The　surface　modification　of　three-dimensional　(3D)　materials　is　an　efficient　method　for　adjusting　their　interfacial　defect　concentration,　electronic　conductivity　and　content　of　functional　groups　with　extensive　applications　in　catalysis,　electrode　materials　and　bioengineering.　In　this　work,　a　multiphase　iron　nanocrystals　consisting　of　Fe3C,　Fe　and　FeN　nanoparticles　encapsulated　in　hierarchical　structure　of　graphite　carbon　(denoted　as　Fe/Fe3C/FeN@GC)　is　synthesized　for　the　first　time　by　a　novel　high　temperature　plasma　method.　Meanwhile,　more　defects　and　functional　groups　are　introduced　by　surface　modification　of　graphite　carbon　layer　of　Fe/Fe3C/FeN@GC　with　controllable　CO2　(low　temperature)　plasma.　Benefiting　from　the　advantages　of　multiple　heterogenous　interface　and　the　abundant　interfacial　polarization　relaxation　that　represent　strong　electromagnetic　(EM)　wave　dissipation　as　well　as　an　applicable　impedance　matching,　the　optimized　Fe/Fe3C/FeN@GC　demonstrate　superior　microwave　absorption　(MA)　properties.　The　minimum　reflection　loss　(RL)　achieves　−54.4　dB　(more　than　99.9%　MA)　at　17.6　GHz　with　a　thin　thickness　of　1.8　mm,　and　the　maximum　effective　absorption　bandwidth　(EAB,　RL　3C/FeN@GC　composites　with　strong　absorption,　broad　EAB,　light　mass　(only　filling　content　of　30　wt%)　and　ultrathin　thickness　are　prospective　candidate　for　high　performance　EM　wave　absorbers.　©　2024　Elsevier　Ltd