Ultrathin Magnetic–Conductive Heterostructures for Absorption‐Dominant Electromagnetic Interference Shielding

Ultrathin, lightweight, and mechanically compliant electromagnetic interference (EMI) shields are increasingly required for high‐frequency electronic devices. However, conventional metallic shields predominantly rely on conductivity‐driven reflection mechanisms, which become ineffective at reduced thickness and may cause severe secondary radiation. Ferrites provide high‐frequency magnetic losses that promote absorption; however, their low electrical conductivity often limits their absolute shielding effectiveness, thereby hindering their broader application in EMI shielding. Here, amorphous–nanocrystalline magnetic heterostructures were fabricated via magnetron sputtering, combining a soft magnetic CoFeB metallic glass layer with Ni–Zn ferrite in two configurations: co‐sputtered composite films (CSFs) and CoFeB/NiZnFeO multilayer films (MLFs). The CSFs exhibit a non‐monotonic shielding behavior with increasing ferrite content, revealing an optimal trade‐off among impedance matching, conductive reflection, and ferrite‐enabled magnetic dissipation. When deposited on flexible polyimide substrates, ferrite incorporation shifts the shielding mechanism toward absorption dominance (up to ∼72.5% for the MLFs and ∼90% for the CSFs) and promotes stable performance across the X and Ku bands. Due to their nanoscale thickness and low areal density, the CoFeB films and MLFs deliver high specific electromagnetic shielding efficiencies of 6.41 × 10 4 and 2.56 × 10 4 dB cm 2 g −1 , respectively. To further enhance the absolute shielding effectiveness without a large thickness increase, the MLFs were integrated with a conductive MXene film to form MLFs@MXene composites, achieving ∼60 dB shielding across the X and Ku bands (∼30 dB higher than the MXene film) at a total thickness of ∼21 μm. These results present a scalable co‐design strategy that integrates heterostructure engineering and substrate selection to synergistically exploit electric and magnetic losses for broadband, high‐frequency EMI shielding.