Synthesis of self-forming core/shell nanoparticles of magnetic metal/nonmagnetic oxide

Various kinds of self-forming core/shell nanoparticles with a core consisting of magnetic metal and a protective shell consisting of nonmagnetic metal oxide (magnetic metal: Co or FeCo; nonmagnetic metal: Al or Si) were synthesized. The synthesis conditions and compositions of the materials were controlled in order to obtain homogeneous core/shell nanoparticles with high dispersivity. The process of applying carbon coating to the nanoparticles using thermal plasma and subsequently removing the carbon shell by heating in hydrogen was effective for obtaining relatively homogeneous core/shell nanoparticles with crystalline grain sizes of approximately 20 nm. Partial oxidation after the carbon-coating process suppressed aggregation of the core/shell nanoparticles and promoted phase segregation of the oxide shells from metallic cores. Similar effects were also achieved by either increasing the amount of nonmagnetic metal or adding a small amount of another nonmagnetic metal to the metal. The bulk nanocomposite of the self-forming core/shell nanoparticles with high dispersivity that were obtained by phase segregation of oxide shells from metallic cores exhibited low magnetic losses with a high ferromagnetic resonance frequency in the gigahertz band.