  We use a pure spin current originating from the spin Hall effect to generate
a spin-orbit torque (SOT) strongly reducing the effective damping in an
adjacent ferromagnet. Due to additional microwave excitation, large spin-wave
amplitudes are achieved exceeding the threshold for 4-magnon scattering, thus
resulting in additional spin-wave signals at discrete frequencies. Two or more
modes are generated below and above the directly pumped mode with equal
frequency spacing. It is shown how this nonlinear process can be controlled in
magnonic waveguides by the applied dc current and the microwave pumping power.
The sudden onset of the nonlinear effect after exceeding the thresholds can be
interpreted as spiking phenomenom which makes the effect potentially
interesting for neuromorphic computing applications. Moreover, we investigated
this effect under microwave frequency and external field variation. The
appearance of the additional modes was investigated in the time-domain
revealing a time delay between the directly excited and the simultaneously
generated nonlinear modes. Furthermore, spatially resolved measurements show
different spatial decay lengths of the directly pumped mode and nonlinear
modes.
