Magnetic Excitation Spectrum of Ca$_{2-x}$Sr$_{x}$RuO$_{4}$

We have studied the concentration dependence of the magnetic excitation spectrum in single crystal samples of the layered perovskite ruthenates, Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ for 2$>$x$>$0.4. For large x, the spectrum is similar to that observed in pure Sr$_{2}$RuO$_{4}$ with incommensurate excitations strongly peaked in Q at ($\pm $0.3, $\pm $0.3, q$_{z})$, consistent with Fermi-surface nesting wavevectors. As the concentration approaches the x=0.5 quantum critical point, the spectrum becomes broadly distributed in \textbf{Q} with a sharp upturn at $\pm $0.3 in both $h$ and $k$ and a flat distribution of scattering across the 2d ferromagnetic zone center. Possible interpretations of this scattering and qualitative similarity to the excitation spectrum of Sr$_{3}$Ru$_{2}$O$_{7}$, also in close proximity to a quantum critical point, will be discussed. ORNL is managed by UT-Battelle for the US DOE under contract DE-AC05-00OR22725.