  Ferromagnetic insulators (FMIs) are one of the most important components in
developing dissipationless electronic and spintronic devices. However, since
ferromagnetism generally accompanies metallicity, FMIs are innately rare to
find in nature. Here, novel room-temperature FMI films are epitaxially
synthesized by deliberate control of the ratio of two B-site cations in the
double perovskite Sr2FeReO6. In contrast to the known ferromagnetic metallic
phase in stoichiometric Sr2FeReO6, a FMI state with a high Curie temperature
(Tc~400 K) and a large saturation magnetization (MS~1.8 {\mu}B/f.u.) 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 excess Fe. The emerging FMI state by controlling
cations in the epitaxial oxide perovskites opens the door to developing novel
oxide quantum materials & heterostructures.
