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 (SrTiO3)(5)-(SrRuO3)n-(SrTiO3)(5) (STO5-SROn-STO5) 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 STO5-SRO2-STO5 heterostructure is nearly stoichiometric, metallic, and ferromagnetic with T-C similar to 128 K, even though it lacks the characteristic bulk-SRO octahedral tilts and matches the cubic STO structure. In contrast, the STO5-SRO1-STO5 heterostructure features Ru-Ti intermixing in the RuO2 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.