Biharmonic pumping technique was used to study four-photon nonlinear respon
se of ultrathin ferromagnetic metal films (Ni, thickness down to 17 nm) and
some reference samples (Bi and Au). In the case of Ni films, we found spec
ific two-photon resonant feature on a side wing of dependence of self-diffr
action efficiency versus frequency detuning of biharmonic pumping component
s. We interpret this feature as a manifestation of Ni ferromagnetism.