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

Perovskite-related transition-metal oxides exhibit properties ranging from insulating to superconducting as well as unusual magnetic phases, and cobaltates, in particular, have been known for their propensity for spin-state transitions. Nonmagnetic LaCoO$_{3}$ and paramagnetic LaNiO$_{3}$ are parent compounds for the La(Co$_{1-x}$Ni$_{x})$O$_{3}$ (LCNO) family, which, for intermediate Ni content x, exhibits ferromagnetism. The local electronic structure and the ferromagnetic interaction in LCNO have been studied by x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD). XAS indicates a mixed-valence state for both Co and Ni, with both valences changing systematically with increasing x. Simultaneously, a spin-state redistribution towards HS (Co site) and LS (Ni site) occurs, and temperature-dependent spin-state transitions are increasingly suppressed. XMCD identifies the element-specific contributions to the magnetic moment and interactions. A simple model based on a double-exchange-like mechanism between Co$^{3+}$ HS and Ni$^{3+\, }$HS can qualitatively account for the evolution of ferromagnetism in the LCNO series.