  Acoustic and optical ferromagnetic resonance (FMR) in the interlayer exchange
coupled Fe$_{20}$Ni$_{80}$/Co bilayer have been investigated. In the optical
mode, unexpected increasing tendencies of peak value at the resonance frequency
has been observed under an increasing magnetic field. We presented analytical
calculations with which the exchange coupling between Co and Fe$_{20}$Ni$_{80}$
layers, the magnetization and the in-plane uniaxial anisotropy are taken into
account, to interpret the increasing of the maximum values of the optical
permeability. Both experimental measurements and theoretical calculation show
that such tendencies are dependent on the layer thickness t, and that there is
a critical field above which the optical peak value begins to decrease. These
results might help us to understand the mechanism of interlayer exchange
coupling induced optical FMR and might enlighten us to find new possibility of
high frequency applications of magnetic materials.
