revealed by Raman scattering under pressure*

At ambient pressure, the spin-orbit coupling induced Mott insulator Sr₂IrO₄ shows canted antiferromagnetic (AFM) ordering with a Néel temperature of 240 K. The evolution of the magnetic structure with tuning parameters such as chemical doping and engineered strain has been well studied, but the behavior under hydrostatic pressure remains unsettled. Here, we present optical Raman measurements of low frequency magnons on Sr₂IrO₄ single crystals to investigate the complex magnetic evolution under high pressure. While at ambient pressure the J_eff=1/2 magnetic moments are mostly projected within the a-b plane, the spin correlation along the c-axis increases under pressure between layers. Starting approximately at 3 GPa, the AFM experience a spin-flop transition of first-order nature similar to the behavior in magnetic fields and of Sr₃Ir₂O₇. The phase coexistence region extends to 11 GPa, before spins are fully rearranged from ordering in the basal plane to along the c-axis. Above 20 GPa, quasielastic Raman scattering arising from spin fluctuations appear, suggesting the onset of a disordered magnetic phase.