Short-range magentic correlations and dynamic orbital ordering in the thermally activated spin state of LaCoO$_3$

The cobalt perovskites La$_{1-x}$Sr$_x$CoO$_3$ show intriguing spin, lattice, and orbital properties similar to the ones observed in colossal magnetoresistive manganites. The x=0 parent compound is a non-magnetic insulator at low temperatures, but shows evidence of a spin-state transition of the cobalt ions above 50K from a low-spin to an intermediate or high-spin configuration. Using high resolution, inelastic neutron scattering, we observe a distinct low energy excitation at 0.6meV coincident with the thermally induced spin state transition observed in susceptibility measurements. The thermal activation of this excited spin state also leads to short-range, dynamic ferro- and antiferromagnetic correlations. These observations are consistent with the activation of a zero-field split intermediate spin state as well as the presence of dynamic orbital ordering of these excited states. \\ \\ Work supported by US DOE BES-DMS W-31-109-ENG-38 and NSF DMR-0454672