Room-temperature ferromagnetic insulating state in highly cation-ordered Sr₂Fe_1+xRe_1-xO₆ films

Ferromagnetic insulators with high Curie temperatures are critical components for developing quantum electronic/spintronic devices. However, since ferromagnetism generally accompanies metallicity, ferromagnetic insulators have been very limited in nature. Here, we report a highly insulating ferromagnetic state found in 3d-5d double perovskite Sr₂Fe_1+xRe_1-xO₆ (-0.2<x<0.2) epitaxial films by modifying the Fe/Re ratio. Beyond the original ferromagnetic metallic ground state in a stoichiometric film, Fe-rich films showed demanding ferromagnetic insulating states with three orders of higher room-temperature resistance than that of the metallic film, a high Curie temperature about 400 K, and a large saturated magnetization about 1.8 uB/f.u. In this presentation, we discuss about the growth, related physical properties, and the origin of the emerging ferromagnetic insulating state based on x-ray and optical spectroscopy.