  We use inelastic neutron scattering to probe magnetic excitations of an
optimally electron-doped superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$
above and below its superconducting transition temperature $T_c=25$ K. In
addition to gradually opening a spin pseudo gap at the antiferromagnetic
ordering wavevector ${\bf Q}=(1/2,1/2,0)$, the effect of superconductivity is
to form a resonance centered also at ${\bf Q}=(1/2,1/2,0)$ but at energies
above the spin pseudo gap. The intensity of the resonance develops like a
superconducting order parameter, similar to those for hole-doped
superconductors and electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$. The
resonance is therefore a general phenomenon of cuprate superconductors, and
must be fundamental to the mechanism of high-$T_c$ superconductivity.
