Alloy Element Adjustment and Heat Treatment Combination in Enhancing Electromagnetic Wave Absorption Properties of FeCoNiCu Medium Entropy Alloy

Fe1Co0.8Ni1Cu_x quaternary medium-entropy magnetic alloy particles with varied Cu element contents were prepared with liquid-phase reduction. Characterization of the microstructural features, static magnetic properties, and electromagnetic wave absorption behavior of these particles was carried out through SEM, VSM, and vector network analysis combined with heat treatment processes. The results demonstrate that the synthesized Fe1Co0.8Ni1Cux alloy particles exhibit a spherical morphology. Increasing Cu content leads to a gradual reduction in particle size, with average particle diameters decreasing to the range of 10–40 nm. Additionally, elevated annealing temperatures enhance the crystallinity of the alloy particles, causing particle aggregation and moderate growth. With the increase in Cu content, CoFe2O4 and CuFe2O4 phases are formed during annealing, resulting in a decrease in saturation magnetization intensity. Conversely, higher annealing temperatures promote crystallinity, thereby increasing saturation magnetization as well as residual magnetization and coercivity, exhibiting an initial rise followed by a decrease trend with increasing Cu element content. The real and imaginary components of the dielectric constant of Fe1Co0.8Ni1Cux alloy particles increase with Cu content, indicating improved dielectric loss capacity. However, the relationship between dielectric loss and Cu content is not linear, and post-heat treatment dielectric loss peaks are higher than in untreated samples. Multiple magnetic loss absorption peaks were observed in the imaginary component of permeability under different annealing conditions. Polarization relaxation is identified as the primary mechanism for dielectric loss, while eddy current loss dominates magnetic loss in some degree. The attenuation constant of heat-treated Fe1Co0.8Ni1Cux particles is higher than that of untreated particles, while the impedance matching value is lower. Under 600 °C annealing conditions, Fe1Co0.8Ni1Cu0.5 exhibits a minimum reflection loss (RLmin) of −40.26 dB, with a maximum absorption bandwidth of 4.72 GHz observed with coating thickness of 1.7 mm.