Persistent short-range spin fluctuation in rare-earth substituted square lattice Sr₂Ir_0.89 Tb_0.11O₄

Sr₂IrO₄, a prototypical J_eff = 1/2 square lattice system, is widely studied for novel physics. While transition metal substitution effects are well explored, rare-earth doping at the Ir⁴⁺ site is less explored. Here we present a detailed magnetization and neutron diffraction study on Sr₂Ir_1-xTb_xO₄ (x = 0.11). The system tends to adopt an in-plane antiferromagnetic configuration with a “+ + + +” stacking sequence. Strong spin fluctuations persist down to the lowest measured temperature, consistent with the concave shape order parameter. A pronounced magnetic diffuse scattering rod develops along the L direction at low temperature, indicating the formation of short-range magnetic correlations with a characteristic length of ~ 24(2) angstroms. This work provides a new pathway to exotic quantum phases in the strongly spin-orbit-coupled iridates.