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 FeNi3-high-entropy oxide (HEO) biphasic composites on reduced graphene oxide under low oxygen partial pressure. By tuning thermodynamic driving forces and kinetic barriers, the FeNi3 metastable fraction is precisely regulated from 0% to 80.4%. Density functional theory shows that FeNi3 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.