Magnetic properties of bilayer Sr3 Ir2 O7: Role of epitaxial strain and oxygen vacancies

Using ab initio methods, we investigate the modification of the magnetic properties of the m=2 member of the strontium iridates Ruddlesden-Popper series Srm+1IrmO3m+1, bilayer Sr3Ir2O7, induced by epitaxial strain and oxygen vacancies. Unlike the single-layer compound Sr2IrO4, which exhibits a robust in-plane magnetic order, the energy difference between in-plane and out-of-plane magnetic orderings in Sr3Ir2O7 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, which is driven by the crossover between the intralayer J1 and interlayer J2 magnetic exchange interactions upon compressive strain. While J1 is essentially insensitive to strain effects, the strength of J2 changes by one order of magnitude for tensile strains ≥3%. In addition, our study clarifies that the unusual in-plane magnetic response observed in Sr3Ir2O7 upon the application of an external magnetic field originates from the canting of the local magnetic moments due to oxygen vacancies, which locally destroy the octahedral networks, thereby allowing for noncollinear spin configurations.