Mixed spin states for strain-robust ferromagnetism in SrCoO₃ thin films

Epitaxial strain in transition-metal oxides can induce dramatic changes in electronic and magnetic properties. A recent study on epitaxially strained SrCoO_3 thin films revealed persistent ferromagnetism even across a metal-insulator transition. This challenges the current theoretical predictions, and the nature of the local spin state underlying this robustness remains unresolved. Here, we employ high-resolution resonant inelastic x-ray scattering (RIXS) at the Co-L_3 edge to probe the spin states of strained SrCoO_3 thin films. Compared with CoO_6 cluster multiplet calculations, we identify a ground state composed of a mixed high- and low-spin configuration, distinct from the previously proposed intermediate-spin state. Our results demonstrate that the robustness of ferromagnetism arises from the interplay between this mixed spin state and the presence of ligand holes associated with negative charge transfer. Our results establish a nontrivial magnetic ground state and offer new pathways for designing robust ferromagnetic phases in correlated oxides.