  The electro-magnetic 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
magneto-transport measurements. The experimental data demonstrate that the
STO5-SRO2-STO5 heterostructure is stoichiometric, metallic, and ferromagnetic
with TC ~ 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 non-stoichiometry in the Ru sublattice in the n = 1
case opens an energy gap and induces antiferromagnetic ordering. Thus, the
results indicate that the observed non-stoichiometry is the cause of the
observed loss of metallicity and ferromagnetism in the n = 1 case.
