Spin-State Transition in ${\mathrm{LaCoO}}_3$: Direct Neutron Spectroscopic Evidence of Excited Magnetic States

A gradual spin-state transition occurs in ${\mathrm{LaCoO}}_3$ around $T\sim 80–120\mathrm{K}$, whose detailed nature remains controversial. We studied this transition by means of inelastic neutron scattering and found that with increasing temperature an excitation at $\sim 0.6\mathrm{meV}$ 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 $g\sim 3$, the second interpretation is definitely favored.

Figure 1

Temperature evolution of the INS profiles measured in ${\mathrm{LaCoO}}_3$. The solid circles correspond to the ${\mathrm{LaAlO}}_3$ nonmagnetic reference compound at $T=50\mathrm{K}$. The lines are the result of least-squares fits using Gaussian functions to describe the line shape of the transition. For clarity, an offset has been added to the various curves.

Figure 2

Observed (circles) and fitted (solid line) temperature dependence of the integral intensity of the INS peak at 0.6 meV. The resulting level scheme is shown in the inset.

Figure 3

The magnetic inelastic scattering at $T=50\mathrm{K}$ in 0 (circles) and 6 T (boxes) applied field.

Figure 4

Schematic diagram of the excitation spectrum of high-spin (left) and intermediate-spin (right) ${\mathrm{Co}}^{3+}$ as a function of trigonal distortion (see text). The $\tilde{J}=1$ state in the left panel is split by trigonal distortion into a lower singlet and an upper doublet. Similarly, the $\tilde{J}=2$ state in the right panel is split into a doublet, a singlet, and a doublet.