We describe the magnetic properties of gold-coated iron nanoparticles, and the effects of low pH and heat treatment. Acicular iron and spherical iron based nanoparticles were coated with a thin layer of gold. The morphology and magnetic properties of magnetic particles were examined using transmission electron microscopy and alternative gradient magnetometry. While the small spherical particles had relatively uniform layers coatings, the larger acicular particles had many gold clusters decorating the surface. The original acicular iron nanoparticles had a specific magnetic moment of 145 emu/g and a coercivity of 1664 Oe. Corrosion tests showed good corrosion resistance for gold-coated commercial iron particles even in a 1.0×10−3 M HCl solution at 80 °C for 12 h, compared with uncoated particles.

Monodispersed Co/CoO cluster assemblies with the mean cluster sizes of 6 and 13 nm have been prepared by a plasma-gas condensation type cluster beam deposition apparatus. We measured the effects of the oxygen gas flow rate during deposition, temperature, and cluster size on the coercivity and hysteresis loop shift induced by field cooling. The large exchange bias field ͑10.2 kOe͒ and coercivity ͑5 kOe͒ were observed at 5 K for the monodispersed Co/CoO cluster assembly with dϭ6 nm. The correlations between unidirectional anisotropy and uniaxial anisotropy, training effect and magnetic relaxation can be interpreted by the hypothesis of a spin disorder in the interfacial layer between the antiferromagnetic CoO shell and the ferromagnetic Co core.

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Articles you may be interested inElectron transport properties in Nb and NbN cluster-assembled films produced by a plasma-gas-condensation cluster source Nanometer-sized Cr clusters in the size range of 7.6-13 nm have been produced by a plasma-gas-condensation-type cluster deposition apparatus, which combines a grow-discharge sputtering technique with an inert gas condensation technique. We have studied influences of the Ar gas pressure, P Ar , and the Ar gas flow rate, V Ar , on the size distribution of Cr clusters by transmission electron microscopy. Monodispersed Cr clusters are formed at both low P Ar and low V Ar . At low P Ar , Cr clusters nucleate and grow only in the liquid-nitrogen-cooled growth region, and the deposition rate is rather low. At high P Ar , on the other hand, a large amount of Cr clusters are formed even near the sputtering source, and the nucleation and growth occur over a wide region between the sputtering source and the growth region. Under this condition, the deposition rate is relatively high. Consequently, the formation mechanism of the monodispersed clusters is similar to that of monodispersed colloidal particles: The nucleation and growth processes are definitely separated and the coagulation of growing particles is prohibited. In the present experiments, these conditions are effectively attained by using a carrier gas flow and liquid-nitrogen-cooling of the cluster growth region.