Charles J. O’Connor

Magnetic properties of ultrafine cobalt ferrite particles

We have studied magnetic properties of a diluted system of ultrafine cobalt ferrite nanoparticles (d∼3.3 nm). From the peak of the zero-field-cooled measurements, we obtained the blocking temperature TB of about 90.5 K and it is virtually independent of the applied magnetic field up to 5 kOe. At the superparamagnetic region T>TB, the system follows the modified Curie-law variation of the magnetic susceptibility χ=χo+C/T. We observed that the saturation magnetization follows a spin-wavelike temperature dependence at temperature above 10 K. In spite of the cubic structure for cobalt ferrite, at 2 K, the reduced remanence Mr/Ms is equal to 0.46 which is close to the theoretical value of 0.5 expected for noninteracting uniaxial single-domain particles with the easy axis randomly oriented. From the ac susceptibility measurements at different frequencies, we obtained a linear dependence of the logarithm of the experimental time window τex as function of inverse blocking temperature (1/TB). The fitting results i...

Magnetism of nanophase metal and metal alloy particles formed in ordered phases

In general, the intrinsic magnetic properties of a single metallic elemental can be increased by forming alloys containing one or two additional metals. In this article, metallic cobalt, cobalt/platinum alloys, and gold-coated cobalt/platinum nanoparticles have been synthesized in reverse micelles of cetyltrimethlyammonium bromide. Magnetic characterization of all samples demonstrate that the particles containing platinum and gold exhibit a higher blocking temperature and larger coercivities relative to pure cobalt nanoparticles of the same size. The dc susceptibility of a sample of 15 nm cobalt nanoparticles exhibit a blocking temperature of 70 K and coercivity, Hc, of 1800 G at 2 K. When equimolar quantities of cobalt and platinum were combined and reduced in the reverse micelle, the blocking temperature increased to 130 K and Hc at 2 K is reported as 2700 G. When additional platinum is added, however, the blocking temperature dropped to 100 K and coercivity at 2 K decreased to 2000 G. Addition of a gol...

magnetic properties of iron and iron platinum alloys synthesized via microemulsion techniques

In this article, metallic iron, and iron/platinum alloys nanoparticles have been synthesized via chemical assembly and magnetically characterized. Fabrication of iron and iron/platinum particles was achieved by reducing 0.1 M aqueous metal salts confined in the polar portions of inverse micelles of cetyltrimethylammonium bromide with borohydride. The dc susceptibility of a sample of 8 nm iron nanoparticles exhibits a blocking temperature of 54 K and coercivity of 200 G at 10 K. The presence of the gold coatings prevented oxidation and allowed the samples to be manipulated without additional precautions to prevent oxidation. Two iron/platinum alloys have been synthesized and verified by x-ray powder diffraction and transmission electron microscopy. Magnetic characterization is performed using superconducting quantum interference device magnetometry.

Atomic structure and magnetic properties of MnFe2O4 nanoparticles produced by reverse micelle synthesis

Using the aqueous cores of reverse micelles as nanoreactors, nanoparticles (d∼10 nm) of the mixed ferrite MnFe2O4 were produced. Seven processing trials were performed where the concentration of ammonium hydroxide, reaction temperature, and the oxidizing agent were varied. All trials result in Mn-ferrite particles with varying chemistry and structure. The Mn concentration in the resulting ferrite is strongly enhanced by both the presence of H2O2 as an oxidizing agent and a surplus of ammonium hydroxide. The increased Mn concentration correlates with a higher fraction of octahedrally coordinated Mn cations. When near-stoic amounts of ammonium hydroxide are used, the resulting ferrites are nearly stoichiometric with a more equitable distribution of Mn cations on the octahedral and tetrahedral sublattices. In all ferrite nanoparticles, the Mn cations have a preference for octahedral site occupancy that is larger than the 20% measured in bulk Mn-ferrite. We attribute the cation filling trends to the stabiliza...

Fabrication and properties of magnetic particles with nanometer dimensions

Novel nanocrystalline inorganic materials ranging from metals to ternary metal oxides and fluorides have been synthesized in aqueous solutions with restricted environment of reverse micelles. This technique allowed also sequential synthesis leading to a core–shell type structure. Methods of synthesis of nanoparticles of magnetic metals Fe, Co and alloys FePtX and CoPtX coated with gold, metal ferrites MFe2O4 (M=Mn, Fe and Co) and antiferromagnetic fluoromanganites NaMnF3 and KMnF3 have been developed. The synthesized materials were characterized using X-ray diffractometry, TEM microscopy with EDAX analysis and SQUID magnetometry. Novel physical properties of these materials are discussed with emphasis given to the differences between course and fine-grained magnetic materials.

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.

Ultrafine NiFe2O4 powder fabricated from reverse microemulsion process

NiFe2O4 ultrafine powder with high crystallinity has been prepared through a reverse microemulsion route. The composition in starting solution was optimized, and the resulting NiFe2O4 was formed at temperature of around 550–600 °C, which is much lower than that observed from the solid-state reaction. Magnetic investigation indicates that samples are soft-magnetic materials with low coercivity and with the saturation magnetization close to the bulk value of Ni ferrite.

A new method for the synthesis of magnetoliposomes

A new method for the synthesis of magnetoliposomes, i.e., nanosized magnetic particles coated by a phospholipid membrane, is presented. Magnetoliposomes are prepared by directly using the phospholipid vesicles as nanoreactors for the precipitation of the magnetic particles. The magnetoliposomes have been characterized using transmission electron microscopy imaging and x-ray powder diffraction. The magnetic properties of the magnetoliposomes have been investigated with a superconducting quantum interference device magnetometer. Our results indicate that the magnetoliposomes contain approximately spherical maghemite nanoparticles averaging 25 nm in diameter. The occurrence of a phospholipid bilayer surrounding the magnetic particles is confirmed both by transmission electron micrographs of samples negatively stained with uranyl acetate and by digital fluorescence imaging microscopy measurements of magnetoliposomes labeled with fluorescein. The temperature dependence of the zero field cooled and field cooled...

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.

Anomalous magnetoresistance in NiMnGa thin films

The origin of anomalous negative magnetoresistance and its temperature dependence in polycrystalline Ni–Mn–Ga films prepared by pulse laser deposition was studied. The investigation of structural, transports, magnetic, and ferromagnetic resonance properties of the films suggests contributions of different mechanisms in magnetotransport. At low magnetic fields the main contribution to magnetoresistance is due to the transport between the areas with different orientation of magnetic moments, while at high fields it is an electron scattering of in spin-disordered areas.