Joan A Wiemann

Microemulsion-processed bismuth nanoparticles

Abstract Bismuth (Bi) is classic semi-metal with a low carrier density, small carrier effective masses, very long mean free path and highly anisotropic Fermi surface. In this work, we present a technique for the synthesis of bismuth nanometer-sized particles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be obtained with ultrafine particle with ∼20 nm in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band at ∼268 nm.

Nanocrystalline bismuth synthesized via an in situ polymerization-microemulsion process

Nanometer-sized bismuth particles were prepared using an inverse microemulsion method. To prevent air-oxidation, an in situ polymerization technique using methyl methacrylate (monomer) and 2-hydroxyethyl methacrylate (co-monomer) with cross-link agent was employed, and polymeric network was formed around the water droplets. Our characterizations reveal that very highly crystalline bismuth particles on the order of 20 nm can be obtained within this polymeric network. Comparison with a similar microemulsion procedure without polymer indicates that the polymeric network protected the bismuth particles against oxidation, especially during post-synthesis annealing. The current investigation demonstrates a feasible route to produce single phase, air-sensitive metal nanocrystallites using this modified microemulsion technique.

Fabrication of pseudo-spin-valves and 100 nm sized periodic elements for magnetic memory application

Abstract Co–Cu–Co pseudo spin-valves (PSV’s) and Co–Cu multilayers have been fabricated and studied for the giant magnetoresistance (GMR) effect. Attention is focused on optimizing the low field response. They are deposited on Si (001) wafers without removing the natural silicon-dioxide layer, using S-research sputtering guns in high vacuum. A buffer layer of Cr–Cu was used before depositing the PSVs. Realization magnetic switching at low field for the soft layer and high field for the hard-layer makes the structure suited for MRAM application. The low and high switching fields are typically 100 and 2200 Oe, respectively. The magnetic measurement using SQUID magnetometry and VSM confirms the switching fields in the hysteresis loops of the magnetization. The GMR multilayers have a low switching field of 60 Oe at low temperatures. They are patterned into two-dimensional 100 nm sized periodic dot-array using ultraviolet interferometric lithography. The purpose is to develop processes of fabricating GMR structures over large areas with nanometer feature sizes for high-density memory devices. The coercive field for the patterned structures is enhanced by a factor of four at low temperatures and by a small fraction at higher temperatures. The magnetization remains to be in plane as in un-patterned case.

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.

THICKNESS DEPENDENCE OF GIANT MAGNETORESISTANCE EFFECT IN GRANULAR CU-CO THIN FILMS

This work investigates the magnetic and transport properties of Cu–Co thin films with a special reference to their dependence on the film thickness. Such dependencies of the giant magnetoresistance (GMR) effect in silver-based magnetic alloys, such as Ag–Fe, Ag–Co, and Ag–FeNi films, have recently been found, and they were interpreted within the framework of surface spin-flipping scattering. This article reports on similar thickness dependence in the Cu-based alloy, although the spin-orbit interaction in Cu films is much weaker than in Ag films. A reduction of the GMR in the thinnest samples by a factor of 6, compared to the value of as-deposited bulk samples (8.6% in 50 kOe and at 5 K), was accompanied by an increase in resistivity by no more than 50%. A novel vapor-mixing technique of simultaneous sputtering from two sources was used to deposit Cu80Co20 granular thin films of the 20% nominal Co volume fraction. The thickness of the films, ranging from 8 to 400 nm, was measured by the small-angle x-ray r...

Colossal magnetoresistance and magnetic phase transitions in bulk Nd0.7Ba0.3MnO3

Perovskite-type Nd0.7Bao0.3MnO3 has been synthesized and its magnetic and magnetotransport properties studied in the temperature range from 5 K to 300 K. X-ray diffraction indicates that the crystal-symmetry is orthorhombic with a=0.5508(1) nm, b=0.5496(2) nm, and c=0.7764(3) nm. There exist two magnetic phase transitions. It first undergoes a paramaganetic-to-ferromagnetic phase transition at Curie temperature, TC=115 K, and then partial ordering of Nd magnetic moments develops at TNd=40 K as temperature is further decreased. Resistivity measurements indicate corresponding insulator-to-metal transition near TC and a reentrant metal-to-insulator transition near 40 K. Colossal magnetoresistance exists over a wide temperature range, MR=[R(H)-R(0)]/R(0) is over 90% below 120 K. The maximum MR (98%) appears at 95 K, close to the paramagnetic-to-ferromagnetic phase transition.

Formation and structure of LaNi2 intermetallic compound under high pressure

The Cubic LaNi2 Laves phase has been synthesized under high pressure. The effects of temperature and pressure on the stability of the Laves phase have been studied. High pressure also induces the phase transitions from intermetallic compounds La2Ni3 and LaNi2.286 to the Laves phase.

COLLOIDAL BISMUTH NANOPARTICLES: SYNTHESIS AND UV-VIS ABSORPTION

Quantum-confined Bi structures (quantum wells and quantum wires) are currently being studied for their varies novel properties. We now pursue the next level of quantum confinement in bismuth, quantum dots. In this work, we present a technique for the synthesis of bismuth nanoparticles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be formed with ultrafine particle in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band centered at ~268 nm.

Magnetic properties of (γ‐Fe2O3)80Ag20 nanocomposites prepared in reverse micelles

The magnetic properties of nanoparticles of γ‐Fe2O3 prepared by reverse micelles have been studied by dc magnetization, transverse ac susceptibility, and Mossbauer spectroscopy. The nanoparticles of γ‐Fe2O3 in the nanocomposite (γ‐Fe2O3)80Ag20 exhibit superparamagnetic behavior. The blocking temperatures determined by the three methods indicate the superparamagnetic nature of (γ‐Fe2O3)80Ag20 above 70–80 K and show correlation with measuring time. The average particle diameter obtained by transmission electron microscopy of the γ‐Fe2O3 particles is ∼10nm and that of the Ag particles is ∼20nm. The average particle size determined from the magnetic analyses for the γ‐Fe2O3 particles is ∼12nm. Mossbauer spectra obtained between 4.2 and 295 K clearly reveal the presence of superparamagnetic relaxation at temperatures above ∼80K. The Mossbauer spectra reveal at most 1% of paramagnetic Fe2+ ions in the 295-K spectrum.