Fundamental Thickness Limit of Itinerant Ferromagnetic SrRuO3 Thin Films

We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO3 that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO3 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-0). Namely, in the thin film geometry, the hybridized Ru d(yz,zx) orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of N-0.