Candace T. Seip

Magnetic properties of a series of ferrite nanoparticles synthesized in reverse micelles

Nanoscale particles of the general formula MFe/sub 2/O/sub 4/ (M=Co, Mn, Fe) were synthesized in reverse micelles of twin-tailed anionic bis(2-ethylhexyl) sodium sulfosuccinate (AOT) in isooctane. The size of the particles was controlled by adjusting the AOT/water molar ratio. Particle sizes were confirmed using XRD and uniformity was determined by SEM. Magnetic measurements, carried out using a SQUID Susceptometer, indicated superparamagnetic behavior. FC and ZFC demagnetization experiments indicate blocking temperatures of 46 K, 30 K, and 7 K for for MFe/sub 2/O/sub 4/ M=Co, Mn, Fe respectively. Below blocking temperatures, the nanoparticles demonstrate hysteresis with coercivities of H/sub c/=6000G, 3800G, and 500G for the Co, Mn, and Fe ferrites.

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...

The fabrication and organization of self-assembled metallic nanoparticles formed in reverse micelles

Abstract Metallic iron nanoparticles are synthesized in reverse micelles of cetyltrimethyl-ammonium bromide (CTAB) using hydrazine as a reducing agent. Once the iron nanoparticles have formed inside the micelle, an aqueous solution of HAuCl 4 is added to the iron/CTAB mixture. Addition of the aqueous gold solution increases the size of the reverse micelle and the Au(III) is reduced to Au(0) via excess hydrazine. Because gold and iron grow with complementary crystal structures, the metallic gold forms a coating on the outer surface of the iron particles. The gold shells on the iron particles provide functionality and thin films of the gold coated particles have been made by self-assembly reactions between the gold surface of the particle and thiol functionalized substrates.

Preparation and characterization of Ni nanoparticles in an MCM mesoporous material

Abstract An aluminosilicate with the MCM-41 structure (A1MCM-41) was used as a host for the synthesis of nickel metal nanoparticles. Initially, ion exchange in aqueous solutions allows the introduction of nickel cations into A1MCM-41, and then reduction with sodium borohydride produces nanometer-sized nickel metal particles. Products (Ni-AlMCM-41) were characterized by elemental analysis, transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and magnetic susceptibility. Ni-AlMCM-41 was found to have a Ni:Al:Si ratio of ca. 1:4:28. The nickel metal particles were 1–2 nm in diameter and showed superparamagnetic behavior with a blocking temperature (T b ) of 5 K.

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.

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...

Nanophase Magnetic Materials: Synthesis and Properties

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, CoFe2O4, Fe and Fe/Au 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 (e.g., iron nanoparticles with gold coatings). Lyotropic liquid crystal media also offer template effects for the synthesis of non-spherical magnetic nanostructures. 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.

Low-Temperature Multistep Topotactic Routes to New Mixed-Valence Perovskites

Multistep topotactic routes are exploited to introduce mixed-valency into mixed-metal oxides at low temperatures (≤ 350°C). A new set of single- and triple-layered perovskites, Na 1-x-y Ca x/2 LaTiO 4 and Na 2-x+y Ca x/2 La 2 Ti 3 O 10 , respectively, has been prepared by a combination of ion exchange and reductive intercalation. The single-layer series are metastable compounds. The magnetic and electronic behavior of the triple-layer titanate is consistent with Anderson localization effects, while those of the single-layer materials are more complex; samples demonstrate “Hurd-like” conductivity and an unusual magnetic response. The details of the synthesis and characterization of these materials are presented and their magnetic and electronic behavior discussed.

Magnetotransport and magnetism in granular EuS-Co and EuS-Ag nanocomposites prepared by mechanical alloying

Magnetic and magnetotransport properties have been studied in nanocomposites (EuS)-Co and (EuS)-Ag prepared by mechanical alloying. For (EuS)/sub 70/Co/sub 30/, the large negative magnetoresistance (/spl sim/50%) found at 20 K, just above the Curie temperature (T/sub c/) of EuS, arises from the suppression of spin fluctuation in EuS. Below T/sub c/, the positive magnetoresistance (/spl sim/15%), which is uncharacteristic of ferromagnetic semiconductors, is probably due to defect-induced magnetic fluctuation. On the other hand, (EuS)/sub 70/Ag/sub 30/, where nonmagnetic silver replaces cobalt, exhibits negative magnetoresistance below T/sub c/, which is possibly related to the spin dependent scattering commonly seen in GMR materials. While (EuS)/sub 70/Co/sub 30/ shows semiconducting conductivity, (EuS)/sub 70/Ag/sub 30/ is metallic. Reentrant spin glass behavior has been observed in EuS-Ag system, where the rise of antiferromagnetic coupling is due to the defects and disorder caused by mechanical milling.

Magnetic Properties of Cobalt and Manganese Ferrite Nanoparticles Synthesized Reverse Micelles

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