Dielectric loss enhancement induced by microstructure of CoFe₂O₄ foam for realizing broadband electromagnetic wave absorption

CoFe₂O₄ has been widely used for electromagnetic wave absorption by virtue of high Snoek limit, strong anisotropy and suitable saturation magnetization, yet the inherent shortcomings such as low dielectric loss, high density and magnetic agglomeration limit the pursuit for ideal absorbents. In this study, a microstructure regulation strategy is recommended to resolve the inherent disadvantages of pristine CoFe₂O₄ via a sol-gel auto-combustion method. A series of CoFe₂O₄ foams (S0.5, S1.0 and S1.5) constructed by 2D curved surfaces were obtained by changing the ratio of citric acid to Fe³⁺, in which the electromagnetic parameters were adjusted by morphology regulation. Thanks to the proper impedance matching and conductance loss provided by moderate complex permittivity, the effective absorption bandwidth (EAB) of S0.5 is as high as 7.3 GHz, exceeding most of CoFe₂O₄-based absorbents. Moreover, the EAB of S1.5 reaches 5.0 GHz (8.9-13.9 GHz) covering most of X band, which is ascribed to intense polarization proffered by lattice defects and heterogeneous interface. The 3D foam structure overcomes the high density and magnetic agglomeration issues of CoFe₂O₄ nanoparticles, and meanwhile, the good conductivity of 2D curved surfaces can effectively elevate the complex permittivity so as to ameliorate the dielectric loss of pure CoFe₂O₄. This study provides a fresh idea for the theoretical design and practical production of lightweight and broadband pure ferrite.