  The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa$_2$(Cu$_{0.97}$Ni$_{0.03}$)$_3$O$_7$ (T$_c$=80 K) and
YBa$_2$(Cu$_{0.99}$Zn$_{0.01}$)$_3$O$_7$ (T$_c$=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E$_r \simeq$40 meV in
the pure system) shifts to lower energy with a preserved E$_r$/T$_c$ ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.
