X-ray absorption and magnetic circular dichroism of ${\text{LaCoO}}_3$, ${\text{La}}_{0.7}{\text{Ce}}_{0.3}{\text{CoO}}_3$, and ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{CoO}}_3$ films: Evidence for cobalt-valence-dependent magnetism

Epitaxial thin films of undoped ${\text{LaCoO}}_3$, of electron-doped ${\text{La}}_{0.7}{\text{Ce}}_{0.3}{\text{CoO}}_3$, and of hole-doped ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{CoO}}_3$ exhibit ferromagnetic order with a transition temperature $T_{\text{C}}\approx 84\text{K}$, 23 K, and 194 K, respectively. The spin-state structure for these compounds was studied by soft x-ray magnetic circular dichroism and by near-edge x-ray absorption fine structure at the $\text{Co}{L}_{2,3}$ and $\text{O}K$ edges. It turns out that superexchange between ${\text{Co}}^{3+}$ high-spin and ${\text{Co}}^{3+}$ low-spin states is responsible for the ferromagnetism in ${\text{LaCoO}}_3$. For ${\text{La}}_{0.7}{\text{Ce}}_{0.3}{\text{CoO}}_3$ the ${\text{Co}}^{3+}$ ions are in a low-spin state and the spin and orbital moments are predominantly determined by a ${\text{Co}}^{2+}$ high-spin configuration. A spin blockade naturally explains the low transition temperature and the insulating characteristics of ${\text{La}}_{0.7}{\text{Ce}}_{0.3}{\text{CoO}}_3$. For ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{CoO}}_3$, on the other hand, the magnetic moments in the epitaxial films originate from high-spin ${\text{Co}}^{3+}$ and high-spin ${\text{Co}}^{4+}$ states. Ferromagnetism is induced by $t_{2g}$ double exchange between the two high-spin configurations. For all systems, a strong magnetic anisotropy is observed, with the magnetic moments essentially oriented within the film plane.

Figure 1

(Color online) $\phi =0°$ and $\phi =65°$ $\text{Co}{L}_{2,3}$ NEXAFS spectra of LCO taken at 20 K. No spectral changes occur with sample orientation, reflecting the isotropic nature of the unoccupied electronic density of states.

Figure 2

(Color online) Comparison of the $\phi =0°$ $\text{Co}{L}_3$ NEXAFS spectra of LCO, LCCO, and LSCO taken at 20 K. The inset shows the corresponding $L_2$ spectra. The spectral shape of both edges changes upon doping.

Figure 3

(Color online) $\text{Co}2p$ XAS multiplet calculations of ${\text{Co}}^{3+}$ LS, ${\text{Co}}^{3+}$ HS, ${\text{Co}}^{3+}$ IS, ${\text{Co}}^{2+}$ HS, ${\text{Co}}^{4+}$ LS, and ${\text{Co}}^{4+}$ HS. In addition the simulated (solid line) and measured (dotted line) spectra of LCO, LCCO, and LSCO are also depicted. For clarity, the spectra for the different configurations are offset.

Figure 4

(Color online) $\phi =0°$ and $\phi =65°$ $\text{Co}{L}_{2,3}$ SXMCD spectra of LCO taken at 20 K. A clear anisotropy between normal and grazing incidence is found. The magnetic moments are essentially caused by high-spin ${\text{Co}}^{3+}$ states.

Figure 5

(Color online) $\phi =0°$ and $\phi =65°$ $\text{Co}{L}_{2,3}$ SXMCD spectra of LCCO taken at 20 K. In contrast to LCO, a reduced anisotropy between $\phi =0°$ and $\phi =65°$ spectra is found. The magnetic moments are predominantly determined by ${\text{Co}}^{2+}$.

Figure 6

(Color online) $\phi =0°$ and $\phi =65°$ $\text{Co}{L}_{2,3}$ SXMCD spectra of LSCO taken at 20 K. For comparison the $\phi =0°$ and $\phi =65°$ $\text{Co}{L}_{2,3}$ data of LCO are included. The magnetic moments are caused by ${\text{Co}}^{3+}/{\text{Co}}^{4+}$ high-spin states.

Figure 7

(Color online) $\text{O}K$ SXMCD and absorption of LCO, LSCO, and LCCO taken at 20 K. The absorption intensity is strongly doping dependent. A strong XMCD effect is found for LCO and LSCO while it is small for LCCO. The shaded areas in the absorption data exemplify that the SXMCD signal is limited to the $t_{2g}$ range. Inset: spectral weight for $e_g$ and $t_{2g}$ levels in a simple ionic model (for details, see text).

Figure 8

(Color online) Scheme showing the relevant hopping process for (a) LSCO, (b) LCCO, and (c) LCO. (a) For the $t_{2g}$ double exchange in LSCO an electron moves from a ${\text{Co}}^{3+}$ HS site to a ${\text{Co}}^{4+}$ HS neighbor. (b) For LCCO the spin blockade inhibits the transfer of an electron from the ${\text{Co}}^{2+}$ HS to the ${\text{Co}}^{3+}$ LS site. (c) For strained LCO correlated $t_{2g}$ and $e_g$ hopping processes between ${\text{Co}}^{3+}$ LS and ${\text{Co}}^{3+}$ HS neighbors lead to a ferromagnetic superexchange. Due to the correlated hopping of $t_{2g}$ and $e_g$ electrons, spin states are interchanged without a net transfer of charge. In the sketch, only the local $t_{2g}$ and $e_g$ hopping processes for the transformation of a ${\text{Co}}^{3+}$ LS into a ${\text{Co}}^{3+}$ HS ion are considered (Refs. 59, 60).