Fundamental thickness limit of itinerant ferromagnetic SrRuO${}_3$ thin films

We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO${}_3$ that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO${}_3$ films remain metallic even for a thickness of 2 unit cells (uc), but the Curie temperature, T${}_C$, starts to decrease at 4 uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T${}_C$ decrease to a decrease in the density of states (N${}_o$). Namely, in the thin film geometry, the hybridized Ru-d${}_{y}z,zx$ orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of N${}_o$.