Renormalization of Commensurate Magnetic Peak in Ni-doped La1.85Sr0.15CuO4

We have studied the magnetic excitations in impurity doped La1.85Sr0.15Cu1−yAyO4 (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 the incommensurability δ = 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 broad commensurate peak at Ecross = 15 meV. Compared to the impurity free sample with a similar Sr-concentration x = 0.16, Ecross for Ni:y = 0.029 is decreased by nearly the same factor for the reduction in Tc. This is very similar to the shift of the resonance energy (Eres) in Ni-doped YBa2Cu3O7. These common impurity effects on the shift of Ecross and Eres suggest the same magnetic origin for the resonance peak in YBa2Cu3Oδ and that for a crossing point of upward and downward dispersions in the La2−xSrxCuO4. We propose that the sudden change in the dispersion is better described by a crossover from incommensurate spin fluctuations to a gapped spin wave rather than a hourglass-like dispersion.