Electromagnetic properties of Fe-Co granular composite materials containing acicular nanoparticles

Electromagnetic properties of acicular (needle-like) Fe₇₆Co₂₄ nanoparticle composite materials have been studied in microwave frequency range up to 20 GHz. The Fe₇₆Co₂₄ particles are commercially available acicular Fe₇₆Co₂₄ nanoparticles with an approximate length and diameter of 100 and 25 nm, respectively. The Fe₇₆Co₂₄ nanocomposites were prepared by embedding the Fe₇₆Co₂₄ nanoparticle in an appropriate resin. Since the metallic Fe₇₆Co₂₄ nanoparticles have an oxidized surface, even high particle content composites at 78 vol.%, which is in the percolated state, does not show metallic conduction; a low frequency plasmonic state with the negative permittivity spectrum was not observed. Meanwhile, the negative permeability spectrum caused by the magnetic resonance in Fe₇₆Co₂₄ alloy was obtained in the high particle content composites. From the measurement of the complex permeability spectra under the external dc magnetic field, it was clarified that the gyromagnetic spin rotation mainly contributes to the permeability spectrum of nanocomposites due to extremely small quantity of domain walls in the acicular nanoparticles. This result suggests that the negative permeability spectrum was caused by the gyromagnetic spin resonance. By the comparison of the complex permeability spectrum between the acicular Fe₇₆Co₂₄ nanocomposite and the spherical Fe₅₀Co₅₀ microcomposite, the gyromagnetic spin resonance frequency of the acicular nanocomposite tends to locate higher than that of the spherical microcomposite owing to the demagnetizing field effect. Therefore, it can be concluded that the negative permeability frequency band of the acicular nanocomposite is higher than that of the spherical microcomposite at the same particle content. (paper)