Tri-layer Co@Co_xFe_{1− x} @Fe@Fe₃O₄thorny core–shell composite particles and their electromagnetic absorption properties

By the combination of chemical liquid-phase reduction, in situ self-oxidation and thermal treatment, tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny core–shell composite magnetic particles were synthesized, and their microstructure, static magnetic properties, and electromagnetic wave absorption performance were investigated. Results show that the tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles largely retained the morphology of thorny Co@Fe@Fe₃O₄ particles, but some particles are aggregated, resulting in an increase in the particle size. The tri-layer Co@Co_xF_1−x@Fe@Fe₃O₄ thorny particles exhibit a typical core–shell structure with an internal core and 200–300 nm thick external coating. The center of the particle contains Co, while the outer layer comprises Fe and O. With heat treatment, a discernible transition phase of CoFe and Co₇Fe₃ was formed at the Co/Fe interface. With the increase in self-oxidation temperature, the specific saturation magnetization of the Co@Co_xF_1−x@Fe@Fe₃O₄ particles exhibits a slight increase before a downward trend, while the coercivity decreases slightly and then increases. At an oxidation temperature of 70 °C, the tri-layer Co@Co_xFe_1−x@Fe@Fe₃O₄ thorny particle samples exhibit optimal absorption performance, with a minimum reflection loss of −24.42 dB at a coating thickness of 1.6 mm and a maximum effective absorption bandwidth of 4.80 GHz.