Magnetism and metal-insulator transition in oxygen deficient SrTiO$_3$

TL;DR

This study uses first-principles calculations to explore how oxygen vacancies, doping, and strain influence magnetism and electronic properties in SrTiO$_3$, revealing conditions for stable local magnetic moments and potential device applications.

Contribution

It demonstrates that free-carrier density and strain control magnetism in oxygen-deficient SrTiO$_3$, highlighting the importance of doping and lattice distortions for magnetic state stability.

Findings

Isolated vacancies are non-magnetic double donors.

Doping can stabilize single donor states with local moments.

Strain tuning affects the energy of local magnetic moments.

Abstract

First-principles calculations to study the electronic and magnetic properties of bulk, oxygen-deficient SrTiO$_3$ (STO) under different doping conditions and densities have been conducted. The appearance of magnetism in oxygen-deficient STO is not determined solely by the presence of a single oxygen vacancy but by the density of free carriers and the relative proximity of the vacant sites. We find that while an isolated vacancy behaves as a non-magnetic double donor, manipulation of the doping conditions allows the stability of a single donor state, with emergent local moments coupled ferromagnetically by carriers in the conduction band. Strong local lattice distortions enhance the binding of this state. The energy of the in-gap local moment can be further tuned by orthorhombic strain. Consequently we find that the free-carrier density and strain are fundamental components to obtaining trapped spin-polarized electrons in oxygen-deficient STO, which may have important implications in the design of optical devices.