(FeCo)3Si?SiOx core?shell nanoparticles fabricated in the gas phase

A method of fabricating core?shell nanoparticles by using an integrated nanoparticle deposition technique in the gas phase is reported. The principle of the method is based on nanoparticle growth from the vapour phase, during which elements showing lower surface energies prefer to form the shells and elements showing higher surface energies prefer to stay in the cores. This method was applied successfully to the Fe?Co?Si ternary system to fabricate core?shell-type nanoparticles. The nanoparticles were exposed in air after collection to achieve oxidation. The analysis results based on transmission electron microscopy (TEM), Auger electron spectroscopy (AES), x-ray diffraction (XRD), and a superconducting quantum interference device (SQUID) showed that the core parts are magnetic materials of body-centred cubic (bcc) structured (FeCo)3Si of 15?nm in diameter, and the shell parts are amorphous SiOx of 2?nm in thickness. These core?shell-type nanoparticles show a magnetic anisotropy constant of about 7 ? 105?erg?cm?3 and a saturation magnetization of around 1160?emu?cm?3, which is much higher than that of iron oxide. After annealing at 300 ??C in air, (FeCo)3Si?SiOx core?shell-type nanoparticles showed a little bit of a drop in magnetic moment, while pure FeCo nanopariticles totally lost their magnetic moment. This means that the shells of SiOx are dense enough to prevent the magnetic cores from oxidation.