Programmable Metastable Engineering of FeNi3–High-Entropy Oxide Heterostructures for Ultrawideband Electromagnetic Absorption

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.