Local atomic and electronic structures of epitaxial strained LaCoO${}_3$ thin films

We have examined the atomic and electronic structures of perovskite lanthanum cobaltite (LaCoO${}_3$) thin films using Co $K$-edge x-ray absorption fine structure (XAFS) spectroscopy. Extended XAFS (EXAFS) demonstrates that a large difference between in-plane and out-of-plane Co-O bond lengths results from tetragonal distortion in highly strained films. The structural distortions are strongly coupled to the hybridization between atomic orbitals of the Co and O atoms, as shown by x-ray absorption near edge spectroscopy (XANES). Our results indicate that increased hybridization is not the cause of ferromagnetism in strained LaCoO${}_3$ films. Instead, we suggest that the strain-induced distortions of the oxygen octahedra increase the population of $e_g$ electrons and concurrently depopulate $t_{2g}$ electrons beyond a stabilization threshold for ferromagnetic order.

Figure 1

In-plane and out-of-plane (a) Co $K$-edge EXAFS spectra [$k^2\chi \left(k\right)$], and (b), (c) Fourier filtered first shell contributions to the EXAFS [$k^2\tilde{\chi }\left(k\right)$] of LCO films on STO and LAO substrates. In (b), fits to $k^2\tilde{\chi }\left(k\right)$ using the phase and amplitude functions from a LCO powder are also shown. Note the definitive structural changes in the film on STO as compared to the film on LAO, seen in (c).

Figure 2

(a) Co and Ti $K$-edge XANES of LCO films and the STO substrate measured with the x-ray wave vector and polarization vector in-plane along $\langle 100\rangle$ and $\langle 110\rangle$ directions at room temperature. The vertical line shows the position of the electron binding energy ($E_0$). (b) Co $K$-edge XANES of LCO films measured with the x-ray wave vector (polarization vector) along the [010] in-plane direction and the polarization vector (wave vector) along the [001] out-of-plane direction at 15 K. Note the reduced intensity of the hybridization feature in the film on STO, seen in the inset plots.