Enhanced MHz Magnetic Properties of Easy‐Plane FeSi _3.5 Soft Magnetic Composites via Acid Etching‐Induced Dual Coating

Abstract Easy‐plane soft magnetic composites (SMCs) have been proven to be a type of soft magnetic material that is highly likely to be matched with the high‐frequency operating conditions of third‐generation semiconductors. In this study, FeSi 3.5 easy‐plane powders are surface‐modified with phosphoric acid and subsequently subjected to annealing. This process successfully synthesizes easy‐plane FeSi 3.5 powders featuring a dual‐layer coating. Furthermore, the formation of the Fe 2 (SiO 3 ) 3 layer not only suppresses excessive corrosion of FeSi 3.5 by phosphoric acid but also reduces lattice mismatch between the substrate and the outer coating layer. Based on Bertotti's classical theory, the results indicate that this process provides excellent insulation while facilitating the release of internal strain and lowering defect concentration resulting in a significant reduction in hysteresis loss and eddy current loss. Hysteresis loss and eddy current loss decrease from 3404.1 to 2362 kW m −3 and from 414.1 to 89.4 kW m − 3 at 20 mT, 5 MHz, respectively. This study develops a novel method for soft magnetic material design by acid etching and annealing process to construct a dual‐coating, thereby further optimizing the comprehensive magnetic properties of FeSi 3.5 easy‐plane SMCs.