Inelastic neutron scattering study of phonons and magnetic excitations in ${\mathrm{LaCoO}}_3$

We have investigated the phonon and the magnetic excitations in ${\mathrm{LaCoO}}_3$ by inelastic neutron scattering measurements. The acoustic phonon dispersions show some characteristic features of the folded Brillouin zone (BZ) for the rhombohedrally distorted perovskite structure containing two chemical formula units of ${\mathrm{LaCoO}}_3$ in the unit cell. We observed two transverse optical (TO) phonon branches along $\left(\delta \delta \delta \right)$, consistent with previously reported Raman active $E_g$ modes which show remarkable softening associated with the spin-state transition [Ishikawa et al., Phys. Rev. Lett. 93, 136401 (2004)]. We found that the softening takes place in the TO mode over the whole BZ. In contrast, the acoustic phonons show no anomalous softening associated with the spin-state transition. The low-energy paramagnetic scattering at 8 K is weak, increasing towards a maximum at $E\gtrsim 15\mathrm{meV}$, consistent with excitation of the nonmagnetic low-spin to magnetic intermediate-spin state of ${\mathrm{Co}}^{3+}$ ions.

Figure 1

The inelastic neutron scattering intensities per 1000 k monitor for ${\mathrm{LaCoO}}_3$ at $\mathit{Q}=(1-\delta ,1+\delta ,1+\delta )$ at $T=8$ and 200 K with $E_f=14.7\mathrm{meV}$, after being corrected by the temperature factor. The solid lines are fits to the data using a Gaussian line shape. The arrows indicate the peak position of the acoustic phonons. The dotted lines are the background level.

Figure 2

The inelastic neutron scattering intensity per 1200 k monitor at $\mathit{Q}=(3-\delta ,1+\delta ,1+\delta )$ at $T=8$ and 200 K with $E_f=30.5\mathrm{meV}$. The phonon scattering intensity is corrected by the temperature factor. The solid lines are fits to the data using a Gaussian line shape. The arrows indicate the peak positions of the optical phonons. The dotted lines are the background level.

Figure 3

Temperature dependence of the phonon energy at $\mathit{Q}=(2.50,1.50,1.50)$. The Raman data are from Ref. 18. The dashed lines are guides to eyes.

Figure 4

The phonon dispersion curves of ${\mathrm{LaCoO}}_3$ along (a) $\left(\delta \delta \delta \right)$, (b) $\left(0\delta \delta \right)$ and (c) $\left(\delta 00\right)$. The lines are guides to eyes. Inset in Fig. 4c shows the trajectories for the scans. The hexagon and the rectangle are the ${\left(01\overline{1}\right)}_c$ section of the Brillouin zones for the rhombohedral and the pseudocubic unit cells, respectively.

Figure 5

The energy spectra of the inelastic neutron scattering intensities at ${\mathit{Q}}_1=(1.04,0.04,0.04)$ (a) at 8 and (c) 120 K, and ${\mathit{Q}}_2=(2.96,0.04,0.04)$ (b) at 8 and (d) 120 K. The solid lines are the Bragg (or some incoherent) scattering tail. The dotted lines are the phonon component including the background intensity shown by the dash-dotted lines.

Figure 6

The magnetic scattering intensity at ${\mathit{Q}}_1=(1.04,0.04,0.04)$ at 8 K.

Figure 7

The magnetic scattering intensity obtained by $I\left(120\mathrm{K}\right)–I\left(8\mathrm{K}\right)$ measured at (a) ${\mathit{Q}}_1$ and (b) ${\mathit{Q}}_2$. Polarized neutron scattering data (Ref. 3) are also shown. The solid lines denote the resolution-convoluted scattering function (Ref. 3). The dotted line shows the phonon scattering intensity.