Programmable Metastable Engineering of FeNi 3 –High‐Entropy Oxide Heterostructures for Ultrawideband Electromagnetic Absorption

ABSTRACT Electromagnetic wave absorbing (EWA) materials remain limited by steady‐state thermodynamic design, restricting the use of non‐equilibrium thermodynamic–kinetic competition to tailor intrinsic electromagnetic properties. Here, we report a carbothermal‐reduction‐driven metastable engineering strategy for programmable FeNi 3 –high‐entropy oxide (HEO) biphasic composites on reduced graphene oxide under low oxygen partial pressure. By tuning thermodynamic driving forces and kinetic barriers, the FeNi 3 metastable fraction is precisely regulated from 0% to 80.4%. Density functional theory shows that FeNi 3 is kinetically favored due to lower migration barriers, while HEO is thermodynamically stabilized by lower Gibbs free energy, enabling controllable phase coexistence. Fe–N site modification further enhances interfacial polarization and magneto‐dielectric coupling, improving impedance matching and attenuation. The optimized composite achieves a minimum reflection loss of −50.6 dB in the Ku band at low filler loading. Integrated into flexible metamaterials, the effective absorption bandwidth expands to 35.9 GHz (4.1–40 GHz), with excellent mechanical flexibility and strong radar cross‐section reduction and radio‐frequency shielding. This work establishes a cross‐scale metastable engineering paradigm spanning carrier regulation, phase competition, and metamaterial design for high‐performance electromagnetic absorbers.