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Focuses on a study which measured magnetic ac susceptibility of randomly arranged crystallites and well-oriented iron crystallites as a function of applied magnetic field, temperature and frequency. Information on resonant quantum tunneling of magnetization; Studies made on magnetic nanoparticles; Measurement of magnetization as a function of temperature.

Nanocubes and partially truncated rhombic dodecahedral nanoparticles of Cr have been studied by electron diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. It is found that the nanoparticles of Cr are enclosed by the epitaxial passive oxide films. The oxide films on 100% truncated nanocubes of Cr with only one kind of facets, {1 0 0} facets, are face-centered cubic (fcc) structured Cr2O3 with a lattice constant of 0.407 nm. There are two kinds of oxide films in partially truncated nanoparticles of Cr with two kinds of crystallographic facets, {1 0 0} and {1 1 0}. The same fcc Cr2O3 is found on the {1 0 0} facets while the rhombohedral α-Cr2O3 is found on the {1 1 0} facets. This is similar to the two kinds of oxides, fcc and rhombohedral Fe2O3, which have also been observed in polyhedral nanoparticles of Fe. These passive Cr2O3, found in nanoparticles of Cr which have remained unchanged in water for four years, may have important implications for protective oxide films involving Cr. [Copyright &y& Elsevier]

Abstract Simultaneous enhancement of the strength and ductility in metal-matrix nanocomposites with high content nano-reinforcements is crucial for the development of novel composite materials. Here SiC/Al nanocomposites with ultrafine Al grains and intragranular 10 vol% SiC nanoparticle were prepared by mechanical alloying and hot pressure sintering followed by hot extrusion. The results show that nanocrystalline Al grains and uniformly distributed SiC nanoparticles (SiC np) in the composite powders are obtained after ball-milling. The prior particle boundary (PPB) in the composite powders cannot be fully removed by sintering at 580 °C under 20 MPa, but can be eliminated by subsequent hot extrusion at 450 °C with a ratio 25:1. The extruded nanocomposites possess ultrafine Al grains with diameter 320 nm and intragranular SiC np created by the dynamic recrystallizations (DRX). Such optimal microstructure favors a significant improvement of 284% in yield strength (YS) and 259% in ultimate tensile strength (UTS) compared with monolithic Al, and more interestingly, maintains an elongation up to 12% due to the homogeneous nanoparticles in fine Al grains. Theoretical analysis shows that the dominant strengthening factor is Orowan strengthening related to the high content intragranular SiC np , with second factor grain boundary strengthening due to ultrafine Al grains. [ABSTRACT FROM AUTHOR]

Remarkably oxidation-resistant nanoparticles of iron prepared at Tianjin University have been studied by electron diffraction and high-resolution transmission electron microscopy (HRTEM). HRTEM images clearly show the polyhedral epitaxy of γ-Fe[sub 2]O[sub 3] layers on equivalent {100} facets of the {100} truncated {110} rhombic dodecahedral nanoparticles of iron. Oxide layers on {110} facets are formed by the lateral extension of the epitaxial {100} γ-Fe[sub 2]O[sub 3] layers over adjacent {110} facets. The {110} oxide layers are bent and concave inward. The compressive γ-Fe[sub 2]O[sub 3] layers which are about 4 nm thick give effective protection, according to the theory of Caberra-Mott, to the iron core from further oxidation. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A comprehensive review of the giant Hall effect (GHE) is presented, with emphasis on novel experimental data obtained in Ni–SiO2 and Co–SiO2 films prepared by co-sputtering. GHE is observed close to and on both sides of the metal–insulator transition. From the point of view of microscopic conduction mechanisms, this means a crossover from metallic conductivity with weak localization to tunneling, or hopping, between separate granules across insulating barriers. Magnetic percolation is also interrupted at this concentration of metal, leading to superparamagnetic behavior of the composite and blocking phenomena. Temperature dependencies of magnetization and extraordinary Hall coefficient in the composites near the critical concentration are compared. In single phase magnetic metals and alloys, the extraordinary Hall is believed to be directly proportional to the total magnetization, due to side jumps or skew scattering. In a metal–insulator composite, only those electrons traveling in conduction critical paths can contribute to the Hall signal, thus only magnetization of the material belonging to these paths is important in the Hall measurements. Comparison with the magnetic results leads to new possibilities in understanding both the electronic and magnetic properties of granular nanocomposites. [Copyright &y& Elsevier]

1 vol% SiC nanoparticle reinforced aluminum (nano-SiC p /Al) composite foams and pure Al foams were manufactured by powder metallurgy with foaming agent CaCO 3 in order to explore the effects of nanoparticles. The microstructure, compressive property and energy absorption of these foam materials were investigated. The microstructural characterization indicates that after adding the nanoparticles, the average pore diameter changes from millimeter to micrometer, meanwhile the average cell size decreases by 50.4%. It is found from the compressive experiments that under similar porosities, the addition of 1 vol% SiC nanoparticles induces a significant improvement of 194.5% in yield stress and 69.4% in energy absorption capacity compared with the pure Al foams. The comparative research reveals that a small addition of nanoparticles can not only improve the foam structure, but also significantly strengthen the metal and enhance energy absorption. [ABSTRACT FROM AUTHOR]

Abstract: Iron–iron oxide granular films are fabricated using dc magnetron sputtering. Their structural, magnetic and transport properties are studied in the vicinity of percolation threshold. X-ray photoelectron spectroscopy and transmission electron microscopy confirm the coexistence of iron and FeO x . Magnetic measurement indicates the exchange coupling between the iron and the FeO x matrix. Metal–insulator transition is observed. The non-monotonic temperature dependencies of resistivity occur near the percolation threshold and the mechanism is investigated with the longitudinal and transverse magnetic transport measurement in terms of weak localization. [Copyright &y& Elsevier]

Co nanoparticle films were prepared by plasma–gas-condensation-type particle beam deposition system. High-resolution transmission electron microscopy images show that the Co nanoparticles have a very narrow size distribution with an average diameter of ∼20 nm, and each of the Co nanoparticles is covered with an ∼3 nm layer of CoO. Hysteresis loops of the films after field-cooling in a 5 T magnetic field are greatly shifted, which can be attributed to the exchange bias effect caused by the interfacial exchange coupling between the CoO shell and the Co core. The zero field cooled films show several prominent properties, such as a quite large coercive field, a small remanence and their abnormal dependences on temperature. All these observations can be attributed to the existence of an exchange bias effect within each single Co nanoparticle even without a field-cooling process. [Copyright &y& Elsevier]