Everett E. Carpenter

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...

Magnetic coupling induced increase in the blocking temperature of γ-Fe2O3 nanoparticles

In this article, we report the magnetic properties of surfactant coated γ-Fe2O3 nanoparticles which are pressed under different pressures. With increasing pressure, the sample volume decreases, density increases, and a 55% density change has been achieved. The blocking temperature is increased from 50 to 80 K. Analyzing the data of blocking temperature versus densities, which exhibits linear relationship, and comparing the magnetic properties, the increase in blocking temperature is understood in terms of increased magnetic interactions between neighboring nanoparticles, which is due to the reduced average interparticle distance by the applied pressure.

Dynamic radio-frequency transverse susceptibility in magnetic nanoparticle systems

A novel resonant method based on a tunnel-diode oscillator is used to study the dynamic transverse susceptibility in a Fe nanoparticle system. The magnetic system consists of an aggregate of nanometer-size core (Au)-shell (Fe) structure, synthesized by reverse micelle methods. Static and dynamic magnetization measurements carried out in order to characterize the system reveal a superparamagnetic behavior at high temperature. The field-dependent transverse susceptibility at radio-frequencies for different temperatures reveals distinct peak structure at characteristics fields (±HK,HC) which changes with temperature. It is proposed that relaxation processes could explain the influence of the temperature on the field dependence of the transverse susceptibility.

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.

Synthesis and magnetic properties of gold-iron-gold nanocomposites

Abstract By utilizing the sequential synthesis afforded reverse micelles, nanocomposite materials can be synthesized which have a diamagnetic core surrounded by a thin shell of ferromagnetic material passivated with a second shell of a diamagnet. Using gold as the diamagnetic material and iron as the ferromagnetic material, nanocomposites can be synthesized where there is a thin layer of the magnetic material, which is passivated and protected from oxidation. In this case, all of the spins of the magnetic layer lie within the surface of the particle. Magnetic properties were measured for nanophase particles using SQUID magnetometry. The particles, which consist of a 6 nm core of gold, coated with a 1 nm thick iron layer and passivated with an outer shell of gold, are superparamagnetic with a blocking temperature of 45 K and coercivity at 10 K of 400 Oe. These results are similar to magnetic properties of 8 nm iron particles coated with gold, where blocking temperature is 50 K and coercivity is 400 Oe. This suggests that in nanoparticles the spins that define the outer surface are responsible for the magnetic properties.

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.

Phenomenological magnetic modeling of Au:Fe:Au nano-onions

A new type of materials, the nano-onions, has been shown to exhibit GMR. These nanostructured composites consist of a nonmagnetic core coated with a thin layer of a bulk ferromagnet with a passivating nonmagnetic surface layer. The nano-onion investigated had a 3 nm Au core, a 1 nm Fe layer, and a 2 nm Au coating; all values correspond to the radius. The materials were manufactured using a sequential reverse micelle technique, detailed elsewhere. The sample preparation method produces a powder sample was cold pressed into a pellet. Magnetic investigation of the sample indicated that the material was superparamagnetic with a blocking temperature of 52 K for particles approximately 8 nm in diameter. At 10 K, the coercivity was 420 Oe, indicating a large degree of order. The GMR was measured over the entire temperature range available. At 10 K, in 5 T, a 1% MR was observed. The GMR was modeled using a simple phenomenological magnetic model initially used to study GMR granular thin films. To more accurately m...

Characterization of ferrite nanoparticles by laser desorption/ionization mass spectrometry

Magnetic nanoparticles are important materials used in magnetic storage media. The specific physical properties of nanophase particles can have profound effects on magnetism and will determine the merits of particular particles as components in storage devices. However, obtaining a detailed depiction of the structural features of nanoparticles is a non-trivial process. We have employed laser desorption/ionization mass spectrometry (LDI-MS) to aid in the characterization of nanoscale magnetic particles. Our investigation reports on two types of ferrite nanoparticles, having Fe2O3 and Fe3O4 crystal structures, that were each synthesized in reverse micelles. During positive and negative LDI-MS of both types of particles, iron oxide peaks were consistently produced. Compared to Fe2O3, Fe3O4 particles yielded more peaks at the high m/z end of the mass spectrum, and the relative intensities of higher m/z peaks were typically greater. A negative ion mode depth profiling experiment revealed the capacity of the LDI-MS approach to obtain information regarding changes in nanoparticle composition as a function of sampling depth. Over the course of prolonged irradiation of stationary Fe2O3 nanoparticles, distinctly different desorption/ionization behavior was observed at the particle surface as compared to the core region. The depth profiling experiment was particularly useful for distinguishing surface impurities from core constituents.