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The electromagnetic properties of materials are generally frequency-dependent. Controlling the permittivities and permeabilities of composites is commonly achieved in a narrow frequency range. This work characterizes the electromagnetic properties of epoxy/silver nanocomposites using the transmission/reflection method. The silver nanoparticles serving as electric dipoles enhance the permittivity as the volume fraction increases. On the other hand, the rapidly changing magnetic field induces current on the conducting silver particles, resulting in a negative polarizability. The macroscopic magnetic susceptivities are negative under the long-wavelength limit. The electromagnetic properties are controllable and just slightly change over a very broad frequency range from 8 GHz to 110 GHz.

The epoxy/magnetite nanocomposites express superparamagnetism under a static or low-frequency electromagnetic field. At the microwave frequency, said the X-band, the nanocomposites reveal an unexpected diamagnetism. To explain the intriguing phenomenon, we revisit the Debye relaxation law with the memory effect. The magnetization vector of the magnetite is unable to synchronize with the rapidly changing magnetic field, and it contributes to diamagnetism, a negative magnetic susceptibility for nanoparticles. The model just developed and the fitting result can not only be used to explain the experimental data in the X-band but also can be used to estimate the transition frequency between paramagnetism and diamagnetism.

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