Negative permittivity behavior in Fe50Ni50/Al2O3 magnetic composite near percolation threshold

The physical properties of conductor-insulator composites often take dramatic changes near the percolation threshold (fc), leading to interesting applications, such as double negative material. This phenomenon also will be enlarged by increasing the differences between the properties of the constitutive phases. In this paper, the Fe50Ni50/Al2O3 magnetic cermets with different Fe50Ni50 volume contents (10%, 20%, 30%, 40%) were prepared via hot-pressing sintering. The permittivity of Fe50Ni50/Al2O3 composites has been studied in the radio frequency range. The results indicate that when the Fe50Ni50 content is below fc, the reactance of the composites takes a negative value, which indicates a capacitive character. The metallic Fe50Ni50 grains are isolated in the insulating ceramic matrix, leading to an insulator-like dielectric property. When the volume fraction of Fe50Ni50 reaches 40% which is above fc, the composites manifest an inductive character. In this case, the negative permittivity has been obtained over the whole test frequency range due to the percolation phenomenon, which makes Fe50Ni50/Al2O3 composites as promising candidates for double negative materials.