Oxygen vacancies at the origin of pinned moments in oxide interfaces: the example of tetragonal CuO/SrTiO$_3$

TL;DR

This paper uses first principles calculations to explain the origin of pinned orbital and paramagnetic moments at oxide interfaces, attributing them to oxygen vacancies and structural distortions in a CuO/SrTiO3 heterostructure.

Contribution

It provides a detailed theoretical scenario linking oxygen vacancies to magnetic phenomena observed experimentally at oxide interfaces, highlighting the role of structural distortions.

Findings

Oxygen vacancies induce a paramagnetic 2D electron gas at the interface.

Structural distortions break C4 symmetry, pinning the Cu orbital moment.

The Ti valence remains unchanged despite the magnetic phenomena.

Abstract

Obtaining an accurate theoretical description of the emergent phenomena in oxide heterostructures is a major challenge. Recently, intriguing paramagnetic spin and pinned orbital moments have been discovered by x-ray magnetic circular dichro\"ism measurements at the Cu $L_{2,3}$-edge of a tetragonal CuO/SrTiO$_3$ heterostructure. Using first principles calculations, we propose a scenario that explains both types of moments, based on the formation of oxygen vacancies in the TiO$_2$ interface layer. We show the emergence of a paramagnetic 2D electron gas hosted in the interface CuO layer. It is invisible at the Ti $L_{2,3}$-edge since the valence of the Ti atoms remains unchanged. Strong structural distortions breaking both the local and global fourfold rotation $C_4$ symmetries at the interface lead to the in-plane pinning of the Cu orbital moment close to the vacancy. Our results, and in particular the pinning of the orbital moment, may have implications for other systems, especially monoxide/dioxide interfaces with similar metal-oxygen bond length and weak spin-orbit coupling.