Transition metal oxides exhibit a wide range of tunable electronic properties arising from the complex interplay of charge, spin, and lattice degrees of freedom, governed by their $d$ orbital configurations, making them particularly interesting for oxide electronics and (electro)catalysis. Perovskite oxide heterointerfaces offer a promising route to engineer these orbital states. In this work, we tune the $\mathrm{Co}3d$ orbital occupancy in ${\mathrm{LaCoO}}_3$ from a partial $d^7$ to a partial $d^5$ state through interfacial engineering with ${\mathrm{LaTiO}}_3$, ${\mathrm{LaMnO}}_3$, ${\mathrm{LaAlO}}_3$, and ${\mathrm{LaNiO}}_3$. Using x-ray absorption spectroscopy combined with charge transfer multiplet calculations, we identify differences in the Co valence and spin state for the series of oxide heterostructures. ${\mathrm{LaTiO}}_3$ and ${\mathrm{LaMnO}}_3$ interfaces result in interfacial charge transfer towards ${\mathrm{LaCoO}}_3$, resulting in a partial $d^7$ orbital occupancy, while a ${\mathrm{LaNiO}}_3$ interface results in a partial Co $d^5$ occupancy. Strikingly, a ${\mathrm{LaAlO}}_3$ spacer layer between ${\mathrm{LaNiO}}_3$ and ${\mathrm{LaCoO}}_3$ results in a Co $d^6$ low-spin state. These results indicate that the Co spin state, like the valence state, is governed by the interfacial environment. High-resolution scanning transmission electron microscopy imaging reveals a clear connection between strain and spin configuration, emphasizing the importance of structural control at oxide interfaces. Overall, this work demonstrates that interfacial engineering simultaneously governs orbital occupancy and spin state in correlated oxides, advancing spin-engineering strategies in correlated oxides and offering new insights for the rational design of functional oxide heterostructures.

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• Multilayer thin films
• Perovskites
• Solid-solid interfaces
• Epitaxy
• Scanning transmission electron microscopy
• X-ray absorption spectroscopy