Essential role of magnetic frustration in the phase diagrams of doped cobaltites

Doped perovskite cobaltites (e.g., ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_x{\mathrm{CoO}}₃$) have been extensively studied for their spin-state physics, electronic inhomogeneity, and insulator-metal transitions. Ferromagnetically interacting spin-state polarons emerge at low $x$ in the phase diagram of these compounds, eventually yielding long-range ferromagnetism. The onset of long-range ferromagnetism ($x\ensuremath{\approx}0.18$) is substantially delayed relative to polaron percolation ($x\ensuremath{\approx}0.05$), however, generating a troubling inconsistency. Here, Monte Carlo simulations of a disordered classical spin model are used to establish that previously ignored magnetic frustration is responsible for this effect, enabling faithful reproduction of the magnetic phase diagram.