Charge-Transfer-Induced Multivalent States with Resultant Emergent Magnetism in Transition-Metal Oxide Heterostructures

Transition-metal oxide (TMO) heterostructures provide fertile grounds for creating and manipulating intriguing properties and functionalities. At the interface of TMO heterostructures, electronic reconstructions generally occur via charge transfer and lead to an extraordinary spectrum of emergent phenomena but unattainable in their bulk constituents. However, the basic mechanism of charge transfer at the interface is not fully determined or even understood in heterostructures, which may hide the underlying mechanisms and intriguing physics. Herein, an intrinsic charge transfer and resultant exotic ferromagnetism are unambiguously observed in the heterostructures between the nonmagnetic LaCoO₃ (LCO) and SrTiO₃ (STO). Combining element-specific X-ray absorption spectroscopy and atomic multiplet fitting, direct evidence of charge transfer-induced multivalence of cobalt ions, interactions of which would contribute to the novel magnetism beyond the intuition, in concert with first-principles density-functional-theory calculations, is demonstrated. Beyond LCO/STO system, a more broadly applicable principle for the heterostructures between 3d TMO and STO where charge transfer and resultant multivalence or conducting interfaces are coexistent is establish. This study represents an advance that the electronic reconstruction and the multiple electron configurations of 3d transition metal ions will constitute a powerful tool for the designs of functional materials and creations of unconventional physical properties.