Orbital-ordered ferromagnetic insulating state in tensile-strained SrCoO${}_3$ thin films

At ambient pressure, bulk SrCoO${}_3$ is a ferromagnetic (FM) metal in cubic perovskite structure. By contrast, magnetic properties of epitaxial SrCoO${}_3$ thin films, especially at high tensile strain ($\epsilon \gtrsim 3$%), remain unclear: Previous calculations had predicted antiferromagnetic (AFM) states more energetically favorable in this regime, but recent experiments indicated a FM insulating state. In this work, using first-principles calculations, we perform an extensive search for the structural, spin, magnetic, and orbital states of SrCoO${}_3$ thin films. Our calculations indicate that at $0<\epsilon \lesssim 2.5$%, SrCoO${}_3$ favors a FM half-metallic state with intermediate-spin ($t_{2g}^5{e}_g^1$-like) Co exhibiting $d^6\underset{‾}{L}$ character. At $\epsilon \gtrsim 2.5$%, a FM insulating state with high-spin ($t_{2g}^4{e}_g^2$-like) Co dominates. This FM insulating state is achieved via complicated orbital ordering, cooperative Jahn–Teller distortion, and octahedral tilting about all three crystal axes.