The Understanding of Size‐Dependent Magnetic Loss Mechanisms Based on Surface Atoms Moment to Tune Electromagnetic Wave Response

ABSTRACT Magnetic loss is a key mechanism in electromagnetic wave (EMW) absorption, but it is still a challenge to understand the relationships between the size effect and magnetic loss ability. Here, we synthesized Fe ₃ O ₄ @CeO ₂ core‐shell composites to reveal the above relationship. Our synthesis started with the preparation of Fe ₂ O ₃ nanocubes, followed by coating CeO ₂ shell by hydrothermal technology. After undergoing H ₂ reductive annealing, Fe ₃ O ₄ @CeO ₂ core‐shell nanocubes were obtained with their magnetic core size fixed to 155 ± 10 nm, 305 ± 10 nm, 510 ± 15 nm, and 705 ± 15 nm, respectively, where CeO ₂ shell can stabilize the magnetic cores against agglomeration and adjust the dielectric feature. Within 2–6 GHz, the smallest‐sized Fe ₃ O ₄ @CeO ₂ represents the highest magnetic loss ability by the natural resonance for the largest number of surface atoms. Within 6–14 GHz, the exchange resonance from the Fe ²⁺ ‐O‐Fe ²⁺ super‐exchange interactions is suppressed by the increased oxygen vacancies and super‐exchange distances for the small‐sized particles. As a result, the largest‐sized Fe ₃ O ₄ @CeO ₂ achieved an effective absorption bandwidth of 5.7 GHz (8.1–13.8 GHz) for the medium‐frequency application. Our work clarifies the key role of surface atom moment in magnetic loss, offering guidance for understanding the relationship among size‐mechanism‐performance.