Microwave absorption of FeCo-C core–shell nanoparticles with tunable thickness of C shells and the underlying mechanism

In order to address the key question of how the thickness of the C shell affects the microwave absorption properties of metal-C core–shell nanoparticles, FeCo-C core–shell nanoparticles, with identical metal cores but varied carbon shell thicknesses, were synthesized by simply annealing FeCo-C core–shell nanoparticles in air. The free electronic-polarization theory was employed, and thus the polarization storage dependence of the loss plot, i.e., ${\epsilon }_p^{\prime }-{\epsilon }_p^{"}$ was introduced to elucidate the underlying microwave absorption mechanism. It was found that permittivities, conduction and polarization losses degrade as the carbon shell becomes thinner. Meanwhile, the thickness of the C shells was well adjusted to tune the magnetic resonance frequencies and intensities of the nanoparticles. Notably, a thicker C shell enhances the complete polarization relaxation process and increases polarization loss. Due to optimal dielectric and magnetic properties, FeCo-C nanoparticles exhibit an optimal reflection loss value up to −72.2 dB and an effective absorption bandwidth of 7.6 GHz at 3.5 mm. These results indicate that the synthesized FeCo-C core–shell nanoparticle is a promising candidate for microwave absorption applications. Furthermore, the introduction of ${\epsilon }_p^{\prime }-{\epsilon }_p^{"}$ plot is expected to have a significant impact on the field of microwave absorption.