Co₇Fe₃ and Co₇Fe₃@SiO₂ Nanospheres with Tunable Diameters for High-Performance Electromagnetic Wave Absorption

Ferromagnetic metal/alloy nanoparticles have attracted extensive interest for electromagnetic wave-absorbing applications. However, ferromagnetic nanoparticles are prone to oxidization and producing eddy currents, leading to the deterioration of electromagnetic properties. In this work, a simple and scalable liquid-phase reduction method was employed to synthesize uniform Co₇Fe₃ nanospheres with diameters ranging from 350 to 650 nm for high-performance microwave absorption application. Co₇Fe₃@SiO₂ core-shell nanospheres with SiO₂ shell thicknesses of 30 nm were then fabricated via a modified Stöber method. When tested as microwave absorbers, bare Co₇Fe₃ nanospheres with a diameter of 350 nm have a maximum reflection loss (RL) of 78.4 dB and an effective absorption with RL > 10 dB from 10 to 16.7 GHz at a small thickness of 1.59 mm. Co₇Fe₃@SiO₂ nanospheres showed a significantly enhanced microwave absorption capability for an effective absorption bandwidth and a shift toward a lower frequency, which is ascribed to the protection of the SiO₂ shell from direct contact among Co₇Fe₃ nanospheres, as well as improved crystallinity and decreased defects upon annealing. This work illustrates a simple and effective method to fabricate Co₇Fe₃ and Co₇Fe₃@SiO₂ nanospheres as promising microwave absorbers, and the design concept can also be extended to other ferromagnetic alloy particles.