Design of Broadband Microwave-absorbing Metamaterial Structures Based on Superparamagnetism

With the advancement of 5G communication technology, microwave transmission and reception devices have become ubiquitous in daily life, leading to serious issues of electromagnetic radiation pollution. Microwave absorbing materials, which can eliminate or attenuate the impact of incident electromagnetic waves, are regarded as a key technological approach to addressing this problem. Current research on microwave absorbers focuses primarily on developing dielectric materials with broadband absorption capabilities. Compared to conventional dielectric-type absorbers - limited by their unitary magnetic permeability - magnetic materials offer higher refractive indices and better impedance matching with free space. However, the inherent Snoek limit of magnetic materials indicates that their magnetic permeability inevitably decreases at high frequencies, which restricts their application in high-frequency regimes. In contrast, superparamagnetic materials, due to the formation of single-domain magnetic structures at the nanoscale, introduce an additional superparamagnetic relaxation mechanism dominated by magnetic moment reversal. This endows them with a unique broadband response, offering significant potential for the design of broadband absorbing metamaterials. This study employs superparamagnetic nanoparticles as the base medium and draws inspiration from the structural coloration mechanism in optics to design a broadband metamaterial absorber. In nature, the micro/nanostructures found on the surfaces of certain animals and plants can form impedance-matching elements, leading to macroscale anti-reflective structural colors. Building upon this theoretical foundation, superparamagnetic composite media were fabricated and their electromagnetic parameters were experimentally characterized. Based on the measured parameters, two subwavelength metamaterial architectures - honeycomb and cicada wing-inspired structures - were numerically simulated. The results demonstrate that the designed structures exhibit broadband microwave absorption characteristics.