Renormalization of commensurate magnetic peak in Ni-Doped La _1.85Sr_0.185CuO₄

We have studied magnetic excitations in impurity-doped La _1.85Sr_0.15Cu_1-y,A_yO₄ (A = Ni or Zn) by neutron scattering. The dispersion for Zn:y = 0.017 is similar to that for the impurity-free sample: incommensurate peaks with incommensurability σ - 0.12 ± 0.01 (rlu) do not change their positions up to 21 meV. On the other hand, for Ni:y - 0.029, two incommensurate peaks observed at low energies suddenly change into a single broad commensurate peak at E _cross - 15 meV. Compared with that for the impurity-free sample with a similar Sr concentration of x - 0.16, E_cross for Ni:y - 0.029 is decreased by nearly the same factor as the reduction in TE_c. This is very similar to the shift of resonance energy (E_res)in Ni-doped YBa₂Cu₃O₇. These common impurity effects on the shifts of E_cross and E_res suggest the same magnetic origin of the resonance peak in YBa₂Cu₃O_6+x and that of the crossing point of upward and downward dispersions in La _2-xSr_xCuO₄. We propose that the sudden change in the dispersions is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than an hourglass-like dispersion.