Spin-state transition in LaCoO3:: Direct neutron spectroscopic evidence of excited magnetic states

A gradual spin-state transition occurs in LaCoO3 around T similar to 80-120 K, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at similar to 0.6 meV appears, whose intensity increases with temperature, following 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. We further discuss the nature of the magnetic excited state in terms of intermediate-spin (t(2g)(5)e(g)(1), S=1) versus high-spin (t(2g)(4)e(g)(2), S=2) states. Since the g factor obtained from the field dependence of the inelastic neutron scattering is g similar to 3, the second interpretation is definitely favored.