Local electronic structure and ferromagnetic interaction in La(Co,Ni)O$_{\mathrm{3}}$

Perovskite-related transition-metal oxides exhibit a wide range of properties from insulating to superconducting as well as many peculiar magnetic phases, and cobaltites, in particular, have been known for their proximity to spin-state transitions. How this changes with partial substitution by Ni is the topic of the present study. The local electronic structure and the ferromagnetic interaction in La(Co$_{\mathrm{1-x}}$Ni$_{\mathrm{x}})$O$_{\mathrm{3}}$ has been studied by x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD). XAS clearly indicates a mixed-valence state for both Co and Ni, with both valences changing systematically with Ni content, x. While the gradual spin-state transition of Co$^{\mathrm{3+}}$ from low-spin (LS) to high-spin (HS) is preserved for low x it is suppressed in the high Ni-content samples. Regarding the spin configuration of Ni we find it stabilized in a ``mixed'' spin state, unlike the purely LS state of Ni in LaNiO$_{\mathrm{3}}$. XMCD identifies the element-specific contributions to the magnetic moment and interactions. In particular, we find that it must be the coexistence of the HS state in both Co$^{\mathrm{3+}}$ and Ni$^{\mathrm{3+\thinspace }}$that induces $t_{\mathrm{2g}}$-based ferromagnetic interaction via the double-exchange mechanism.