Enhanced microwave absorption through effective coupling modulation of core-shell-like CoNi/Co@NC nano composite structures

By a combination of liquid-phase deposition, high-temperature pyrolysis, and chemical liquid-phase reduction, five sets of nano CoNi/Co@NC composite particles with varying molar ratios of Co to Ni were successfully synthesized. A systematic investigation was conducted on their microstructure, crystallographic properties, static magnetic characteristics, and electromagnetic wave absorption capabilities. The results indicate that all samples inherited the polyhedral morphology of the Co@NC precursor, with CoNi alloy particles deposited on the surface of Co@NC as a nucleation substrate. Diffraction peaks were observed at 2θ = 44.2°, 51.6°, and 76°, corresponding to the (111), (200), and (211) crystal facets of the face-centered cubic structures of Co, Co_0.5Ni_0.5, and Ni, respectively. The specific saturation magnetization (M_s) and remanent magnetization (M_r) of the samples initially decreased and then increased with decreasing Co:Ni ratios, with the CN-91 sample (Co:Ni = 1:9) exhibiting the highest M_s and M_r, measuring 84.13 emu/g and 7.61 emu/g, respectively. The coercivity (H_c) fluctuated across different compositions, with the CN-37 sample (Co:Ni = 3:7) displaying the highest H_c, indicating superior soft magnetic properties. The real part of the complex permittivity (ε′) of the samples declined overall with increasing frequency, while the imaginary part (ε'') remained relatively stable; the real part of the complex permeability (μ′) was stable across the 2-18 GHz range, and fluctuations in the imaginary part (μ'') were observed in the 14-18 GHz frequency band. All samples exhibited distinct polarization relaxation and conductivity losses, with the magnetic loss mechanisms including resonance loss and eddy current loss. By the modification with nano CoNi particles, the CN-91 sample (Co:Ni = 1:9) achieved an effective absorption bandwidth of 4.96 GHz (13.08-18 GHz) at a thickness of 1.3 mm, while the CN-37 sample (Co:Ni = 3:7) attained a minimum reflection loss of −60.583 dB at a thickness of 4.4 mm.