<p>Fe-Si soft magnetic composites (SMCs) typically suffer from deteriorated permeability stability and increased eddy-current loss under high-frequency operating conditions, which limits their application in advanced power electronics. In this work, Fe−6.5 wt.% Si SMCs coated with Al₂O₃ insulating layers were fabricated by high-energy ball milling followed by spark plasma sintering (SPS). The influence of Al₂O₃ content on the microstructure, electrical resistivity, and magnetic properties was systematically investigated over a frequency range below 500&#xa0;kHz. The results show that the introduction of Al₂O₃ effectively enhances the electrical insulation between Fe-Si particles, leading to increased resistivity and reduced high-frequency core loss. Scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS) line-scan results indicate that, as the Al₂O₃ content increases from 2.5 wt.% to 7.5 wt.%, the coating layer on the surface of Fe-Si particles evolves from incomplete coverage to a more continuous Al₂O₃ insulating coverage. This structural evolution is accompanied by improved permeability stability and a substantial reduction in magnetic loss. Among the investigated samples, the SMC containing 7.5 wt.% Al₂O₃ achieves the most favorable balance between permeability stability and core-loss reduction. The improved magnetic properties are mainly attributed to the enhanced interparticle insulation and the suppression of eddy-current effects provided by the Al₂O₃ coating. These results provide useful insights into the optimization of Fe-Si soft magnetic composites for high-frequency applications.</p> Graphical Abstract <p></p>