Engineering Magnetic Response via Multilevel Architectures With Ultrathin Microwave Absorption and Lower Heat Generation

Electromagnetic absorber patches with limited thickness are highly desirable for wearable and flexible electronics, yet strong absorption in thin layers is commonly achieved by increasing dielectric or magnetic loss, which often causes excessive heat generation under GHz‐frequency excitation. A key challenge therefore lies in enhancing the real part of permeability (µ′) without a concomitant increase in the imaginary part (µ″). Here, we report a magnetic‐field‐induced hierarchical assembly strategy for constructing Co‐based multilevel one‐dimensional architectures constructed by cobalt@carbon core–shell (Co@C) nanorods as building blocks. Unlike conventional single‐scale assemblies, the resulting multilevel structure helps decouple permeability enhancement from magnetic loss across different length scales. The one‐dimensional organization amplifies shape anisotropy, resulting in a desirable high‐µ′/low‐µ″ magnetic characteristic, with µ′ increased by 1.36‐fold while µ″ remains nearly unchanged. Together with interfacial‐polarization‐dominated attenuation, this architecture delivers a minimum reflection loss of −33.9 dB at an ultrathin thickness of 1 mm. Meanwhile, the nanoscale Co@C units suppress eddy‐current effects, and the reduced magnetic loss further mitigates electromagnetic‐to‐thermal conversion and temperature rise during microwave exposure. This work provides a practical structural design strategy for microwave absorbers in thickness‐constrained and thermally sensitive applications.