Spin-state-driven metal-insulator transition in $(\mathrm{La},\mathrm{Sr})\mathrm{Co}{\mathrm{O}}_3$ under high-pressure

Using Co $K\beta$ x-ray emission spectroscopy (XES) we have investigated the effect of pressure on the Co spin state of a single crystal of metallic, ferromagnetic ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_3$. The analysis of the XES spectra up to $14.4\mathrm{GPa}$ at 300 and $34\mathrm{K}$ clearly shows a gradual pressure-induced spin-state transition from a high- or intermediate-spin to a low-spin state. These results provide microscopic evidence that the recently observed pressure-induced metal-insulator transition in $(\mathrm{La},\mathrm{Sr})\mathrm{Co}{\mathrm{O}}_3$ is driven by a simultaneous spin-state transition. We further show that under high pressures the low-spin state is relatively stable against temperature, in contrast to that in the $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ case. This is attributed to an enhancement of the crystal field splitting at high pressures.

Figure 1

(Color online) Co $K\beta$ emission spectra at $34\mathrm{K}$ and different pressures up to $14.4\mathrm{GPa}$. The XES spectrum of $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ at $17\mathrm{K}$ (black solid line) is shown as a reference for the LS $(S=0)$ state. Inset shows a blowup of the satellite region. The intensity of the satellite structure is suppressed at $14.4\mathrm{GPa}$.

Figure 2

(Color online) Co $K\beta$ emission spectra at $300\mathrm{K}$ and different pressures up to $14.4\mathrm{GPa}$. Inset shows a blowup of the satellite region.

Figure 3

(Color online) Left scale: pressure dependence of the integrated absolute difference (IAD) in ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_3$ up to $14.4\mathrm{GPa}$ at 300 and $34\mathrm{K}$ as deduced from the analysis of the Co $K\beta$ emission spectra (see text). Dotted lines through the data points are guides to the eyes. Right scale: pressure dependence of the Co magnetic moment $\left({\mu }_{\mathrm{Co}}\right)$ as obtained from magnetization measurements on ${\mathrm{La}}_{0.82}{\mathrm{Sr}}_{0.18}\mathrm{Co}{\mathrm{O}}_3$ under pressure up to $1\mathrm{GPa}$. (Ref. 19)