Finding a hidden state opens a path to dissipationless quantum electronics

Researchers discovered that a hidden ferromagnetic insulating (FMI) state emerged in Sr₂Fe_1+xRe_1-xO₆ films when the material’s local ionic order was disturbed by a small modification in cation ratio.¹ This work provides a promising new route to developing oxide quantum devices, as the high Curie temperature (T_c) FMI state is rare in nature but critical for realizing quantum electronic and computing devices that may operate at room temperature without dissipating energy.

The investigators used pulsed laser epitaxy to grow highly cation-ordered Sr₂Fe_1+xRe_1-xO₆ thin films and further control the ratio between iron (Fe) and rhenium (Re) ions. Compared to the ferromagnetic metallic ground state found in stoichiometric Sr₂FeReO₆, the change in 3d and 5d transition metal ratio created by Fe surplus and Re deficit in Sr₂Fe_1+xRe_1-xO₆ was found to play an important role in inducing the novel FMI state with a high T_c of approximately 400 K. Soft X-ray and optical spectroscopies as well as scanning transmission electron microscopy were employed to understand the metal-insulator transition (MIT) with robust ferromagnetism. Excess Fe³⁺ ions and emergent Re⁶⁺ ions contributed to the MIT. This work demonstrates the versatile functionality of 3d-5d double perovskites, in which strong electron correlation from 3d orbitals and large spin-orbit coupling from 5d orbitals cooperate to change local magnetic state, opening the door to developing novel oxide materials for quantum technologies.