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We have studied the magnetic excitations in impurity dopedLa$_{1.85}$Sr$_{0.15}$Cu$_{1-y}$A$_{y}$O$_{4}$ (A=Ni or Zn) by neutronscattering. The dispersion for Zn:$y=0.017$ is similar to that for the impurityfree sample: incommensurate peaks with the incommensurability$\delta=0.12\pm0.01$ (rlu) do not change their positions up to 21 meV. On theother hand, for Ni:$y=0.029$, two incommensurate peaks observed at low energiessuddenly change into a broad commensurate peak at $E_\mathrm{cross}=15$ meV.Compared to the impurity free sample with a similar Sr-concentration $x=0.16$,[B. Vignolle {\it et al.} Nature Physics {\bf 3} (2007) 163],$E_\mathrm{cross}$ for Ni:$y=0.029$ is decreased by nearly the same factor forthe reduction in $T_{c}$. This is very similar to the shift of the resonanceenergy ($E_\mathrm{res}$) in Ni-doped YBa$_{2}$Cu$_{3}$O$_{7}$.[Y. Sidis {\itet al.}: Phys. Rev. Lett. {\bf 84} (2000) 5900]. These common impurity effectson the shift of $E_\mathrm{cross}$ and $E_\mathrm{res}$ suggest the samemagnetic origin for the resonance peak in YBa$_{2}$Cu$_{3}$O$_{\delta}$ andthat for a crossing point of upward and downward dispersions in theLa$_{2-x}$Sr$_{x}$CuO$_{4}$. We propose that the sudden change in thedispersion is better described by a crossover from incommensurate spinfluctuations to a gapped spin wave rather than a hourglass-like dispersion.

Renormalization of Commensurate Magnetic Peak in Ni-doped La$_{1.85}$Sr$_{0.15}$CuO$_{4}$