It is well accepted that the microwave absorption performance (MAP) of carbon nanotubes (CNTs) can be enhanced via coating magnetic nanoparticles on their surfaces. However, it is still unclear if the magnetic coating structure has a significant influence on the microwave absorption behavior. In this work, nano-Fe₃O₄ compact-coated CNTs (FCCs) and Fe₃O₄ loose-coated CNTs (FLCs) are prepared using a simple solvothermal method. The MAP of the Fe₃O₄-coated CNTs is shown to be adjustable via controlling the Fe₃O₄ nanocoating structure. The results reveal that the overall MAP of coated CNTs strongly depends on the magnetic coating structure. In addition, the FCCs show a much better MAP than the FLCs. It is shown that the microwave absorption difference between the FLCs and FCCs is due to the disparate complementarities between the dielectric loss and the magnetic loss, which are related to the coverage density of Fe₃O₄ nanoparticles on the surfaces of CNTs. For FCCs, the mass ratio of CNTs to Fe³⁺ is then optimized to maximize the effective complementarities between the dielectric loss and the magnetic loss. Finally, a comparison is made with the literature on Fe₃O₄-carbon-based composites. The FCCs at the optimized CNT to Fe³⁺ ratio in the present work show the most effective specific RL_min (28.7 dB·mm^-1) and the widest effective bandwidth (RL < -10 dB) (8.3 GHz). The excellent MAP of the as-prepared FCC sample is demonstrated to result from the consequent dielectric relaxation process and the improved magnetic loss. Consequently, the structure-property relationship revealed is significant for the design and preparation of CNT-based materials with effective microwave absorption.

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