Dajie Zhang

Encapsulated iron, cobalt and nickel nanocrystals; Effect of coating material (Mg, MgF2) on magnetic properties

Abstract Nanometer sized core/shell particles of[Mg]Fe, [MgF 2 ]Fe, Co, and Ni, where Mg or MgF 2 are the shell materials, have been prepared by a low temperature matrix method. Phase segregation was accomplished within each individual particle by controlled heating to yield the completely oxide free core/shell particles, which were sealed in glass or quartz tubes and studied by SQUID magnetometry. For [Mg]Fe samples with very small Fe crystallites, enhanced anisotropy energies were observed. For core/shell particles, saturation magnetization [Ms] values for Fe, Co, and Ni were found to be both size and temperature dependent. Indeed, M(T) behavior is richly dependent on size in the nanometer range, and on the type of surface interface. Quantitative assessment and discussion of Bloch exponent and Bloch constant variations as iron crystallite size change are discussed.

Size and Interface Dependent Magnetic Properties of Encapsulated Metal Clusters

The properties of magnetic nanoparticles is an area of current active interest with many problems of fundamental importance [1–4]. The nanoscale places particles at the single domain size where the coercivity can have a maximum or below this size where the particles eventually become superparamagnetic with no coercivity [5]. The density of states of various excitations in nanoparticles is affected by size through coarsening and/or introduction of a finite wavelength cut off. And, perhaps most importantly, nanoparticles have a large fraction of surface atoms which may have properties distinctly different than those in the bulk. Moreover, the exposed nature of the surface makes it susceptible to interfacial materials which might modify the properties of the surface atoms.

Synthesis and Characterization of Iron-Magnesium Bimetallic Particles

The clusters of two immiscible metals, iron and magnesium, were prepared by atom clustering in cold pentane(Solvated-Metal-Atom-Dispersion). Very small α-Fe crystallites embedded in a matrix of magnesium and organic fragments were obtained. Stable Fe-Mg particles containing small α-Fe crystallites in a range of 3 to 20 nm in size were formed by heat-treatment followed by oxidative passivation. Fe-Mg bimetallic materials were also prepared by a vacuum co-evaporation method. The Fe-Mg SMAD particles have over-all sizes in a range of 40 to 85 nm, and specific surface areas in a range of 17 to 26 m2/g, while the vacuum co-evaporated Fe-Mg materials are considered as bulk materials.