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.

众所周知，复杂过渡金属氧化物的异质结构会引发异常的涌现量子态，这些量子态是由界面处的对称性破坏和其他不连续性引起的。在这里，我们报告了通过插入单个等价 SrO 层（称为 δ 掺杂）在超薄 CaRuO _{3薄膜中出现了不寻常的、与厚度相关的特性。}虽然块状 CaRuO ₃是金属且非磁性的，但厚度小于或等于约 15 个晶胞 (uc) 的薄膜是绝缘的，但仍然是非磁性的。然而，δ-掺杂到这种 CaRuO _3的中间薄膜诱导绝缘体到金属的转变和具有强磁阻行为的不寻常的铁磁性。原子分辨成像和密度泛函理论计算表明，整个 δ 掺杂薄膜保留了块状 CaRuO ₃正交结构，而明显的结构和电子变化高度集中在 SrO 层附近。结果突出了CaRuO ₃中磁不稳定性的微妙性质以及可以改变它的微妙影响，特别是A位阳离子在电子和磁性结构中的作用以及钌酸盐中的晶格畸变。它还提供了一种通过对其结构和组成进行高度局部修改来设计材料系统的实用方法，这可能为氧化物电子学的设计提供新途径。