Insulator-to-Metal Transition of Cr₂O₃ Thin Films via Isovalent Ru³⁺ Substitution

In transition-metal oxides, the vast combinations of crystal structures and metal ions provide a fertile ground for tailoring their electronic properties. Using a thin-film growth technique creates an additional interface-related degree of freedom in the artificial heterostructures. However, in corundum-type and crystallographically related ternary compounds, the scarcity of oxides applicable to a lattice-matched electrode layer often impedes the electrical characterization of these heterostructures. Here, we report the synthesis of a thin film of a new corundum-type oxide that exhibits metallic conduction. By substituting Ru into the antiferromagnetic insulator Cr₂O₃ via pulsed laser deposition, we fabricated thin films of (Cr_1–xRu_x)₂O₃ not known in bulk. The structural, electrical, and magnetic characterizations showed that the antiferromagnetic insulator became a paramagnetic metal at Ru contents x greater than approximately 0.40 while preserving the host corundum structure. Spectroscopic analyses revealed the formation of Ru 4d-derived bands at the Fermi level and the presence of Ru³⁺, which is a valence state of Ru rarely observed in oxides. The stabilization of the unusual Ru³⁺ state is attributed to the charge neutrality constraint in the robust Cr₂O₃-based framework as well as the kinetics-driven vacuum deposition process.