Room-Temperature Perovskite Ferromagnetic Insulator via Three-Dimensional Tensile Strain.

Ferromagnetic insulators are receiving ever-increasing research activities driven not only by the unique advantage of low power loss during spin-wave-based information processing but also by the potential to construct next-generation spintronic devices. However, either the exceedingly rare candidates or the low Curie temperature far below room temperature greatly hinder their practical application. Here, through the modulation of a novel three-dimensional (3D) tensile strain, a room-temperature ferromagnetic insulating state with a Curie temperature as high as 594 K is achieved in self-assembled LaCoO_{3}:MgO nanocomposite thin films. Atomically resolved electron microscopy quantifications identify the 3D strain state of the thin film, where the +2.6% out-of-plane and +2.1% in-plane tensile strains are attributed to the interphase mismatch between the LaCoO_{3} and MgO building blocks and epitaxial constraint, respectively. Combined with the assessment of electronic states and theoretical analysis, we correlate the strain state with the resulting ferromagnetic insulating property and clarify the underlying mechanisms, by which the emergent strain states break the degeneracy of crystal-field splitting and tailor the on-site Coulomb interactions and spin configuration. These findings underscore the efficacy of a three-dimensional strain strategy in engineering the long-desired high-temperature ferromagnetic insulators via the manipulation of strong spin-lattice coupling, providing a promising approach for the exploitation of exotic functionalities in correlated oxides.