Orbital Engineering through Strain‐Induced Spin–Orbit–Lattice Coupling in Sr 2 FeReO 6 Double Perovskites

ABSTRACT Ferromagnetic 3 d ‐5 d double perovskites exhibit appealing physical properties due to their unique electronic band structures, which result from the hybridization of 3 d and 5 d orbitals. Sr 2 FeReO 6 (SFRO) exhibits half‐metallicity, a high Curie temperature (∼400 K), and spin–orbit–lattice coupling, making it a promising candidate for spintronic applications. Here, we demonstrate that epitaxial strain modulates the oxygen‐octahedral tilt configuration and the associated Re 5 d orbital polarization in SFRO thin films, leading to pronounced changes in magnetic anisotropy and electronic transport. Atomic‐resolution STEM analysis and first‐principles calculation reveal distinct octahedral configurations under compressive and tensile strain, accompanied by an orbital‐dependent reorganization of the Fe–O–Re hopping network. The resulting redistribution of Re t 2g states near the Fermi level (E F ), further supported by X‐ray linear dichroism, is associated with a reversal of the magnetic easy axis from in‐plane to out‐of‐plane. Under tensile strain, the reconstructed low‐energy electronic states are accompanied by suppressed optical conductivity and semiconducting‐like transport, in contrast to the metallic‐like behavior under compressive strain. These findings establish epitaxial strain as an effective means of controlling coupled lattice, orbital, magnetic, and electronic degrees of freedom in 3 d –5 d double perovskites.