Antiferromagnetic instability in Sr$_3$Ru$_2$O$_7$: stabilized and revealed by dilute Mn impurities

X-ray Absorption Spectroscopy (XAS) and Resonant Elastic Soft X-ray Scattering (RESXS) studies have been performed on Mn-doped Sr$_3$Ru$_2$O$_7$, both on the Ru and Mn L-edges, to investigate the origin of the metal insulator transition. Extensive simulations based on our experimental findings point toward an intrinsic antiferromagnetic instability in the parent Sr$_3$Ru$_2$O$_7$ compound that is stabilized by the dilute Mn impurities. We show that the metal-insulator transition is a direct consequence of the antiferromagnetic order and we propose a phenomenological model that may be applicable also to metal-insulator transitions seen in other oxides. Moreover, a comparison of Ru and Mn L-edge data on 5\% Mn doped system reveals that dilute Mn impurities are generating much more intense signal than Ru which is occupying 95\% of the lattice sites. This suggests the embedding of dilute impurities as a powerful mean to probe weak and, possibly, spatially inhomogeneous order in solid-state systems. In collaboration with: Y. Yoshida (AIST), J. Geck, D.G. Hawthorn (UBC), M.W. Haverkort, Z. Hu, C. Sch\"{u}\ss ler-Langeheine (Cologne), R. Mathieu, Y. Tokura, S. Satow, H. Takagi (Tokyo), J.D. Denlinger (ALS).