Emergent high-temperature insulating ferromagnetism in Sr₄Fe₅CoO_{13-< /sub> epitaxial thin films}

High-temperature ferromagnetic insulators play a crucial role in a wide range of emerging magnetoelectricity phenomena and hold the potential to become fundamental components of upcoming spintronic devices. However, the strong interaction between ferromagnetism and metallic properties presents a challenge, impeding the development of high-temperature ferromagnetic insulators based on oxides. Heterostructures or superlattice materials, especially those containing perovskite layers, offer a forward-looking solution. In this work, high-quality Sr4Fe(6)O(13) (SFO) and cobalt-doped SFO (Sr4Fe5CoO13-delta , SFCO) thin films were grown on Nb-SrTiO3(001) substrates using pulsed laser deposition technology. The grown SFO films exhibit paramagnetism, possibly due to the transition of their film structure from the orthorhombic to the tetragonal phase. In contrast to SFO films, high-quality SFCO thin films exhibit significant ferromagnetism at room temperature, with Curie transition temperature as high as 800 K. This phenomenon is mainly attributed to the formed Dzyaloshinskii-Moriya interactions between Fe-O-Co and increased lattice distortions caused by Co-doping. In contrast, the Curie transition temperature of the SFCO film is slightly higher than that of the SFCO ceramics. This enhancement is likely due to surface effects, where an increase in surface energy introduces additional energy barriers at the film surface and interface, thereby enhancing the thermal stability of the film. These characteristics advance the research of high-temperature magnetic insulators and broaden the operating temperature range of spintronic devices based on ferromagnetic insulators.