The covalently binding of yeast alcohol dehydrogenase (YADH) to magnetic nanoparticles via carbodiimide activation was studied. The magnetic nanoparticles Fe3O4 with a mean diameter of 10.6 nm were prepared by co-precipitating Fe2+ and Fe3+ ions in an ammonia solution and treating under hydrothermal conditions. Transmission electron microscopy (TEM) micrographs showed that the magnetic nanoparticles remained discrete and had no significant change in size after binding YADH. X-ray diffraction (XRD) patterns indicated both the magnetic nanoparticles before and after binding YADH were pure Fe3O4. Magnetic measurement revealed the resultant magnetic nanoparticles were superparamagnetic characteristics, and their saturation magnetization was reduced only slightly after enzyme binding. The analysis of Fourier transform infrared (FTIR) spectroscopy confirmed the binding of YADH to magnetic nanoparticles and suggested a possible binding mechanism. In addition, the measurement of protein content revealed that the maximum weight ratio of YADH bound to magnetic nanoparticles was 0.125, below which the binding efficiency of YADH was almost 100%. The kinetic measurements indicated the bound YADH retained 62% of its original activity and exhibited a 10-fold improved stability than did the free enzyme. The maximum specific activities and Michaelis constants were also determined.

Synthesis and Magnetic Properties of Fe3O4 Nanoparticles

Abstract: Magnetic Fe3O4 nanoparticles with size below 10 nm have been prepared by the aqueous phase coprecipitation method. The Fe3O4 nanoparticles show typical superparamagnetism. Comparison is made between the dispersed sample and the powder sample, and the results are discussed.

Synthesis, self-assembly, and magnetic properties of fexcoypt100-x-y nanoparticles

Abstract: FexCoyPt100-x-y alloy nanoparticles were prepared by the simultaneous reduction of cobalt acetylacetonate and platinum acetylacetonate and the thermal decomposition of iron pentacarbonyl. The relative amounts of iron, cobalt, and platinum in the particles depended on the amount of iron, cobalt and platinum charged to the reaction. As prepared, the particles were superparamagnetic and had a distorted fcc structure. The average particle diameter was 3.5 nm and the size distribution was very narrow. The particles could be dispersed in hydrocarbon solvents and formed films consisting of hexagonal close-packed particles on carbon-coated copper TEM grids. The films were sputter coated with amorphous carbon and then annealed at temperatures ranging from 550 to 700 °C to transform the particles to the tetragonal (L10) phase. The coercivity of the annealed films increased with increasing annealing temperature. For films with a similar degree of transformation to the tetragonal phase, increasing the cobalt content ...

Preparation of Narrow Size Distribution Superparamagnetic γ-Fe2O3 Nanoparticles in a Sol−Gel Transparent SiO2 Matrix

Abstract: γ-Fe2O3/SiO2 nanocomposites have been prepared using a sol−gel procedure, starting from iron nitrate and triethyl orthosilicate. The addition of acids to the sols resulted in a way to increase part...

Magnetic properties of CoFe2O4 nanoparticles synthesized through a block copolymer nanoreactor route

Abstract: The development of self-assembled magnetic CoFe2O4 nanoparticles within polymer matrices at room temperature is reported. Diblock copolymers consisting of poly (norbornene) and poly (norbornene-dicarboxcylic acid) (NOR/NORCOOH) were synthesized. The self-assembly of the mixed metal oxide within the NORCOOH block was achieved at room temperature by processing the copolymer nanocomposite using wet chemical methods. Morphology and magnetic properties were determined by superconducting quantum interference device magnetometry, transmission electron microscopy, wide angle x-ray diffraction, and 57Fe Mossbauer spectroscopy. The CoFe2O4 nanoparticles are uniformly dispersed within the polymer matrix, and have an average radius of 4.8±1.4 nm. The nanocomposite films are superparamagnetic at room temperature and ferrimagnetic at 5 K.

On-chip manipulation and magnetization assessment of magnetic bead ensembles by integrated spin-valve sensors

Abstract: Manipulation and detection of magnetic beads on a semiconductor chip opens up new perspectives for analysis of magnetically labeled specimens in biomechanical micro-electromechanical systems for biological applications. Sensitive spin-valve sensors were integrated with magnetic field generating conductors to assess the behavior of ensembles of superparamagnetic nanoparticles 300 nm in diameter that contain 75%–80% magnetite. The spin-valve multilayer including a nanooxide layer achieves 8% magnetoresistance (MR) for an integrated device of 2×16 μm2. Motion of the magnetic particles towards and across the sensor is achieved by two tapered magnetic field generating current conductors. The spin-valve sensor detects the stray magnetic field that emanates from the ensemble of magnetic particles. We study the transients in the magnetic signal on the order of 1% MR. These results lead to a model that describes magnetization configurations of the cluster of beads.

Magnetic properties and Mossbauer spectra of urban atmospheric particulate matter: a case study from Munich, Germany

Abstract: Summary Due to the high concentrations of iron in anthropogenically-derived atmospheric particulate matter (PM), magnetic techniques are ideal for the study of the behaviour of PM. However, to undertake these studies it is necessary to have a thorough understanding of the magnetic mineralogy of PM. This study reports a detailed examination of the magnetic mineralogy of PM samples collected in Munich. Several different approaches have been utilised in the characterisation; from standard mineral magnetic techniques such as magnetic hysteresis and microscopy to non-standard techniques such as Mossbauer spectroscopy. In particular, the study has focused on the smallest magnetic grains (< 30 nm), which are mostly likely to be inhaled into the human lung. These have been characterised using low-temperature magnetic and Mossbauer measurements, in conjuncture with the standard frequency dependency of magnetic susceptibility measurements. The primary magnetic minerals were found to be maghemite and metallic iron. The mean magnetic grain size range for these two minerals was 0.1-0.7 � m. The magnetic signature was estimated to come from 60-70% maghemite and the rest from metallic iron. A significant percentage of maghemite in the grain-size range 10-16 nm was identified, and there was evidence for metallic iron in the grain-size range 5-8 nm. This latter fraction is of particular importance as transition metal particles < 10 nm in diameter are known to catalyse the production of free radicals which are known to cause many lung complaints such as asthma. It was difficult to give accurate estimates of the total magnetic content by mass due to the mixed mineralogy, however, the absolute limits for the total magnetic content was estimated to be between 0.02-1.2 %. It is suggested that the maghemite comes from automobiles and the metallic iron from the street-trams which run near the sampling site.

Review Modern magnetic materials in data storage

Abstract: The current status of the technology of magnetic recording as used in disk drives is reviewed. The emphasis is on the magnetic materials used in the application and on some of the technical problems that may limit the increase in areal density. The new technology of magnetic random access memory (MRAM), which has evolved from the magnetic recording application, is also reviewed. A wide range of magnetic materials is essential for the advance of magnetic recording and the MRAM technology. For the magnetic-recording application the requirements are for high-magnetization, soft magnetic materials for write heads, new antiferromagnetic alloys with high blocking temperatures, large coupling to ferromagnetic films and low susceptibility to corrosion for pinning films in giant magnetoresistive sensors, and for the MRAM application, the requirement is for new ferromagnetic alloys with large values of tunneling polarization ratio. A significant limitation to magnetic recording is found to be the inconsistent demands on media thickness: small media thicknesses are required for large values of signal-to-noise ratio, while large values of thickness are required to reduce the impact of the superparamagnetic effect, which results in the potential for data loss over time. Both of these requirements are discussed. Multilayer ferromagnetic films for recording surfaces are shown to allow both large signal-to-noise ratio and adequate resistance to data loss.

Microemulsion-Assisted Synthesis of Tunable Superparamagnetic Composites

Abstract: Tunable superparamagnetic materials have been obtained by the thermal annealing of iron oxide/silica composites synthesized using microemulsions with a high content of the iron oxide precursor. The crystallochemical characteristics of the powders obtained after controlled hydrolysis within the reverse micelle nanocavities and their thermal evolution up to the formation of the γ-Fe2O3 crystalline phase were studied. The thermally annealed composites clearly exhibited superparamagnetic behavior with the value of the saturation magnetization easily tuned by changing the iron oxide content and the annealing temperature. Information on the influence of particle size and interparticle interactions on the magnetic properties of the composites was inferred from zero field and field cooling measurements and also from the decay of the reduced remanence with temperature. It was observed that the blocking temperature increased with content and particle size of γ-Fe2O3. We have also determined a strong dependence of t...

Magnetic characterization and recording properties of patterned Co/sub 70/Cr/sub 18/Pt/sub 12/ perpendicular media

Abstract: A lithographically patterned magnetic medium is one of the proposed routes to magnetic recording at a density beyond that thought to be possible using conventional recording media due to thermal instability caused by superparamagnetism. Using a focused ion beam to pattern a granular Co/sub 70/Cr/sub 18/Pt/sub 12/ film, we have fabricated sub-80-nm size islands that are single domain and with a narrowed switching field distribution and an enhanced thermal stability. Magnetic isolation of the islands is shown to be a result of vanishing of magnetic remanence and coercivity in the irradiated region and not a result of sputtering. Recording measurements using a quasi-static giant magnetoresistive head demonstrate the sensitivity to detect single 80-nm islands. The readback jitter from the patterned region is dramatically reduced compared to that measured for continuous media at the same linear density.