La displacement driven Double-exchange like mediation in Titanium $d_{xy}$ ferromagnetism at the LaAlO$_3$/SrTiO$_3$

TL;DR

This paper proposes that La atom displacement and hybridization mediate ferromagnetism at LaAlO3/SrTiO3 interfaces through a double-exchange like mechanism, independent of oxygen vacancies.

Contribution

It introduces a novel mechanism involving La displacement and hybridization that explains ferromagnetism in LAO/STO interfaces without oxygen vacancies.

Findings

La displacement can induce ferromagnetism at the interface.

Hybridization between La d_{z^2} and O p_{x,y} mediates magnetic interactions.

Surface modifications can enhance or switch the ferromagnetic state.

Abstract

The epitaxial atomistic interfaces of two insulating oxides, LaAlO$_3$ (LAO)/SrTiO$_3$ (STO), have attracted great interest owing to rich emergent phenomena \cite{hwang12:nmat,coey13:mrs,ohtomo04:} such as interface metallicity \cite{ohtomo04:,nakagawa06:}, thickness dependent insulator-metal transition \cite{huijben06:nmat}, superconductivity \cite{N.Reyren08312007}, ferromagnetism \cite{brinkman.07:natmat}, and even their coexistence \cite{bert11:nphys,li11:nphys,dikin11:prl}. However, the physics origin of ferromagnetic ordering in the $n$-type LAO/STO interface is in debate. Here we propose that the polar distortion of La atom can ignite the ferromagnetism at the interface even without oxygen vacancy. The induced hybridization between La $d_{z^2}$ and O $p_{x,y}$ states can mediate double-exchange like interaction between Ti $d_{xy}$ electrons. We further suggest that the structural and electrical modification of the outermost surface of LAO or switching the polarization direction of ferroelectric overlayers on LAO/STO can promote such La displacement.