CoNi and FeCoNi fine particles prepared by the polyol process: Physico-chemical characterization and dynamic magnetic properties

Abstract

Spherical and monodisperse Co _x Ni_{100 − x} and Fe _z [Co _x Ni_{100 − x} ]_{1 − z} particles are synthezied by the polyol process over a wide size range (lying from a few micrometers to a few tens of nanometers). The whole physico-chemical characterizations, i.e. dark-field image by TEM, SAED, density, saturation magnetization and chemical analysis, are consistent with a “core-shell” model. In the Co _x Ni_{100 − x} system, the particles are constituted by a ferromagnetic, almost pure and dense core surrounded by a thin coating composed of metal oxides and metallo-organic phases. On the contrary, in the Fe[Co _x Ni_{100 − x} ]_{1 − z} system, the ferromagnetic core is polycrystalline, slightly porous and retains impurities in higher content, the superficial layer having almost the same composition as in the Co-Ni system, but being twice more thick. The microwave permeability of the Co _x Ni_{100 − x} and Fe _z [Co _x Ni_{100 − x} ]_{1 − z} particles, previously insulated by a superficial treatment and then mechanically compacted, is investigated in the 100 MHz-18 GHz frequency range. Whatever the composition, submicrometer-sized particles show several narrow resonance bands which are interpreted as non uniform exchange resonance modes. Iron-based particles have lower resonance frequencies than iron-free powders; they also have higher permeability levels despite their lower cristallinity and their higher impurity content. A mild thermal treatment allows to increase this permeability by eliminating the metallo-organic impurities without modifying the morphology of the particles.