Spin-state transitions and magnetic polaron in lightly doped La$_{1-x}$Sr$_{x}$CoO$_{3}$.

Using the inelastic neutron scattering (INS) technique, we identified the energy levels of the thermally excited states of Co$^{3+}$ ions in both LaCoO$_{3}$ and La$_{0.998}$Sr$_{0.002}$CoO$_{3}$. In LaCoO$_{3}$ an excitation at \textit{$\sim $}0$.$6 meV appears at T$>$30K, whose intensity follows the bulk magnetization. Within a model including crystal field interaction and spin-orbit coupling we interpret this excitation as originating from a transition between thermally excited states located about 120 K above the ground state. Since the $g$-factor obtained from the field dependence of the INS is \textit{g$\sim $ }3, we interpret this state as a high-spin state of Co$^{3+ }$. The lightly doped material \textit{x$\sim $}0$:$002 exhibits paramagnetic properties at low temperatures. An INS peak at energy transfer \textit{$\sim $}0$.$75 meV was observed in it already at $T $= 1$:$5 K. We propose that the holes introduced in the LS state of LaCoO$_{3}$ by doping are extended over the neighboring Co sites, forming thus magnetic polaron and transforming all the involved Co ions (e.g. 6 of them) to the high-spin state. Similarly to LaCoO$_{3}$, we interpret the INS transition at 0.75 meV as that on these high-spin Co$^{3+}$ ions.