Symmetry-mismatch-induced ferromagnetism in the interfacial layers of CaRuO${}_3$/SrTiO${}_3$ superlattices

By modifying the entangled multi-degrees of freedom of transition-metal oxides, interlayer coupling usually produces interfacial phases with unusual functionalities. Herein, a symmetry-mismatch-driven interfacial phase transition from paramagnetic to ferromagnetic state is reported. By constructing superlattices using CaRuO${}_3$ and SrTiO${}_3$, two oxides with different oxygen octahedron networks, the tilting/rotation of oxygen octahedra near interface is tuned dramatically, causing an angle increase from ≈150° to ≈165° for the Ru-O-Ru bond. This in turn drives the interfacial layer of CaRuO${}_3$, ≈3 unit cells in thickness, from paramagnetic into ferromagnetic state. The ferromagnetic order is robust, showing the highest Curie temperature of ≈120 K and the largest saturation magnetization of ≈0.7 µ${}_B$ per formula unit. Density functional theory calculations show that the reduced tilting/rotation of RuO${}_6$ octahedra favors an itinerant ferromagnetic ground state. This work demonstrates an effective phase tuning by coupled octahedral rotations, offering a new approach to explore emergent materials with desired functionalities. (© 2023 Wiley‐VCH GmbH)