Room-Temperature Ferromagnetic Insulating State in Cation-Ordered Double-Perovskite Sr₂ Fe_{1+ x} Re_{1- x} O₆ Films

Ferromagnetic insulators (FMIs) are one of the most important components in developing dissipationless electronic and spintronic devices. However, FMIs are innately rare to find in nature as ferromagnetism generally accompanies metallicity. Here, novel room-temperature FMI films that are epitaxially synthesized by deliberate control of the ratio between two B-site cations in the double perovskite Sr₂ Fe_{1+ x} Re_{1- x} O₆ (-0.2 ≤ x ≤ 0.2) are reported. In contrast to the known FM metallic phase in stoichiometric Sr₂ FeReO₆ , an FMI state with a high Curie temperature (T_c ≈ 400 K) and a large saturation magnetization (M_S ≈ 1.8 µ_B f.u.^-1 ) is found in highly cation-ordered Fe-rich phases. The stabilization of the FMI state is attributed to the formation of extra Fe³⁺ Fe³⁺ and Fe³⁺ Re⁶⁺ bonding states, which originate from the relatively excess Fe ions owing to the deficiency in Re ions. The emerging FMI state created by controlling cations in the oxide double perovskites opens the door to developing novel oxide quantum materials and spintronic devices.