Emergent ferromagnetism and insulator-metal transition in δ-doped ultrathin ruthenates

Abstract

Heterostructures of complex transition metal oxides are known to induce extraordinary emergent quantum states that arise from broken symmetry and other discontinuities at interfaces. Here we report the emergence of unusual, thickness-dependent properties in ultrathin CaRuO3 films by insertion of a single isovalent SrO layer (referred to as δ-doping). While bulk CaRuO3 is metallic and nonmagnetic, films thinner than or equal to ~15-unit cells (u.c.) are insulating though still nonmagnetic. However, δ-doping to middle of such CaRuO3 films induces an insulator-to-metal transition and unusual ferromagnetism with strong magnetoresistive behavior. Atomically resolved imaging and density-functional-theory calculations reveal that the whole δ-doped film preserves the bulk-CaRuO3 orthorhombic structure, while appreciable structural and electronic changes are highly localized near the SrO layer. The results highlight delicate nature of magnetic instability in CaRuO3 and subtle effects that can alter it, especially the role of A-site cation in electronic and magnetic structure additional to lattice distortion in ruthenates. It also provides a practical approach to engineer material systems via highly localized modifications in their structure and composition that may offer new routes to the design of oxide electronics.