Above 400 K robust ferromagnetic insulating phase in hydrogenated brownmillerite iron oxide films with distinct coordinate

Ferromagnetic insulators (FMIs) with excellent optical transparency are highly appealing materials for advanced magneto-optical and spintronic devices. However, their applications have been substantially hindered for decades due to the limited availability of FMIs with low Curie temperature T_c and frustrated optical transparency. Herein, we reported that hydrogenated BaFeO_2.5 films via facile and effective hydrogen plasma treatment exhibit consecutive structural transformations, accompanying with robust ferromagnetic insulating states with T_c > 400 K and desirable optical transparency with spectral range from visible to infrared. We elucidate the effect of reconfigurations of Fe-O coordinate geometry with distinct crystal structures on the emergent electronic properties of hydrogenated BaFeO_2.5 films by combining experimental measurements and theoretical calculations. These findings underscore the importance of engineering polyhedral coordinate of perovskite-derived oxides in surmounting the inherent trade-off between ferromagnetism and electric insulation and open up opportunities for manipulating multifunctional electronic materials.