${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, a prototypical $J_{\mathrm{eff}}=1/2$ square lattice system, is widely studied for novel physics. While transition metal substitution effects are well explored, rare-earth doping at the ${\mathrm{Ir}}^{4+}$ site is less explored. Here we present a detailed magnetization and neutron diffraction study on ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-x}{\mathrm{Tb}}_x{\mathrm{O}}_4$ $(x=0.11)$. 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 $\sim 24\left(2\right)\AA$. This work provides a new pathway to exotic quantum phases in the strongly spin-orbit-coupled iridates.

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• Spin-orbit coupling
• Iridates
• Neutron scattering