The origin of insulating and non-ferromagnetic SrRuO3 monolayers

TL;DR

This study combines experimental and theoretical methods to investigate ultrathin SrRuO3 heterostructures, revealing how stoichiometry and structural tilts influence their metallic and magnetic properties.

Contribution

It provides new insights into the role of stoichiometry and structural distortions in determining the electronic and magnetic behavior of ultrathin SrRuO3 layers.

Findings

STO5-SRO2 heterostructure is metallic and ferromagnetic at 128 K.

STO5-SRO1 heterostructure shows Ru-Ti intermixing, loss of metallicity and ferromagnetism.

Non-stoichiometry in SRO1 induces an energy gap and antiferromagnetic order.

Abstract

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.