Evolution of Structural and Electronic Phase Behavior in Sr$_3$(Ir$_{1-x}$Mn$_x$)$_2$O$_7$

Sr$_3$Ir$_2$O$_7$ is a rare example of a spin-orbit coupling (SOC) induced $J_{eff}=1/2$ Mott antiferromagnet. Here, we present an investigation of the evolution of the electronic and structural properties of Sr$_3$(Ir$_{1-x}$Mn$_x$)$_2$O$_7$, with $x$ up to 44\%. Sr$_3$Mn$_2$O$_7$ is an antiferromagnetic insulator due to exchange splitting of the $t_{2g}$ orbitals\footnote{J.F. Mitchell \emph{et al.}, \emph{J. Appl. Phys.} {\bf 85}, 4352 (1999)}. Nevertheless, hole-doping the B-site of Sr$_3$Ir$_2$O$_7$ with Mn should substantially renormalize the SOC required for the appearance of the Mott state. Our results from neutron diffraction and magnetic susceptibility show a suppression of the magnetic ordering temperature without a substantial increase in the ordered moment. While all measured samples remained insulating, the inflection in the resistivity at $T_N$ is rapidly suppressed upon Mn substitution. Additionally, the monoclinic lattice distortion observed in Sr$_3$Ir$_2$O$_7$ is also substantially reduced. Mn-doping therefore represents an intriguing new pathway for the suppression of the Mott state, while also allowing for an investigation of the interplay between nuclear structure and electronic properties in the Ruddlesden-Popper iridates.