Anisotropic Interfaces Support the Confined Growth of Magnetic Nanometer‐Sized Heterostructures for Electromagnetic Wave Absorption

The fabrication of nanometer‐sized magnetic heterostructures with controlled magnetic components and specific interfaces holds great significance in the field of electromagnetic (EM) wave absorption. However, the process of achieving these structures still poses significant challenges. Here, abundant magnetic heterostructures are successfully fabricated by utilizing the surface energy anisotropy differences of the nonasymmetric hammer‐shaped interface to support the nucleation and growth of magnetic heterostructure components while effectively inhibiting their aggregation. Through a confined growth strategy via in situ growth of FeOOH and sequentially precise thermal treatments, dispersion of the heterostructures is achieved at the nanometer scale, while also observing a high degree of chemical stability due to occurrence of a charge‐compensation effect at the interface. Consequently, a series of magnetic heterostructures are obtained via phase translations of FeOOH precursors. The nanometer‐sized heterostructures demonstrate multilevel interfacial polarization effects. Furthermore, the hierarchical core–shell structure of the heterostructures promotes anisotropic polarization absorption. As a result, the multiple interfaces and nanometer‐sized Fe/Fe 3 O 4 @SiO 2 @Fe‐2 heterostructures demonstrate improved EM wave attenuation performance. Remarkably, they achieve an absorption bandwidth of 9 GHz at a thickness of 1.8 mm. A novel avenue is introduced here for investigating the intricate relationship between structure and properties in magnetic heterostructures.