Neutron scattering study of correlated phase behavior in Sr ₂IrO₄

Neutron diffraction measurements exploring the magnetic and structural phase behaviors of the candidate J_eff=1/2 Mott insulating iridate Sr₂IrO₄ are presented. Comparisons are drawn between the correlated magnetism in this single-layer system and its bilayer analog Sr ₃Ir₂O₇, where both materials exhibit magnetic domains originating from crystallographic twinning and comparable moment sizes. Weakly temperature-dependent superlattice peaks violating the reported tetragonal space group of Sr₂IrO₄ are observed, supporting the notion of a lower structural symmetry arising from a high-temperature lattice distortion, and we use this to argue that moments orient along a unique in-plane axis demonstrating an orthorhombic symmetry in the resulting spin structure. Our results demonstrate that the correlated spin order and structural phase behaviors in both single-layer and bilayer Sr_n+1Ir _nO_3n+1 systems are remarkably similar and suggest comparable correlation strengths in each system.