From Magnetic Fibers to Paper Architectures: A Lightweight Strategy for Robust Low-Frequency Microwave Absorption

Efficient low-frequency microwave absorption is a long-standing challenge: carbonaceous absorbers demand bulky, centimeter-scale thicknesses, while conventional magnetic absorbers rely on high loadings of dense magnetic filler- making both impractical for compact, weight-sensitive systems. Here, we integrate the magnetic material directly into the structural fiber rather than dispersing it as filler: each filament is metallized with a dense, micrometer-thick Fe_0.64Ni_0.36 (Invar-type) alloy coating through a scalable, continuous process, and the functionalized fibers are assembled directly into paper by wet-laid papermaking. The resulting fibers form a hierarchical conductive network that couples strong dielectric dissipation with the permeability needed to relieve the impedance-mismatch bottleneck of low-frequency absorbers, while preserving the mechanical properties of neat aramid paper. The paper with the optimized composition achieves effective S-band absorption at a magnetic loading of only 21.3 wt.%. Free-space reflectance measurements confirm a reflection loss of -21.8 dB and an effective absorption bandwidth of 1.0 GHz at a thickness of 3.3 mm. By uniting scalable fiber metallization with established papermaking, this work opens a practical route to thin, lightweight, low-frequency absorbers and, more broadly, to multifunctional fiber-based materials.