Thickness dependent itinerant ferromagnetism in ultrathin Ba-doped SrRuO3 films

Abstract

The electronic and magnetic properties in $AB{\mathrm{O}}_{3}$ perovskite oxides are extremely sensitive to lattice structure, but also to dimensionality, such as the thickness in thin film-form. Here, we report the thickness-dependent electromagnetic properties of ultrathin epitaxially stabilized ${\mathrm{Sr}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}\mathrm{Ru}{\mathrm{O}}_{3}$ ($x=0.08$, 0.2) thin films on a $\mathrm{SrTi}{\mathrm{O}}_{3}$ (001) substrate. The results reveal that the barium doping $(0.08\ensuremath{\le}x\ensuremath{\le}0.20)$ reduces $\mathrm{Ru}{\mathrm{O}}_{6}$ orthorhombic distortions existing in $\mathrm{SrRu}{\mathrm{O}}_{3}$ and induces a tetragonal distortion, as evidenced by out-of-plane lattice expansion. A metal-to-insulator transition, accompanied by a ferromagnetic to nonmagnetic transition occurs with reducing film thickness from 10 to 3 unit cells for both $x=0.08$ and 0.2, regardless of the doping level. The results suggest that the effects of compositional vacancies and surface/interface contributions introduced via dimensional confinement are more dominant than $A$-site chemical disorder or structural distortion for the loss of metallicity and ferromagnetism in ultra-thin epitaxial films.