Regulating Resonance‐Related Dynamic Magnetic Response through In Situ MnO Formation in FeCoNiCr 0.4 Mn 0.6 High‐Entropy Alloy for Low‐Frequency Microwave Absorption

Efficient microwave absorption in the S–C band requires the strength and frequency distribution of resonance‐related magnetic loss to be regulated while sufficient permeability is retained. The key is therefore to balance control of the resonance‐related dynamic magnetic response with preservation of the magnetic contribution from the ferromagnetic matrix. Here, FeCoNiCr 0.4 Mn 0.6 high‐entropy alloy (HEA) powders were subjected to wet milling and low‐temperature annealing to produce anisotropic flakes while enabling controlled in situ oxidation of the intrinsic Mn. The resulting MnO/HEA interfaces were accompanied by changes in the local strain state and magnetic anisotropy environment of the adjacent ferromagnetic matrix, providing a structural basis for the evolution of resonance linewidth and effective damping. At 300°C, limited MnO formation produced a moderate effective damping state while maintaining relatively high permeability and sufficient ferromagnetic‐phase contribution, enabling effective absorption over 3.2–4.9 GHz at a thickness of 3.5 mm. At 400°C, further MnO formation increased the fraction of the non‐ferromagnetic phase and weakened the microwave magnetic response. These results demonstrate that controlling the oxidation degree of intrinsic alloying elements can balance interfacial regulation, dynamic magnetic relaxation, and ferromagnetic‐phase retention, providing a feasible strategy for low‐frequency microwave absorption in corrosion‐resistant HEAs.