Title: A novel and facile route for the in situ formation of composites with dual coupling interactions for considerable millimeter wave absorption performance

Abstract: A hard/soft magnetic exchange coupling or ferroelectric–ferromagnetic coupling has been proved to be beneficial for improving microwave absorption properties. Herein, we propose a BaZr_xFe_12−xO₁₉/Fe₃O₄/BaZrO₃ composite to integrate the advantages of the two couplings. However, the synthesis of BaFe₁₂O₁₉/Fe₃O₄ in situ still remains a challenge due to the different degrees of oxidation resistance. In this work, by delicately exploiting the reducibility of graphene at high temperatures, the BaZr_xFe_12−xO₁₉/Fe₃O₄/BaZrO₃ composite was successfully prepared in situ by a hydrothermal process combined with a subsequent heat treatment at 600 °C. The two integrable couplings provide good impedance matching and attenuation characteristics for the material system, making it reach a considerable millimeter wave absorption performance around an atmospheric window of 35 GHz with a broad bandwidth of >7.36 GHz (32.64–40.00+ GHz) and a strong RL of −25.4 dB under an extremely thin thickness of 1.00 mm. The strategy provides a novel and facile pathway to design outstanding millimeter wave absorbers.