Ferromagnetic Fluctuations Enhanced by Mn Doping in Sr$_2$RuO$_4$

Sr$_{2}$RuO$_{4}$ is the first experimentally established example of a spin-triplet superconductor [1]; it has attracted a great deal of interest since its discovery in 1994. Like other unconventional superconductors, the superconductivity of Sr$_{2}$RuO$_{4}$ also occurs in close proximity to magnetic instability. Its normal state is characterized by incommensurate antiferromagnetic (AFM) fluctuations associated with Fermi surface nesting. Moreover, the other ruthenate compounds related to Sr$_{2}$RuO$_{4}$ in the Ruddlesden-Popper series are all magnetic. The Sr-based members Sr$_{3}$Ru$_{2}$O$_{7}$, Sr$_{4}$Ru$_{3}$O$_{10}$ and SrRuO$_{3}$ are either metamagnetic or ferromagnetic (FM), whereas the Ca-based members Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$ are AFM. We have investigated the Mn doping effect in Sr$_{2}$RuO$_{4}$ using floating-zone grown single crystal samples and observed significantly enhanced FM fluctuations in the Mn-doped Sr$_{2}$RuO$_{4}$ samples. The system becomes nearly FM with only a few percent Mn doping. This finding suggests that Sr$_{2}$RuO$_{4}$ involves competing, orbital dependent magnetic fluctuations.\\[4pt] [1] A. P. Mackenzie and Y. Maeno, Rev. Mod. Phys. \textbf{75}, 657 (2003).