Observations of exchange resonance modes on submicrometer sized ferromagnetic particles

Spherical, monodisperse, submicrometer-sized particles in the ${\mathrm{Co}}_x{\mathrm{Ni}}_{\text{(100−x)}}$ system have been synthesized by the polyol process over the whole chemical composition range. These fine particles have been insulated by a chemical treatment in solution by a superficial layer of ${\mathrm{MnO}}_2.$ The microwave permeability of these passivated particles, which are mechanically compacted, has been investigated in the frequency range 0.1–18 GHz. The dynamic permeability measurements exhibit several narrow resonance bands. This experimental behavior is in qualitative agreement with Aharoni’s calculation [A. Aharoni, J. Appl. Phys. 69, 7762 (1991)] of exchange resonance modes. The influence of the chemical composition of the ferromagnetic particles upon permeability has been investigated in the microwave range. It is shown that the peaks are shifted towards the high frequencies when atomic Co/Ni ratio increases.