Origin of insulating and nonferromagnetic SrRuO3 monolayers

The electromagnetic properties of ultrathin epitaxial ruthenate films have long been the subject of debate. Here we combine experimental with theoretical investigations of ${(\mathrm{SrTi}{\mathrm{O}}_{3})}^{5}\text{\ensuremath{-}}{(\mathrm{SrRu}{\mathrm{O}}_{3})}^{n}\text{\ensuremath{-}}{(\mathrm{SrTi}{\mathrm{O}}_{3})}^{5} (\mathrm{ST}{\mathrm{O}}^{5}\text{\ensuremath{-}}\mathrm{SR}{\mathrm{O}}^{n}\text{\ensuremath{-}}\mathrm{ST}{\mathrm{O}}^{5}$) heterostructures with $n=1$ and 2 unit cells, including extensive atomic-resolution scanning-transmission electron-microscopy imaging, electron-energy-loss spectroscopy chemical mapping, as well as transport and magnetotransport measurements. The experimental data demonstrate that the $\mathrm{ST}{\mathrm{O}}^{5}\text{\ensuremath{-}}\mathrm{SR}{\mathrm{O}}^{2}\text{\ensuremath{-}}\mathrm{ST}{\mathrm{O}}^{5}$ heterostructure is nearly stoichiometric, metallic, and ferromagnetic with ${T}_{\mathrm{C}}\ensuremath{\sim}128\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, even though it lacks the characteristic bulk-SRO octahedral tilts and matches the cubic STO structure. In contrast, the $\mathrm{ST}{\mathrm{O}}^{5}\text{\ensuremath{-}}\mathrm{SR}{\mathrm{O}}^{1}\text{\ensuremath{-}}\mathrm{ST}{\mathrm{O}}^{5}$ heterostructure features Ru-Ti intermixing in the ${\mathrm{RuO}}_{2}$ layer, also without octahedral tilts, but is accompanied by a loss of metallicity and ferromagnetism. Density-functional theory calculations show that stoichiometric $n=1$ and $n=2$ heterostructures are metallic and ferromagnetic with no octahedral tilts, while nonstoichiometry in the Ru sublattice in the $n=1$ case opens an energy gap and induces antiferromagnetic ordering. Thus, the results indicate that the observed nonstoichiometry is the cause of the observed loss of metallicity and ferromagnetism in the $n=1$ case.