Percolation of frustrated polarons in doped cobaltite La_1-xSr_xCoO_3-δ (LSCO)

Due to fascinating phenomena such as magneto-electronic phase separation and Co ion spin-state transitions, the archetypal cobaltite La_1-xSr_xCoO_3-δ (LSCO) remains of high interest. Replacing La by Sr in the diamagnetic and insulating parent compound LaCoO_3-δ introduces holes and magnetic moments. As Co undergoes spin-state transitions, 7-site spin polarons form at small x with a spin-per-Sr of S=12.5. Further doping leads to a glassy magnetic state and finally to a ferromagnetic (FM) metal at x=0.18. As simple statistical considerations predict a percolation of polarons already at x=0.05, the question arises what suppresses the formation of a macroscopic FM cluster.
We address this question within a microscopic model capturing both competing magnetic interactions between the Co moments in different spin states and spatial inhomogeneity introduced by disorder. Our Monte-Carlo simulations reveal that the origin of the delayed percolation transition lies in the frustration of ferromagnetic polarons via competing magnetic interactions. Our simulations explicitly show how frustrated polarons act as seeds of the observed magneto-electronic phase separated glassy state at intermediate doping 0.05 < x < 0.18, providing a consistent microscopic understanding across the full doping range.