Multi-scale magnetic coupling of Fe@SiO₂@C-Ni yolk@triple-shell microspheres for broadband microwave absorption

Magnetic core@shell and yolk@shell microspheres have received extensive attention; however, the realization of microwave performance enhancement remains a critical challenge for their actual applications. Herein, inspired by multi-scale magnetic coupling interactions, a synchronous in situ reduction process was developed to successfully fabricate Fe@SiO₂@C-Ni (FSCN) yolk@triple-shell microspheres based on the Fe₃O₄@SiO₂ substrate. Owing to the unique multi-scale magnetic coupling interactions in their delicate structure, (i) in each microsphere, abundant Ni NPs with optimized size could affect the density distribution and orientation of the magnetic stray field radiating from the Fe core, and (ii) via the coupling interactions between adjacent composite microspheres, the saturation magnetization was significantly enhanced to support strong magnetic loss capability. Moreover, the special yolk@multi-shell structure offered an optimized impedance balance, facilitating the propoagation of the incoming microwaves into the absorber. Both multiple interfacial polarization and synergistic effects from magnetic units (Fe and Ni) and dielectric shell (SiO₂ and carbon) contributed to electromagnetic wave attenuation. The FSCN composite material exhibited excellent absorption performance with an intense reflection loss (-45.5 dB) and bandwidth absorption (8.2 GHz and 9.8-18 GHz) at the film thickness of only 2 mm. Our new findings provide important design implications for functional spheres and high-performance lightweight microwave absorbers.