Mixed spin states for robust ferromagnetism in strained SrCoO$_3$ thin films

TL;DR

This study uses advanced x-ray scattering to reveal a mixed high- and low-spin state in strained SrCoO$_3$ thin films, explaining their persistent ferromagnetism despite a metal-insulator transition.

Contribution

It identifies a novel mixed spin state in SrCoO$_3$ thin films, challenging previous models and linking it to robust ferromagnetism under strain.

Findings

Identified a mixed high- and low-spin ground state.

Linked the spin state to persistent ferromagnetism.

Provided experimental evidence for a nontrivial magnetic ground state.

Abstract

Epitaxial strain in transition-metal oxides can induce dramatic changes in electronic and magnetic properties. A recent study on the 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. These findings provide direct experimental evidence for a nontrivial magnetic ground state in SrCoO$_3$ and offer new pathways for designing robust ferromagnetic systems in correlated oxides.