[en] Highlights: • The FeCo/CoFe₂O₄ composites were prepared by controlled surface oxidation. • CoFe₂O₄ layers with nanoscale thickness were coated on the surface of the FeCo particles. • A high density of 7.45 g/cm³ was obtained in the compressed composites. • High dc and ac soft magnetic properties were achieved in these composites. FeCo/CoFe₂O₄ micron-nano composites have been fabricated by controllably oxidizing micron-sized FeCo particles. High-purity 60-nm-thick CoFe₂O₄ layers can be formed in situ on the surface of the unoxidized FeCo particles to form a dense FeCo/CoFe₂O₄ core-shell structure. The good intrinsic magnetic properties, including a high saturation magnetization of 210 A·m²·kg^{− 1} and a low coercivity of 11.7 Oe, are obtained in these FeCo/CoFe₂O₄ composites due to the nanoscale thickness of the ferrimagnetic CoFe₂O₄ layers. For traditional soft magnetic composites, low core loss can only be obtained by seriously sacrificing their magnetic induction. In this work, balanced soft magnetic properties with low core loss and high magnetic induction are obtained in the fully compacted FeCo/CoFe₂O₄ composites, which promise great potential applications in high-power and high-frequency magnetic devices. This controlled surface oxidation may also provide an effective surface treatment for metal-based soft magnets to improve their overall performance.