Strategy of adjusting negative permittivity with invariant permeability property in metallic granular percolating composites

The electromagnetic properties including ac conductivity, reactance, permittivity, and permeability of percolating Fe/Epoxy composites are investigated at radio-frequency range. Percolating behavior is observed in the composites. Below percolation threshold, ac conductivity spectra follows the Jonscher's power law indicating the weakened trend of hopping conductive behavior, while the skin effect is dominant above percolation threshold. Plasma-type negative permittivity is attributed to the low frequency plasmonic state explained by Drude model. The frequency region and value of negative permittivity are effectively adjusted by SiO2-coated iron particles' controlling percolating network, while permeability property could be almost kept invariant. Invariant permeability property is attributed to suppressing current loops by SiO2 layers. This strategy with tunable permittivity and invariant permeability provides a method of suppressing the strong electromagnetic coupling effect in intrinsic metamaterials, and can facilitate applications of negative permittivity materials.