Epitaxial strain effects on magnetic ordering and spin-phonon couplings in the (SrMnO$_3$)$_1$/(LaMnO$_3$)$_1$ superlattice from first principles

TL;DR

This study uses first-principles calculations to explore how epitaxial strain affects magnetic phases and spin-phonon interactions in SrMnO3/LaMnO3 superlattices, revealing strain-induced magnetic transitions and enhanced spin-phonon coupling.

Contribution

It provides detailed insights into strain-induced magnetic phase transitions and quantifies spin-phonon coupling variations in the superlattice from first principles.

Findings

Magnetic phase transitions are induced by epitaxial strain.

Oxygen octahedral rotations do not prevent strain-induced magnetic transitions.

Spin-phonon coupling strength increases significantly under tensile strain.

Abstract

We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and polar phonons in the 1:1 SrMnO$_3$/LaMnO$_3$ superlattice from first principles. Magnetic phase transitions of the superlattice induced by epitaxial strain are observed, consistent with previous reports. We find that oxygen octahedral rotations lower the ground state energy but do not destroy the magnetic phase transitions induced by strain. We compute zone center phonon frequencies and eigenvectors as functions of epitaxial strain and magnetic ordering. A substantial increase of the coupling strength between the spin and the lowest-frequency polar mode is observed for tensile strains. This increase can be attributed to a change of character of the lowest mode resulting from different relative couplings of the various polar modes to epitaxial strain. Finally, spin-phonon coupling strengths are computed in a Heisenberg formalism. This analysis directly reveals the changes in exchange couplings due to specific atomic displacements or phonon modes, as well as the nonequivalence of the out-of-plane exchange couplings across LaO layers and across SrO layers, the latter being the result of the artificial structuring in the superlattice.