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 0.64 Ni 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.