Suppression of spin-state transition in epitaxially strained ${\text{LaCoO}}_3$

Epitaxial thin films of ${\text{LaCoO}}_3$ $\left(e\text{-LCO}\right)$ exhibit ferromagnetic order with a transition temperature $T_C=85\text{K}$ while polycrystalline thin ${\text{LaCoO}}_3$ films $\left(p\text{-LCO}\right)$ remain paramagnetic. The temperature-dependent spin-state structure for both $e\text{-LCO}$ and $p\text{-LCO}$ was studied by x-ray absorption spectroscopy at the $\text{Co}{L}_{2,3}$ and $\text{O}K$ edges. Considerable spectral redistributions over temperature are observed for $p\text{-LCO}$. The spectra for $e\text{-LCO}$, on the other hand, do not show any significant changes for temperatures between 30 and 450 K at both edges, indicating that the spin state remains constant and that the epitaxial strain inhibits any population of the low-spin $\left(S=0\right)$ state with decreasing temperature. This observation identifies an important prerequisite for ferromagnetism in $e\text{-LCO}$ thin films.

Figure 1

(Color online) $\text{O}1s$ x-ray absorption spectra of an epitaxial thin film of ${\text{LaCoO}}_3$ taken at different temperatures. Over the whole recorded temperature range, there is no change in the spectral weight distribution.

Figure 2

(Color online) The $\text{O}K$ low-energy region for the $e\text{-LCO}$ and the $p\text{-LCO}$ thin films. Open symbols: $p\text{-LCO}$ film. (The traces for different temperatures are offset for clarity.) Full symbols: $e\text{-LCO}$ film strained by the substrate.

Figure 3

(Color online) Upper panel: $\text{Co}{L}_{2,3}$ NEXAFS taken at different temperatures for $e\text{-LCO}$. No discernible spectral changes occur with temperature. Lower panel: $\text{Co}{L}_{2,3}$ NEXAFS for $p\text{-LCO}$ at 300 K. This trace is remarkably similar to the $e\text{-LCO}$ spectra of the upper panel. (For clarity, the $p\text{-LCO}$ 300 K data in the lower panel are offset from the temperature-dependent $e\text{-LCO}$ spectra in the upper panel.)

Figure 4

(Color online) $\text{Co}{L}_{2,3}$ NEXAFS taken on $p\text{-LCO}$ films (upper panel). Spectral background and a small spectral ${\text{Co}}^{2+}$ contribution centered around 777 eV (cf. lower panel in Fig. 3) were subtracted. The spectrum taken at 50 K exhibits two important changes over the other spectra: a pronounced shoulder at about 781.5 eV, and a significantly higher ratio between maximum and shoulder at the $L_2$ edge. Lower panel: Atomic multiplet simulations using a mixture of LS and HS spectra. The spectra of the upper panel are well described by the choice of parameters.

Figure 5

Fraction of ${\text{Co}}^{3+}$ LS as extracted from the multiplet simulations of the NEXAFS spectra for $p\text{-LCO}$, compared with results from Ref. 16 for single-crystalline LCO. The $e\text{-LCO}$ spectra are very similar to the $p\text{-LCO}$ result at 300 K, and the corresponding LS fraction is included as a dashed line.