Magnetic properties of bilayer Sr$_3$Ir$_2$O$_7$: role of epitaxial strain and oxygen vacancies

TL;DR

This study uses ab initio calculations to explore how epitaxial strain and oxygen vacancies influence the magnetic properties of bilayer Sr3Ir2O7, revealing strain-induced magnetic transitions and the role of vacancies in magnetic response.

Contribution

It provides new insights into the effects of strain and oxygen vacancies on the magnetic behavior of bilayer Sr3Ir2O7, highlighting mechanisms for magnetic transitions and moment canting.

Findings

Epitaxial strain induces a spin-flop transition in Sr3Ir2O7.

The interlayer exchange interaction J2 varies significantly under tensile strain.

Oxygen vacancies cause magnetic moment canting and noncollinear spin configurations.

Abstract

Using {\it ab initio} methods, we investigate the modification of the magnetic properties of the $m=2$ member of the strontium iridates Ruddlesden-Popper series Sr$_{m+1}$Ir$_{m}$O$_{3m+1}$, bilayer Sr$_3$Ir$_2$O$_7$, induced by epitaxial strain and oxygen vacancies. Unlike the single layer compound Sr$_2$IrO$_4$, which exhibits a robust in-plane magnetic order, the energy difference between in-plane and out-of-plane magnetic orderings in Sr$_3$Ir$_2$O$_7$ is much smaller and it is expected that small external perturbations could induce magnetic transitions. Our results indicate that epitaxial strain yields a spin-flop transition, that is driven by the crossover between the intralayer $J_1$ and interlayer $J_2$ magnetic exchange interactions upon compressive strain. While $J_1$ is essentially insensitive to strain effects, the strength of $J_2$ changes by one order of magnitude for tensile strains $\geq$ 3~\%. In addition, our study clarifies that the unusual in-plane magnetic response observed in Sr$_3$Ir$_2$O$_7$ upon the application of an external magnetic field originates from the canting of the local magnetic moments due to oxygen vacancies, which tilt the octahedral networks - thereby allowing for noncollinear spin configurations.