Evolution of Magnetic Oxygen States in Sr-Doped ${\mathrm{LaCoO}}_3$

Magnetism in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ as a function of doping is investigated with x-ray absorption spectroscopy and x-ray magnetic circular dicrhoism at the O $K$ edge, and corresponding first principles electronic structure calculations. For small $x$, the spectra are consistent with the formation of ferromagnetic clusters occurring within a nonmagnetic insulating matrix. Sr-induced, magnetic O-hole states form just above $E_F$ and grow with increasing Sr doping. Density functional calculations for $x=0$ yield a nonmagnetic ground state with the observed rhombohedral distortion and indicates that doping introduces holes at the Fermi level in magnetic states with significant O $2p$ and Co $t_{2g}$ character for the undistorted pseudocubic structure. Supercell calculations show stronger magnetism on oxygen atoms having more Sr neighbors.

Figure 1

XAS at the O $K$ edge. The XAS data were normalized at 550 eV and self-absorption corrections were applied. The data agree well, except at the beginning of the edge (from 527–529 eV) where, with increasing $x$, a large increase in the XAS is observed, corresponding to the addition of empty states.

Figure 2

Difference in the XAS (top) and XMCD (bottom) at the O $K$ edge. A significant XMCD signal is only observed over this same small energy range (shaded region) where change in the XAS occurs and $\mathrm{XAS}(x)-\mathrm{XAS}(0)$ is positive. Above 529 eV, the differences become slightly negative.

Figure 3

Integrals of the difference in O XAS (Fig. 2) from 526–529 eV (magnetic holes) and from 529–532.5 eV (nonmagnetic holes) as a function of $x$; note that the change is not linear. For $x\le 0.15$, the new magnetic holes are associated with FM clusters; over the range 0.15–0.22 where the material becomes fully FM, higher energy holes with no orbital momentum are converted to lower energy magnetic states.

Figure 4

$2\times 2\times 2$ supercell for a Sr doping of $3/8$. Sr (large, light blue); La (large, dark green); Co (medium, blue). Four types of (small) O atoms with varying Sr neighbors: 0 (red, left slant); 1 (magenta, vertical); 2 (pink, horizontal); 3 (yellow, right slant).

Figure 5

Orbitally decomposed DOS for Co $3d$ ($t_{2g}$ and $e_g$) and O $2p$ states from LDA calculations. For each panel top is spin-up, bottom, spin-down. The shaded region for $E\ge E_F$ is expanded on right.

Figure 6

Theoretical O $K$ edge (top), showing a significant increase at the beginning of the edge, and calculated O $K$ edge XMCD signal (bottom).

Figure 7

Calculated hole fractions: magnetic O-hole fraction (red square), nonmagnetic O-hole fraction (green triangle), total O holes (blue circle), total Co holes (gray star).