Supporting materials

Despite pivotal needs in magneto-optical and spintronic devices, the quest for high-temperature ferromagnetic insulators faces stubborn challenges. We report that a facile plasma-assisted hydrogenation technology enables consecutive topotactic transformations with distinct Fe-O polyhedra in BaFeO3-δ films, which exhibits desirable ferromagnetic insulating property with Tc above 400 K and a wide optical-range transparency. This work signifies the successful surmounting the inherent trade-off dilemma between ferromagnetism and electrical insulation, offering a new paradigm in exploration of multifunctional materials for magneto-optical and spintronic applications.