Characterisation and magnetic behaviour of nickel nanoparticles encapsulated in carbon

In the previous papers, we reported the synthesis of nickel nanocrystals encapsulated in an amorphous carbon matrix by a method that consists in the sonochemical decomposition of the Ni(COD)(2) organometalic precursor followed by a heating treatment in vacuum. Both as-prepared and heated samples have been further characterised in the present paper by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) to determine the microstructure and the stability of the nickel nanocrystals for long periods of air storage. It has been shown that carbon is protecting the nickel nanocrystals from oxidation leading to a magnetic behaviour of the sample congruent with the superparamagnetic properties of single domain particles. The analysis of the frequency-dependent ac susceptibility leads to two well-defined distributions of particle size consistent with those observed in TEM experiments. The effective anisotropy constant has been determined for the two particle sizes of nickel nanoparticles. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.