Effect of carrier doping on the formation and collapse of magnetic polarons in lightly hole-doped La 1-x Sr x CoO 3

We investigate the doping dependence of the nanoscale electronic and magnetic inhomogeneities in the hole-doping range $0.002\ensuremath{\leqslant}x\ensuremath{\leqslant}0.1$ of cobalt based perovskites, La${}_{1\ensuremath{-}x}$Sr${}_{x}$CoO${}_{3}$. Using single-crystal inelastic neutron scattering and magnetization measurements we show that the lightly doped system exhibits magnetoelectronic phase separation in the form of spin-state polarons. Higher hole doping leads to a decay of spin-state polarons in favor of larger scale magnetic clusters, due to competing ferromagnetic correlations of Co${}^{3+}$ ions which are formed by neighboring polarons. The present data give evidence for two regimes of magnetoelectronic phase separation in this system: (i) $x\ensuremath{\lesssim}0.05$, dominated by ferromagnetic intrapolaron interactions, and (ii) $x\ensuremath{\gtrsim}0.05$, dominated by Co${}^{3+}$-Co${}^{3+}$ intracluster interactions. Our conclusions are in good agreement with a recently proposed model of the phase separation in cobalt perovskites [C. He et al., Europhys. Lett. 87, 27006 (2009)].