Griffiths phase and critical behavior in layered Sr${}_2$IrO${}_4$ ferromagnet

We report the existence of Griffiths phase (GP) and its influence on critical phenomena in layered Sr${}_2$IrO${}_4$ ferromagnet (T${}_C$ = 221.5 K). The power law behavior of inverse magentic susceptibility, 1/$\chi$(T) with exponent $\lambda = 0.18(2)$confirmtheGPintheregimeT$_C$ $<$ T $\le$ T${}_G$ = 279.0(5) K. Moreover, the detailed critical analysis via modified Arrott plot method exhibits unrealistic critical exponents $\beta$ = 0.77(1), $\gamma$ = 1.59(2) and $\delta =3.06(4)$, in corroboration with magneto-caloric study. The abnormal exponent values have been viewed in context of ferromagnetic-Griffiths phase transition. The GP has been further analyzed using Bray model, which yields a reliable value of $\beta$ = 0.19(2), belonging to the two-dimensional (2D) XYh${}_4$ universality class with strong anisotropy present in Sr${}_2$IrO${}_4$. The present study proposes Bray model as a possible tool to investigate the critical behavior for Griffiths ferromagnets in place of conventional Arrott plot analysis. The possible origins of GP and its correlation with insulating nature of Sr${}_2$IrO${}_4$ have been discussed.