  Using a Magnetic Resonace Force Microscope, we have performed ferromagnetic
resonance (FMR) spectroscopy on parametric magnons created by 4-wave process.
This is achieved by measuring the differential response to a small source
modulation superimposed to a constant excitation power that drives the dynamics
in the saturation regime of the transverse component. By sweeping the applied
field, we observe abrupt readjustement of the total number of magnons each time
the excitation coincides with a parametric mode. This gives rise to
ultra-narrow peaks whose linewith is lower than $5~10^{-6}$ of the applied
field.
