Fe/Si multilayers with strong bilinear and biquadratic couplings were inves
tigated. A complex structure revealed by the Mossbauer spectroscopy corresp
onds to multimode ferromagnetic resonance spectra in a non-saturated state.
Simple dispersion relations for antiferromagnetic coupled bilayer structur
es are shown to be inapplicable to the Fe/Si multilayers with a strong biqu
adratic component to the antiferromagnetic bilinear coupling.