Ferrite/carbon composited materials,especially the bio-derived composited materials possessing both environmental friendliness and outstanding microwave absorption performance,attract numerous attentions for solving the“electromagnetic problem”in the Gigahertz frequency range.In this work,we demonstrate a bio-derived ferrite/carbon material by compositing functional carbonized cotton fibers(CCFs)and Fe3O4 nanoparticles with optimized microwave-absorption properties.By adjusting the carbonization conditions systematically,the Fe3O4 loading contents and the microwave absorption properties can be varied simultaneously-and,indeed,optimized and tuned.The CCFs-Fe3O4 composites exhibited a minimum reflection-loss capacity RL(dB)of-56.8 dB at 10.9 GHz with a thickness of 1.67 mm,and its effective absorption bandwidth(RL(dB)＜-20 dB)was found to broaden to 7.1 GHz.Electromagnetic characterizations,coupled with microstructure analyses,revealed that the enhancement in microwave absorption was triggered by the different microstructures of CCFs-Fe3O4 composites-attributable to the different carbonization processes.These different conditions result in different amounts of Fe3O4 attachment sites and lead to the enhancement of dielectric polarization at localized microstructures.The present work of bio-derived ferrite/carbon materials has important implications in understanding structure-performance relationships in dielectric-magnetic materials,and,meanwhile,could well be extended to a microwave-absorber design approach.