Synergistic magnetic-dielectric cooperation in nano FeCo-Co@NC heterostructures for attracting electromagnetic wave absorption

Nano FeCo/Co@NC composite particles with various Fe:Co molar ratios were synthesized with the combination of liquid-phase deposition, high-temperature pyrolysis, and liquid-phase reduction. The microstructure, static magnetic properties, and electromagnetic wave absorption characteristics of the composites were investigated. The results indicated that all samples retained the polyhedral architecture of Co@NC, with the FeCo alloy depositing on the surface of Co@NC as a nucleation core. As the Fe:Co ratio decreased, the agglomeration of particles was alleviated, resulting in improved dispersion. All samples exhibited typical “S”-shaped hysteresis loops and the saturation magnetization (Ms) initially increased and then decreased as the Fe:Co ratio was reduced, coercivity (Hc) decreased initially and then increased, with the Fe:Co = 5:5 sample exhibiting the lowest Hc. The electromagnetic parameters of the synthesized nano FeCo/Co@NC composite particles exhibited significant compositional dependency and frequency response characteristics. The real part of the complex permittivity (ε’) decreased overall with increasing frequency, the imaginary part (ε’’) remained relatively stable with frequency in samples FC-91 and FC-73, while it gradually decreased in samples with lower Fe:Co ratios, with the FC-37 sample displaying the highest imaginary permittivity. The real part of the complex permeability (μ’) significantly increased in high Fe content samples within the frequency range of 15–18 GHz, the imaginary part of the complex permeability (μ’’) demonstrated an upward trend at lower Fe ratios as the frequency increased. The dominant mechanisms for dielectric and magnetic losses exhibited significant differences among various compositions: high Fe ratio samples were predominantly characterized by polarization relaxation and magnetic resonance, while samples with moderate ratios were primarily governed by conduction and eddy current losses. Among all samples, the Fe:Co = 9:1 sample achieved an effective absorption bandwidth of 5.12 GHz at a thickness of 1.4 mm, with a minimum reflection loss of −16.999 dB at a thickness of 1.6 mm, showcasing optimal electromagnetic wave absorption performance.